Early Oldowan technology thrived during Pliocene environmental change in the Turkana Basin, Kenya

Domain: earth_science
Published: 2025-11-04
Editorial outcome: minor_revision

Abstract
Approximately 2.75 million years ago, the Turkana Basin in Kenya experienced environmental changes, including increased aridity and environmental variability. Namorotukunan is a newly discovered archaeological site which provides a window into hominin behavioral adaptations. This site lies within the upper Tulu Bor and lower Burgi members of the Koobi Fora Formation (Marsabit District, Kenya), presently a poorly understood time interval due to large-scale erosional events. Moreover, this locale represents the earliest known evidence of Oldowan technology within the Koobi Fora Formation. Oldowan sites, older than 2.6 million years ago, are rare, and these typically represent insights from narrow windows of time. In contrast, Namorotukunan provides evidence of tool-making behaviors spanning hundreds of thousands of years, offering a unique temporal perspective on technological stability. The site comprises three distinct archaeological horizons spanning approximately 300,000 years (2.75−2.44 Ma). Our findings suggest continuity in tool-making practices over time, with evidence of systematic selection of rock types. Geological descriptions and chronological data, provide robust age control and contextualize the archaeological finds. We employ multiple paleoenvironmental proxies, to reconstruct past ecological conditions. Our study highlights the interplay between environmental shifts and technological innovations, shedding light on pivotal factors in the trajectory of human evolution.

Introduction
The remarkable ability of humans to inhabit nearly every terrestrial ecosystem is a result of the synergy between biological and technological evolution1. The long term significance of our evolutionary relationship with technology arises from the discovery of stone artifacts within Plio-Pleistocene sediments2,3. This study provides new evidence from the Turkana Basin for the relationship between climatic and environmental shifts and the development of stone tool technologies by hominins. The earliest known assemblages of Oldowan artifacts (~2.9-2.6 Ma) are confined to four localities in eastern Africa4,5 (Fig. 1). The earliest known localities provide insights into technological behaviors at single time horizons in the deep past. Despite advances in our understanding of early human technology, the specific mechanisms through which environmental changes influenced technological evolution in the earliest Oldowan remain poor understood. Demonstrating how early hominins adapted their tool-making practices in response to changing environments would provide new insights into the evolutionary pressures that shaped these innovations. Here, we describe multiple assemblages of stone artifacts from well-constrained horizon, with age estimates at 2.75, 2.58, and 2.44 Ma, from the Koobi Fora Formation in the northeastern portion of the Turkana Basin (Paleontological Collection Area 40, archeological site Namorotukunan, National Museums of Kenya ID: FwJj 52). Detailed knowledge of environmental patterns within this region allows us to explore the interplay between periods of environmental change and the presence of an early Oldowan techno-complex that first evolved in the late Pliocene, and emphasize temporal continuity throughout the Early Pleistocene, in eastern Africa.

The earliest phases of tool manufacture, dating back to over 3.0 million years ago6 (although see Archer7), highlight percussive technology, which is ubiquitous in hominin records and shared with other primates8,9. Tool use, associated with extractive foraging, is a recurring trait in some extant primates10. The oldest systematic production of sharp-edged stone artifacts, known as the Oldowan, is found in the hominin behavioral record at eastern African sites: Ledi-Geraru and Gona in the Afar Basin (2.6 Ma), Ethiopia, and Nyayanga in western Kenya (2.6-2.9 Ma)3,11–13.

In this study, we describe the earliest known Oldowan technologies and their paleoenvironmental context in the Koobi Fora Formation (Fig. 1). The presence of butchery marks on bones within the Namorotukunan assemblage underscores the role of sharp-edged tools in the foraging behavior of these hominins and suggests that the development of Oldowan technology was associated with the exploitation of resources mediated by tool use14. Open habitats such as savannas and grasslands expand at the end of the Pliocene and may have facilitated an adaptive shift in hominins towards a regular exploitation of foods requiring the use of tools (e.g., USOs, bone marrow, meat)15,16. Here we examine three distinct temporal horizons containing evidence of sharp-edged tool technology spanning ~300,000 years. The consistent technological approaches across this interval suggest an enduring technological adaptation in the hominin lineage throughout the late Pliocene and earliest Pleistocene.

Results
Geological context
The Koobi Fora Formation (KFFm.) refers to the sedimentary strata of the Omo Group on the eastern side of Lake Turkana and lies unconformably on Miocene and Pliocene volcanic rocks and associated sediments17,18. The Omo Group encompasses these and other Plio-Pleistocene lacustrine and fluvial deposits that fill a series of alternating half-grabens of the Omo-Turkana Basin, which in turn is part of the larger East African Rift System (EARS19,). Excellent age control throughout the Omo Group (4.02 to ~0.75 Ma) is provided by 40Ar/39Ar dates on volcanic ash layers, K/Ar ages on intercalated basalts and paleomagnetic stratigraphy20. The KFFm. is divided into eight members (see Fig. 1), each delineated by a dated volcaniclastic tephra marker bed at the base of the member (except the basal Lonyumon Mbr19.). In Area 40, where Namorotukunan is located, sedimentary strata are estimated to range roughly between 4.3 to 1.6 Ma21,22, with a gap between 3.0−2.5 Ma represented by the Burgi Unconformity21,23,24. Above this unconformity, a series of lake clays indicate a local change in tectono-sedimentary patterns and the Paleo-Lake Lorenyang transgression in the Turkana basin21,24. To the west of Namorotukunan, across the North Gele Fault19, a younger sedimentary succession is exposed that includes at least the Upper Burgi and KBS Members, as evidenced by the presence of the KBS Tuff within that sequence21.

Geological investigations focused on a 46-meter-thick stratigraphic interval examined in seven sections (Fig. 2). These sections are assigned to local geological units used for mapping around Namorotukunan (Supplementary Fig. 1). The lowermost unit in this framework is a tephra level (TB), identified as the Tulu Bor Tuff21 (Supplementary Table 1). The uppermost level is a sequence of lake clays associated with the hill locally known as Namorotukunan, from which the archeological site name is derived (L2b). This lithological succession comprises three distinctly different sediment packages separated by discrete boundaries (Supplementary Table 2).

The oldest sediment package is characterized by red silts, paleosols, and sands overlying the Tulu Bor Tuff (TB). It consists of poorly developed paleosols with root traces and carbonate-rich silts intercalated with sands and fine gravels. This package is interpreted as a fluvial floodplain (R1 to R4, Fig. 2 and Supplementary Fig. 2; Supplementary Table 2).

The second package is composed of gravels and paleosols, indicating paleochannels and swamp clays associated with a fluvial system (C1-2, C3, C6), characterized by alternating channel and overbank with intervals of paleosols, interpreted as belonging to a floodplain (S3 to C7). All three artifact-bearing horizons are situated within this sedimentary package. The lower part of this interval contains darker paleosols (P0-P3) rich in carbonate concretions and rhizomes, which alternate with sandy gravel deposits that preserve sand casts of plant roots and rhizomes (Supplementary Fig. 2). Brownish-red, poorly developed paleosols characterize the upper part (P4-P6). Some of the upper paleosol levels (e.g., P5) feature large irregular desiccation cracks (Supplementary Fig. 2), filled with well-cemented sands from the overlying bed (e.g., C6). Cut and fill sequences are limited to three local paleo-rivers corresponding to levels C1-2, C3, and C6. The uppermost cut (C6) is the most laterally extensive and shows characteristics of a meandering river. In some areas, this channel eroded underlying sediments down to the paleosol level P3. These cut and fill structures, representing larger paleo-river features, are highly localized and transition laterally into sandy-silty beds interpreted as overbank deposits.

The artifact-bearing horizons occur in the paleoriver system, where currents in the main channel of the ancient, braided river system were robust enough to transport cobbles and pebble-sized clasts that served as the source materials for the stone tools. The paleoriver system included low-energy environments, where slack water and point-bar deposits as well as floodplain soil surfaces provided the geological context for the preservation of artifacts and faunal remains.

The uppermost and third sedimentary package lies above paleosol level P6 and consists of lake clays (L1 to L2b, see Fig. 2, and Supplementary Fig. 2), occasionally featuring sandy units (e.g., LS1) or diatom-rich sediments (e.g., LD) and a single level containing frequent catfish fossil remains (LS1-L2a transition). Previous investigations conducted on the slopes of Namorotukunan Hill (see Fig. 7C in Baldes et al.24) identified the Burgi Unconformity at the base of the L1 bed. However, no age constraints were provided to determine the extent of the time gap represented by this unconformity, leaving its duration, if any, unresolved.

Within the investigated Area 40 sedimentary sequence, four age markers were identified that allowed development of a comprehensive age model for the entire sequence (Fig. 3). The oldest of these markers is a tephra marker bed, which represents the base of the interval. This layer has been identified as Tulu Bor Tuff β21 and is dated by a 40Ar/39Ar feldspar age of 3.44 ± 0.02 million years22,25–27. Geochemical analyses (Supplementary Table 1) also confirm this as the Tulu Bor Tuff β.

Paleomagnetic investigations of all the lithologies present, except for the unconsolidated coarse sands (R1-R4), revealed four distinct magnetic polarity intervals (Supplementary Fig. 3). The lowest part of the section, with a thickness of 16.5 meters, corresponds to a normal polarity zone (NZ1). Subsequently, there is a section ~20 meters thick (RZ1) that features reverse polarities. This is followed by a shorter normal polarity segment (NZ2), around 9 meters thick. The top of the Namorotukunan Hill, less than 1 meter thick, is characterized again by the beginning of another reversed polarity zone (RZ2). In most of Area 40 this unit has been removed by modern erosion.

The Tulu Bor Tuff forms the lower boundary of the sequence at 3.44 Ma, and the Paleo-Lake Lorenyang transgression28,29 marks its upper boundary at around 2.2 Ma. These two markers allow us to correlate the local magnetic polarity sequence observed at Namorotukunan with the established Geomagnetic Polarity Timescale (Supplementary Data 1, Supplementary Data 2 GPTS30). Specifically, NZ1 corresponds with the Gauss (C2An) Chron, RZ1, and RZ2 represent parts of the Matuyama (C2r) Chron, and NZ2 correlates with the Feni excursion (Formerly Réunion excursion)31 or, alternatively, with the Olduvai sub-chron. The Mammoth (C2An.2r) and Kaena (C2An.1r) reverse polarity subchrons from the lower part of the Gauss Chron were not found. These are either located in the sandy intervals S1 and S2, (incompatible with paleomagnetic analysis) or these sandy intervals are associated with disconformities. This limitation impacts our estimations of sedimentation rates in the lower interval but critically, doesn't significantly affect the age estimation of the artifact levels investigated here.

The studied interval includes the Upper Burgi Unconformity (UBU), a surface interpreted in parts of the Koobi Fora Formation as marking a depositional hiatus of several hundred thousand years24. However, the UBU is not clearly expressed in Area 40, where the transition from fluvial to lacustrine facies is conformable and no field evidence for an erosional surface is present. The overlying lake clays match Upper Burgi lithologies described by Baldes24 and the absence of the KBS Tuff in these well-preserved sediments—despite its occurrence ~600 m west of Namorotukunan across the North Gele Fault21—suggests that the Namorotukunan section does not reach the KBS Member. If a hiatus is present, it must be confined to a narrow interval within RZ1, implying a much shorter duration than has been proposed for central Koobi Fora. We thus consider two possibilities: Scenario A, in which sedimentation is continuous, and Scenario B, in which the UBU represents a brief interruption. Both scenarios securely place the archeological levels in the late Pliocene–early Pleistocene and do not alter our behavioral or chronological interpretations.

Considering the correlation of R1 with the Matuyama chron, we are presented with two correlation options for the NZ2 polarity interval: either aligning it with the Feni excursion or the Olduvai sub-chron. We favor the first option—the correlation with the Feni excursion—due to the absence of the KBS Tuff in the lacustrine sediments corresponding to the NZ2 polarity zone. The KBS Tuff is an important and geographically expansive stratigraphic marker that, if present, would support a correlation with the Olduvai subchron. KBS has been described ~600 west of Namorotukunan, across the North Gele fault21, overlaying a thick succesion of lake clays, inconsistent with the section at Namorotukunan. Its absence in Namorotukunan suggests that the NZ2–RZ2 interval more likely corresponds to the Feni excursion.

In addition to the level of the Tulu Bor Tuff β (3.44 ± 0.02 Ma), paleomagnetic data provide three new chronological points for developing the age model for the studied section and obtaining an age estimate for the artifact-bearing horizons at Namorotukunan (Fig. 3). These points are the Gauss-Matuyama reversal (NZ1/RZ1) at 2.610 million years30, the onset of the Feni subchron (RZ1/NZ2) at 2.137 million years, and the termination of the Feni subchron (NZ2/RZ2) at 2.116 million years32.

We present two alternative age models based on the different correlations described above. These age models reflect two possible scenarios: the first scenario (Fig. 3a) assumes continuous sedimentation between gravel levels C7 and lake clays L1, while the second scenario (Fig. 3b) incorporates the Burgi Unconformity at the C7/L1 lithologic boundary, as previously suggested by Kidney21 and Baldes et al.24. This second scenario results in slightly older age estimates for sites Namorotukunan-3 (NMT3) and Namorotukunan-2 (NMT2).

In this paper, we use the first scenario, which provides more conservative (younger) age estimates. It is important to note that these scenarios do not affect the age estimation of the oldest archeological levels presented in this paper.

Archeological findings
Archeological research was carried out within the paleontological collection Area 40 at the Namorotukunan site (36.329 E, 4.399 N). Three excavations (in a ~5000 m2 area), named Namorotukunan-1 (NMT1), Namorotukunan-2 (NMT2), and Namorotukunan-3 (NMT3), corresponding to discrete lithological units S3-C1 (for NMT1), C2-P2 (for NMT2), and C3 (for NMT3) yielded a total of 1290 artifacts: NMT1 (n=198), NMT2 (n=775), and NMT3 (n=317), along with associated fossils (Supplementary Table 3). Artifact-bearing horizons are concentrated in sands and fine gravels (Fig. 4). Artifacts associated with the C3 horizon, (NMT3) appear to have undergone minor post-depositional disturbance as indicated by the absence of the smaller fraction of artifacts and linear orientation for this 2.44 Ma archeological horizon (NMT3; see Supplementary Figs. 4, 5, 6, 7). Abrasion on artifacts is present in some of the artifacts, yet thin sharp edges are preserved in the assemblages (% of assemblage exhibiting abrasion ranges from 18-22%). Geochemical and mechanical damage to the surfaces of many of the fossil bone specimens prevents a detailed taphonomic analysis of bone modification in the faunal assemblage. However, some specimens within the 2.58 Ma assemblage include well-preserved surfaces that possess butchery marks indicating that hominins used tools to extract high-quality dietary resources from large mammalian remains (see Supplementary Fig. 8).

Detailed Descriptions Of The Archeological Finds
Artifacts from Namorotukunan exhibit many features that are diagnostic of anthropogenic conchoidal fracture (e.g., prominent bulbs of percussion, clear striking platforms, contiguous flake scars; although see Proffitt et al.9,33) and are dominated by smaller flakes and simple cores. High proportions of sharp-edged flakes and fragments, comprising 94.2 to 79.4% of the assemblages, indicate sharp edges were the likely focus of this technology. Early Oldowan assemblages tend to have flakes that are of similar dimension to their cores (as opposed to Acheulean sites where cores are much larger) and have fewer flake scars on cores (as compared to younger Oldowan assemblages3,12 (Fig. 5). Various technological attributes indicate that the Namorotukunan assemblages are more similar to other early Oldowan assemblages (e.g., Bokol Dora 1; Nyayanga) than assemblages younger than 1.8 Ma (Fig. 5; and Supplementary Fig. 10,11). Evidence of an understanding of fracture mechanics (e.g., relationship between platform angles and other technological features) is similar between hominins that produced the assemblages at Namorotukunan and older Oldowan assemblages (Supplementary Fig. 11). Cores from the Namorotukunan assemblages also show similar patterns to those seen at other early Oldowan assemblages (>2.3 Ma) where cores are rarely rotated during production (% Cores that are Unifacial; Fig. 5). Although percussive tools are infrequent in the Namorotukunan assemblage, the appearance of heavily battered artifacts (<1% of assemblage; NMT2) confirm the presence of percussive activities (Supplementary Fig. 12).

Comparisons of raw material selectivity demonstrate fine grained chalcedony at proportions that are significantly greater than its presence in nearby conglomerates (Supplementary Fig. 13). We note that although hominins at Namorotukunan appear to be selective in their selection of specific rock types, there is little evidence of transport of stone. Gravels (braided river system deposits) that carried cobbles suitable for artifact manufacture were present in coarser grained sediments directly adjacent to all archeological sites. This level of selectivity is evident in all three assemblages and suggests a persistent preference for materials that fracture consistently (Supplementary Fig. 14).

Paleoenvironmental proxies
The stratigraphic sequence in Area 40 of the KFFm provides a framework for examining the paleoecological conditions from ~3.44 to ~2.0 million years ago. The existence of paleosols, along with accompanying sediments and fauna, facilitates a multiproxy reconstruction of the fluctuating environmental conditions within this period. Pedogenic carbonates, plant wax biomarkers, and phytoliths provide detailed information (Fig. 6, and Supplementary Fig 15 and Supplementary Data 4) on the physiognomic structure of the ecosystems across the late Pliocene and early Pleistocene in the region. The range of values from some paleoenvironmental proxies indicate the presence of diverse habitats within some temporal intervals (e.g., δ13C values of pedogenic carbonates −10 to −2 ‰). However, when fitting linear estimates through the data, there are coincident changes that suggest an increase in C4 vegetation and a decrease in water availability in the gravel and paleosol interval (2.7-2.2 Ma; Supplementary Fig. 16). This is supported by increases in the δ13C values of C31 and C33 n-alkanes that indicate an increase in C4 vegetation and likely, open habitats, in the gravel and paleosol interval.

Phytolith analysis indicates an overall decrease in grass phytoliths that characterize the gravel and paleosol interval (Fig. 6). High incidences of palm and sedge phytoliths before these red silts, paleosols, and sands interval, indicate the presence of substantial standing water in these habitats (3.4–2.6 Ma) that is notably absent in the younger sediments, the gravel and paleosol interval (2.6-2.2 Ma). Significant increases in microcharcoals, associated with landscape-scale wildfires, point to a more seasonally arid habitat beginning around the onset of the gravel and paleosol interval (2.7-2.6 Ma; Fig. 6). Seasonality probably played a key role, as shown by investigations of paleo-fire frequency on more recent timescales in Africa34. Phytoliths from the 2.7-2.2 Ma time interval (Fig. 6) show increased shrub-derived morphotypes and decreased grass phytoliths, further suggesting a shift towards more seasonally arid habitats. This is further supported by the presence of phytoliths of C4 Chloridoideae grasses suggesting xerophytic habitats consistent with arid conditions at this time.

Chemical index of alteration (CIA) and mean annual precipitation (MAP) was calculated based on bulk geochemistry of paleosol samples (see “Methods” and Supplementary Note 2). High CIA values indicate removal of labile cations (Ca2+, Na+ and K+) relative to Al3+ during the chemical weathering of silicate minerals under wet climatic conditions and low values indicate absence of the chemical weathering under dry climate35,36. CIA values (51 to 64) indicate incipient to moderately weathered paleosols. Low CIA values (<55) between 2.9 and 2.7 Ma indicates dry climate (Fig. 6). MAP calculated using geochemical climofunctions for paleosols varies between 231 and 855 mm/year, broadly classifying the climate as semi-arid to semi-humid. MAP follows a similar trend as of CIA and lowest MAP is recorded between 2.7 and 2.6 Ma indicating an increased aridity at this time interval. The timeframe between 2.7 and 2.2 Ma is associated with the relatively low MAP estimates. This pattern is reversed at the return of lacustrine environments (i.e., Paleo-Lake Lorenyang) at the top of the sequence (2.2-2.0 Ma) (Fig. 6).

Environmental magnetic analysis of bulk sediments confirms the reliability of our paleomagnetic approach. The analysis indicates that magnetite is the primary iron oxide, carrying magnetic properties in nearly all samples. However, at the onset of the gravel and paleosol interval (Supplementary Fig. 17), we detected a notable exception: evidence of a second iron mineral, hematite.

Methods
Fieldwork
Excavations at Namorotukunan were conducted in Area 40 (East Turkana Basin, Northern Kenya) between 2013 and 2022. The objective was to excavate known Early Pleistocene localities19 to provide a multidisciplinary context for the Oldowan lithic assemblage found at the site (Fig. 1). All material was excavated according to discrete stratigraphic horizons. Archeological material was mapped to the nearest mm using a total station Leica Builder 505 with a TDS Nomad 900LE using EDM Mobile 5.1bt84. Detailed stratigraphic information for many artifacts was captured using three-dimensional photogrammetric models of artifacts in their in situ context. Excavations were conducted under the auspices of a permit provided by the National Council for Science and Technology of Kenya. Excavations and collections were carried out under the auspices of an Exploration and Excavation Permit provided by the Kenyan Ministry of Heritage, Tourism and Culture. This research was carried out with permissions granted by the National Museums of Kenya and the Ministry of Education, Science, and Technology of the Republic of Kenya (MOEST 13/001/31C 216 and NACOSTI/P/23/31328).

Artifact and faunal analyses
All artifacts were measured using standard caliper and goniometer measures at the Archeology Division of the National Museums of Kenya in Nairobi. Comparisons of technological variables were compiled from the literature following recent reviews3. Analysis of core forms (Fig. 5) included 275 three dimensional models. These models were captured using a structured light scanner (Einscan). Three dimensional models were captured at the National Museum of Kenya (with the exception of the Lomekwi 3 collection which was captured from models available at http:africanfossils.org). Three dimensional models were analyzed using morphometric packages in the R statistical computer language (Morpho, bezier, Rvcg, rgl). Placement of landmarks on three dimensional models was conducted using MeshLab software (2022.10). Variables (e.g., rotation angle between flake scars) were calculated from three dimensional models. Data in this analysis deviate significantly from a normal distribution. As such we calculated differences between groups using a permutation test that compared differences between groups to 10000 resampled simulated assemblages of similar size samples from the entire data set. Significance values are estimated based on the probability of finding similar differences in resampled data sets. Faunal specimens were analyzed in the Archeology Division of the National Museums of Kenya. Modifications to bone surfaces were identified using a 5X hand lens. Further imaging of modifications took place on a Sensofar Nanofocus spinning disk microscope at the Max Planck Institute for Evolutionary Anthropology.

Geological description and stratigraphy
Lithological characterization was conducted in situ across various studied sections, where each section was thoroughly examined to identify and describe the rock types present. Observations focused on key characteristics including texture, color, mineral composition, and structural features (Supplementary Table 2). Systematic sampling was performed, with rock samples collected for detailed laboratory analyses.

To synthesize the geological attributes of the area, seven distinct sections (A-G) were logged, sampled, and measured using a combination of geological field techniques and geochemical methods as well as site fabric analysis. Bedding planes and fault planes were measured using a Brunton compass. The overlapping seven sections were combined in a composite section (Supplementary Fig. 3). These descriptions were then integrated to construct a composite synthetic section that represents the overall geological framework and variability across the study area, facilitating a comprehensive understanding of the geological context and its spatial heterogeneity.

Lateral geological features were documented to assess the horizontal continuity and distribution of rock formations. These observations were crucial for understanding the lateral extent of geological phenomena and their implications on the regional geological setting.

High-detail examination of cut and fill features was undertaken to interpret the processes of erosion and deposition that have influenced the landscape. This included detailed logging of stratigraphic sections to identify sequences of deposition, erosional unconformities, and associated geological processes.

We used paleomagnetism and geochemistry to refine age markers in the section. 160 oriented samples were specifically collected for paleomagnetic studies. Paleomagnetic studies were mainly focused on magnetostratigraphy, aimed at identifying magnetic polarity reversals. Geochemical methods were employed to verify if the signatures of the tephra correspond to well dated tephra in the region. Additionally, 222 unoriented samples were taken for paleoenvironmental reconstruction analyses. We used a multiproxy approach (including Rock-Magnetism, Geochemistry, Phytoliths, etc.) from well dated stratigraphic levels, to describe the paleoenvironmental shifts in the East Turkana environments.

Paleomagnetism
Paleomagnetic samples from the Namorotukunan site were collected across seven trench sections (A–G) (Supplementary Fig. S16; Supplementary Note 1) during six field campaigns conducted between 2012-2022. Using a 2.5 cm-diameter diamond-coated bit powered by hand operated air pump (necessary because water cooling would destroy unconsolidated samples) samples were oriented with a geological compass and inclinometer. After extraction, the samples underwent progressive demagnetization in the laboratory. In the Utrecht paleomagnetic facility 160 samples (Supplementary Fig. S18) underwent dual demagnetization techniques: stepwise alternating-field (AF) demagnetization (0–80 mT) and thermal demagnetization (20–600°C). These methods were employed to isolate primary magnetic components and minimize overprinting.

Samples were measured at the Fort Hoofddijk Paleomagnetic Laboratory (Utrecht University) and at the CENIEH (Centro Nacional de Investigación sobre la Evolución Humana), demagnetization utilized a 2G Enterprises DC SQUID cryogenic magnetometer with an in-line, triaxial, AF demagnetizer for the AF samples, and an ASC thermal demagnetizer (residual field < 20 nT) for the Th samples. The demagnetization results were interpreted using Paleomagnetism.org85, Characteristic Remanent Magnetization (ChRM) directions were typically isolated between 15–45 mT or 150–600°C. Paleomagnetic directions were calculated using principal component analysis86 based on a minimum of four consecutive steps, ensuring the reliability and robustness of the outcomes (refer to Supplementary Data 1).

For rock magnetic insights, a separate sample set underwent Isothermal Remanent Magnetization (IRM) acquisition curve measurements (up to 1.5T) using a MicroMag Model 3900 Series VSM vibrating sample magnetometer (Lake Shore Measurements). These rock magnetic analyses complemented the paleomagnetic investigations, providing a comprehensive understanding of the magnetic properties within the sampled materials.

Tephra analysis
Samples of the Tulu Bor Tuff were collected at (4.399161 N/36.326657 E WGS84) and were based on previous descriptions of the Tulu Bor Tuff in Area 4021. Samples of the Tulu Bor Tuff were analyzed for geochemistry by electron microprobe (EMP) on a JEOL 8900 Superprobe instrument housed at the Carnegie Institution for Science’s Geophysical Laboratory. Samples were measured using wavelength-dispersive x-ray spectroscopy (WDS) using the following analytical setup: accelerating voltage = 12 keV, beam current = 10 nA, and a beam diameter = 10 μm, conditions ideal for reducing alkali loss while obtaining reliable counts for elements such as Fe.

Pedogenic carbonates
Pedogenic carbonates were collected from a 3 km2 region within paleontological Area 40. Samples were collected from paleosols with evidence of pedogenesis such as dish structures and root casts. Samples were collected from trenches that were at least 40 cm beneath the estimated top of the pedogenic horizon. Samples were limited to paleosols with discrete nodules. 47 pedogenic carbonates were collected from this region and results are presented in Fig. 6.

Carbonate nodules were isolated from the surrounding sediment using a rotary diamond bit to remove debris, after which they were manually homogenized in a mortar and pestle. The homogenized carbonate reacted with 100% H3PO4 at 90°C. A dual inlet Thermo MAT253 isotope ratio mass spectrometer analyzed the resulting CO2 gasses for 13C/12C. Sample results were calibrated using an internal Hagit Carrara marble standard and an NBS-19 calcite standard. The average standard precision over multiple runs was 0.5–0.04 ‰ for 13C at Johns Hopkins University.

Plant wax biomarkers
Sediment samples were collected in the field directly onto Al foil. Samples came from at least 40 cm below modern surfaces and were screened to ensure modern plant material (e.g., roots) was absent. In the lab, samples were rinsed with an organic solvent, dichloromethane (DCM), to remove possible surface contaminants. Samples were then wrapped in foil, placed on a steel plate, and gently disaggregated with a hammer into 1–4 cm size pieces. About 200 g of sample was further crushed into a powder using a solvent-rinsed mortar and pestle. An average of 190.8 g (min: 172.6; max: 198.7 g) of sample was extracted using a Dionex Accelerated Solvent Extractor 350 over three consecutive aliquots (~64 g each) of sample packed into 66 ml cells. Soluble lipids were extracted at 100°C and 1600 psi (110.3 bar) over four 10-min static cycles with 9:1 DCM:methanol. A recovery standard was added to the total lipid extract (TLE) after extraction. The TLE was dried down under purified nitrogen and compounds were separated into aliphatic, ketone and ester, polar fractions via silica gel flash column chromatography using hexanes, DCM, and methanol, respectively. The aliphatic fraction contained the n-alkanes.

n-Alkanes were characterized and quantified on an Agilent gas chromatograph (Agilent 7890A GC) equipped with both a mass selective detector (5975C MSD) and flame ionization detector (FID). One microliter of sample dissolved in 100 ml hexane was injected into a multi-mode inlet injector at 60°C (0.1 min hold), which was then ramped to 320°C at 900°C/min and held for the duration of the analysis. Initial GC (gas chromatograph) oven temperature was set at 60°C (1.5 min hold) and ramped to 150°C at 15°C/min, then to 320°C at 48 C/min. A helium-purged microfluidics device downstream of a DB-5 column (30 m length, 250 mm ID) quantitatively split the GC flow to the MSD and FID detectors. Compound identification was done with comparison of mass spectra and retention times to authentic standards, and quantification was done using MSD peak areas using a response factor correction based upon the peak area of known concentrations of internal standards added to the TLE. The carbon preference index (CPI) was calculated using the equation of Marzi et al.87 and average chain length (ACL) was calculated as ∑b iC/∑b C, where C is the concentration of the molecule with chain length i and i=27, 29, 31, 33, 35 for n-alkanes.

Carbon isotope ratios of n-alkanes were analyzed using a GC coupled to a Thermo Delta V isotope ratio mass spectrometer through a combustion interface at the Lamont Doherty Earth Observatory Stable Isotope Laboratory. All sample injections were interspersed with injections of molecular mixtures with known isotopic values (mixes A4 and A5 supplied by Arndt Schimmelmann, Univ. of Indiana) that were used for correction of carbon isotope values. Samples were injected in duplicate or triplicate when feasible.

Phytoliths
To extract phytoliths from sediment, ~1 g of sediment was treated with 10% hydrochloric acid (HCL) to remove the carbonates. Samples were then washed using distilled water and sieved at 250 µm to remove larger particles (>250 µm) and then treated with nitric acid (HNO3) to remove organic matter through an oxidation process. The remaining material was gently sieved through a 53 µm sieve to loosen clay particles adhered to phytoliths for clarity during microscopy analyses. Samples noted to have stubborn clays were further cleaned by repeated sedimentation process and, when necessary, sodium hexametaphosphate was added to deflocculate clays, followed by multiple washing, decanting and centrifuging processes. To float the phytoliths/bio-silica, we added 10 ml of sodium polytungstate with specific gravity of 2.4 g/cm3. Phytoliths were pipetted out to another set of centrifuge tubes for final distilled water washing and recovery.

The recovered residue was dried, and 0.002 g of this was mounted using “Entellen New” mounting media (refractive index 1.53) on a slide and viewed under Carl Zeiss Scope A1 light microscope. Observation, counting and classification was done at 400x magnification. Where necessary, oil immersion was done for Grass Short Silica Cells (GSSCs) morphotypes and photography. Microphotographs were taken by USB 3.0 Digital Camera, THORLABS. Phytolith classification largely followed International Code for Phytolith Nomenclature (ICPN) 1.0 and 2.088 for both non-GSSCs and GSSCs phytoliths. Other classification systems were also consulted89. Only well preserved phytoliths were counted and classified to taxon; weathered phytoliths were classified as indeterminate. Except for one sample, all yielded statistically reliable phytolith counts averaging above 300 morphotypes.

Microcharcoal
Each sample (2 ml) was mixed with 3% sodium hexametaphosphate (NaPO3)6 in a water bath (90°C) for 30 min and mechanically agitated over several days. Lycopodium spore tables were added to calculate pollen and microcharcoal concentrations. Samples settled and decanted (<2 µm), followed by sieving (>125 µm). Separation of pollen and charcoal was done using gentle mechanical agitation over several days followed by heavy liquid separation using lithium heteropolytungstate at specific gravity 2.0. Acetolysis was achieved through a 9:1 mixture of C4H6O3 to H2SO4 and 10% HCl. The samples were mounted on a microscope slide with glycerol (cover: 22×40 mm = inspected area). System microscopy was done at 25x, 40x, 63x. A minimum of 350 pollen grains and 200 microcharcoal were counted.

Paleosol geochemistry
X-ray Fluorescence (XRF) and Loss on Ignition (LOI). Bulk paleosols (n=22) underwent major oxide concentration analysis using the XRF fusion bead method on a Zetium model spectrometer at Laboratório de Caracterização Tecnológica (LCT), Universidade de São Paulo (USP), Brazil. Samples (<200 mesh) were agate-milled to minimize metal contamination. About 1 g of finely powdered samples was fused with lithium tetraborate (Li2B4O7) to form glass beads, eliminating particle size and matrix effects. Measurement accuracy, assessed with physical and analysis replicates and reference materials (GBW07401, GBW07404, Feldspar LCT-05, and Granite LCT-07), surpassed 5% for major oxides and ~7% for minor oxide P2O5. Precision, reflected by relative standard deviation (RSD), was 1% for major oxides and ~2.8% for minor oxide P2O5.

Loss on Ignition (LOI) estimation involved heating ~1 g of finely powdered samples at 1020°C for 2 h using a muffle furnace, presenting LOI (wt%) = 100 × (ash weight 1020°C / sample weight).

Inductively coupled plasma optical emission spectroscopy (ICP OES). ICP OES analysis on selected pedogenic carbonate-free paleosols (n=8) determined CaO* in silicates. Pedogenic carbonate removal involved treatment with 1N HCl, followed by washing, drying, and multi-acid digestion (HF:HNO3:HClO4:HCl). A 300 mg powdered (<200 mesh) sample underwent digestion with 2.4 ml of HCl-HF-HNO3 mixture (2:2:1). Accuracy and precision for CaO* were 2.3% and 0.5% (RSD), respectively. The analysis was performed on an iCap 6300 Duo model Thermo Scientific spectrometer at LCT, USP, using standards GBW07401 and GBW07404 for quality control within the same digestion conditions.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present a newly discovered, site, FwJj 52, Burgi Member, Koobi Fora Formation, Kenya. Three archaeological\nhorizons are described from a critical time period in Oldowan technology, from 2.75-2.44 Ma. In this manuscript, the authors\npresent tool making, geology, chronology, and paleoenvironmental descriptions and results. As the authors state, an issue\nwith early Oldowan technology is that it is difficult to study changes in technological behavior, especially in response to\nenvironmental change. This is because the earliest Oldowan sites provide snapshots, but do not contain continuous\ndeposits. The authors find and describe continuity in tool-making for 300,000 years. The argument presented in this\nmanuscript is that there was climate-driven paleoenvironmental change at the end of the Pliocene, and that there is a\nrelationship between this paleoenvironmental change and the development of stone tool technologies. In addition, the\nauthors argue that there is paleoenvironmental change in the Turkana Basin from 2.75-2.44 Ma, yet the stone tool\ntechnology they describe remains stable during this 300,000 year time period. It is unclear whether the authors are arguing\nfor or against technological innovation being linked to paleoenvironmetal change, or why in some scenarios, for example at\nthe end of the Pliocene, innovative technology is linked to paleoenvironmetal change, whereas during the early Pleistocene\nthere is a stasis in stone tool technology. The authors suggest that paleorivers are related to stone tool technology, and that\nonce rivers replaced a lake on the landscape, stone tool technology persisted. This is explained well. However, it would be\nhelpful to explain and expand on how local changes in vegetation, as evidenced from the well-described carbonates, plant\nwax biomarkers, phytoliths, and faunal remains, are related to the stasis in stone tool technology described by the authors.\nIn order to strengthen the paper, the authors should include section photographs with in situ artifacts. Wall sections with\npoint provenience are shown in Figure 4, but photographs of artifacts in situ should also be included. If figure space is an\nissue, then perhaps section photographs with artifacts can be added to the supplementary material. It is essential in a paper\ndescribing some of the earliest excavated Oldowan technology. The authors refer to controversial stone tools from Lomekwi\nand cite replies that question the context of those artifacts. Section photographs of FwJj 52 with in situ artifacts should be\nincluded here. It would also be helpful to report the distance between the three archaeological sites.\nThe descriptions of stone tools and geology in the main text and supplementary material are thorough and presented well.\nThis manuscript represents a large amount of fieldwork, expertise, and interdisciplinary collaboration. It is a welcome\ncontribution. Some figure and table comments follow.\nFigure and Table Comments:\nFigure 1a should have a scale bar for the Turkana Basin.\nFigure 6j is cropped at the base, Figure 6k the legend and label “k” are cropped and not visible. What do the boxes and\ncrosses represent on Figure 6k? It is not indicated on the legend. In addition, for Figure 6k, the legend says this is showing\nchange, but it looks like this figure is showing a snapshot of modeled climate, not change from time slice 1 to time slice 2. It\nis unclear if we are seeing modeled climate or which areas become more arid from the Pliocene to the Pleistocene. Please\nrevise this figure as it is unclear from both the figure itself and the legend.\nSupplementary Figure 6 a scale bar is needed on the microscope images, and the lowermost inset image is rectangular\nwhile the box is square on the lower color photograph. It looks like the second inset image is not indicated on the correct\narea on the color photograph, since we can see the edge of the bone on the inset image, but the box is not on the edge of\nthe bone in the color photograph.\nMinor comments: Supplementary Table 7 has spelling errors in the names of the fauna. Is “Aepyceotiini” referring to\nAepycerotini? Some “sp.” are italicized while others are not.", "reviewer_2": "Review:\nThe article supplies a 46m composite stratigraphic section that includes three new stratigraphy levels bearing Oldowan tools\nin Turkana basin (East Africa). One of these layers has also vertebrate fossils with evidence of butchery. These\narchaeological sites occurs on sediments that range between 2.75 and 2.44 Ma, a period that were previously thought to be\nabsent in most of the northern portion of the Koobi Fora Fm.\nThe age model for this stratigraphic succession is based on the presence of the Tulu Bor tuff at the base of the sequence\nand four magnetic zones that place the sites in a very old period, up to 2.75 Ma for the oldest level. Two short reverse\nintervals are missing in this section probably because of erosion or included in sandstone beds not sampled in the\npaleomagnetic study. This study is exciting because corresponds to the equivalent time frame to the appearance of the\ngenus Homo and the oldest Oldowan tools ever recovered. These tools represent the oldest tools in the Koobi Fora\nformation, which is very relevant for the area.\nDifferent paleoclimatic proxies have been studied along 46 m of strata recording the period between 3.4 and 2.2Ma which is\na period poorly understood in the basin because of large-scale erosional events. Within this record, three superposed strata\ndated at 2.75, 2.58, and 2.44 Ma show the continuity in Oldowan technology in the area for a period of 0,3 Ma. These proxies\nsuggest a climatic shift at the time of the first appearance of Oldowan tools. This local paleoclimate record can be correlated\nwith other paleoclimatic records in East Africa and Ocean cores, so the authors claim a causal connection between\nenvironmental shifts and the appearance of the Oldowan tool technology in the Turkana Basin. Although this is a\nspeculative hypothesis, the presented data supports the contemporaneity between these two phenomena. Additional future\ndata would validate or not this idea. I think the article supplies important archaeological, chronological, and\npaleoenvironmental information that deserves to be published in Nature Communications. However, I have a list of\ncomments and suggestions that I think would be good to address before publication.\nGeneral Comments:\nFwJj52 is an important archaeological site and will be cited by many colleagues in the future, so probably deserves a name\neasy to remember. So, if the name of this site has not been published before I suggest a simpler name, easier to remember\nand place on a map, if possible with a geographical reference, for example, the name of the closest town, Ileret52?,\nlocal topographic name.\nAge model\nIt is not clear why you correlated NZ2 with the Feni 208 sub-chron (Reunion excursion) and not with Olduvai. Especially\nwhen considering the second scenario with Burgi Unconformity at the C7/L1 lithologic boundary, as previously suggested by\nKidney23 and Baldes et al.26. This would solve two problems, a) sedimentation rate would be more reasonable for this\ninterval, and B) you can correlate the upper lacustrine unit with Lorenyang lake that occurs at Olduvai and not with pre-\nLorenyang lake. This does not necessarily change the age of your sites since an erosional surface exists at this boundary.\nPlease explain better your reasons for this correlation and maybe include this possibility in your model that includes the\nBurgi Unconformity .\nMethods: please specify from the 160 paleomagnetic samples you study, you should indicate the number of samples that\ncorrespond to a single strata (sampling station), and how many stratigraphic levels (sampling stations) you sample. Also,\nconsider placing, supplementary material Figure 3 in the main text, since this is a very important figure.\nIn supplementary info: figures 18 and 19, cite the software you use to prepare the plots\nIn pmag supplementary tables: use only English language in the text\nMinor comments:\nLine 210: Mammoth and Kaena are periods with persistent reverse polarity during 84ka and 123ka, so you should use the\nterm “subchron” or “subchronozone” not “excursion” for Mammoth and Kaena.\nLine 293: is the shift towards arid conditions compatible with the lake phases between 2.2 and 2.4Ma shown in Fig 3 ?\nexplain better\nLine 427: since “Lorenyang lake” occurs in the Turkana lake system, I wonder why is not considered it just as Lake Turkana\nhighstand? (Lorenyang highstand),\nLine: 434 “During the late Pliocene, the EARS witnessed significant landscape and climate transformations such as lake\nregression, decreased woody cover, and aridification (Fig 6; e.g., 17,53,62–67) that significantly transformed vegetation\ncommunities, expanded grasslands, reduced woody cover “\nNo necessary to repeat “woody cover”\nLine: 446 A pivotal event during this period was the closure of the Panama Seaway roughly between 2.8 to 2.5 million years\nago, which had profound implications for Earth's climate and ecosystems 73\nThe reference indicated (73) suggests a late Miocene closure of the Panama isthmus (not Pliocene) based on U/Th zircons\nfrom Panama present in the North Andes, so rephrase please or add additional references.\nLine 579 Using a 2.5 cm-diameter diamond-coated bit powered by a hand pump,\nThis sounds strange, usually battery or gasoline-powered drills are used to collect pmag samples, hand pump should refer\nto the water pump used to lubricate the bit.\nFigure comments:\nFigure 2: This is a nice figure, I suggest placing the stratigraphic sections in chronological order (A, B, C, D etc) and below\nthe geological cross-section to read better the stratigraphy. I suggest placing the edges of the diagram vertically to improve\nthe perspective.\nFigure 3: the letters in the column are too small (ex TB).\nFigure 4: I suggest indicating in the figure the % for tools of specific lithology, (ex. basalt 65%)\nFigure 6: I suggest including the stratigraphic units at the base of the magnetic susceptibility plot, to see the relations of\nchanges in different proxies with lithology.\nIn the plot “Water deficit vs age”, the age numbers are cut. In the figure of “Plio-Pleistocene modeled climate”, the legend\ncan’t be read well.\nThis figure has too much information, I suggest reducing it (50%) for example by leaving the data produced in this study (two\ncolumns with 3 plots each), and placing the other plots as Supplementary information.", "reviewer_3": "Braun and colleagues report on new archaeological sites in northern Kenya, in Area 40, of the Koobi Fora Formation, that\nspan 2.75 to 2.44 Ma and are reported to the early Oldowan. By dealing with a time span that figures the earliest known\ndevelopment of a ‘systematic production of sharp-edged tools’, this paper adds significant insights to our understanding of\nthe adaptation of early toolmakers to changing environments. This is obviously of great significance at both local and global\nscales. The Koobi Fora Formation, with its major unconformities, in particular the Burgi Unconformity which covers much of\nthe 3 to 2 Ma time span, was the only formation of the Omo Group that lacked evidence for stone tool production prior to 2\nMa. Although we might of course have suspected that the early Oldowan groups that occupied the northern and western\nparts of the Turkana Basin (Shungura Fm and Nachukui Fm) might also have been present in its eastern part, it remained to\nbe demonstrated that the Koobi Fora Fm had the potential to preserve late Pliocene / early Pleistocene artifact-bearing\nhorizons. This is convincingly demonstrated for the first time in this article. This changes the paleogeography of the area,\nwhile confirming its status as a key area for exploring patterns of hominin behavioral adaptation during the earliest stages of\nthe Oldowan. The succession of three archaeological horizons in site FwJi 52 of Area 40, located in the northern part of the\nKoobi Fora Formation, adds a temporal dimension with age estimates of 2.75, 2.58 and 2.44 Ma. This enables a discussion\non patterns of behavioral continuity in a fluvial environment that has remained stable over this period. This article represents\na huge amount of multidisciplinary analytical effort, for a new site complex that will definitely enrich our understanding of the\nearly stages of the Oldowan technology. Our recommendations aim to better highlight the potential of these new sites for\ndiscussing patterns of hominin technological adaptation, based on the local environmental settings and associated\nresources at Koobi Fora and more broadly in the Turkana Basin, where discrete trajectories are documented. This would\nbring more originality to the article, which, despite presenting important new archaeological findings, tends to stick to long-\ndebated ideas without providing groundbreaking evidence.\nOne of the main challenges posed by this article is the dating issue, as the marker layers that bracket the archaeological\nhorizons define a particularly long time windows, ranging from 3.44 Ma to 2.2 Ma. Erosional phases and unconformities\noccurred within this time range, although it is not very clear how and where in the sequence they have been identified. The\nauthors point out in p.5 that there is “a gap between 3.0 -2.5 Ma represented by the Burgi Unconformity” in Area 40, a\nstatement which, unless explained, does not conform with what follows. Clarification about the Burgi Unconformity in this\narea is absolutely required to avoid confusion and to provide a solid basis for chronostratigraphic studies.\nWhile we are not qualified to discuss the details of the paleomagnetic investigations, some precautions must be taken in the\ntext; e.g. the age model, as robust as it appears, is based on age estimates, except for the Tulu Bor Tuff at the bottom of the\nsequence, and not on radiometric dates. For this reason, it should be more appropriate to use the term ‘age estimates’ rather\nthan ‘dates’ or ‘dated’ (see for instance the Introduction). Similarly, the three chronological points inferred from the\npaleomagnetic data cannot be taken as local age markers equivalent to the Tulu Bor Tuff, but rather as derived or inferred\nbenchmark ages.\nBased on a combination of local sections and a synthetic stratigraphic log, the geological context is particularly well\ndocumented and illustrated for the entire FwJj52 area. At a more local scale, precision regarding the depositional\nenvironment of the archaeological sites is limited to their location in coarse sands within the gravel and paleosol package.\nPrecisions regarding their position within the drainage system, and which drainage system (e.g. meandering, braided river?),\ncould however be highly relevant for assessing the proximity of the sites to cobble-rich conglomerates that could have\nserved as raw material supply areas. These data would enrich the arguments about raw material availability, as put forward in\nthe discussion. They would also allow for more consistent insights into site taphonomy. We can infer from the\nsedimentological data that the archaeological remains are in secondary position. Demonstrating how the drainage system\naffected depositional and redepositional processes would therefore be highly relevant. For example, what explains the\nstrong vertical dispersion of artifacts as seen in Fig.4, particularly in the older and younger sites? In this context, how can the\nartifacts be considered as deposited on a flat surface (SM Fig5)? What about assemblage integrity and artifact alteration, e.g.\ndegree of abrasion? What might explain limited size sorting and the absence or limited proportions of preferentially oriented\nartifacts in coarse sands (SM Fig5)?\nThe attribution of the assemblages to the early Oldowan seems well established, although pictures or drawings of artifacts\nillustrating a sample of flakes and cores are lacking to fully support this statement. The artifacts shown in Figure 4 lack\ndescriptive captions and they are presented at too small a scale to be really informative. The relevance of data used to\nstatistically discriminate Oldowan, Lomekwian and modern nonhuman primates must be justified, at least in Supplementary\nMaterial. More insight is also needed into the source data used in the PCA plots (see also below). Beyond their affiliation\nwith the early Oldowan, what patterns of continuity can be observed between the three sites? Adding more detail on local\nchronological trends, or lack of thereof, will provide more insight into the notion of continuity beyond the long duration of the\nearly Oldowan technology, which is already well established on a broader eastern African scale. Interpreting the data from\nKF Area 40 as indicative of continuity echoes what has previously been presented as Oldowan ‘stasis’ by Semaw and\ncolleagues. Does the notion of continuity rather than stasis represent a paradigm shift, and on what basis, or is it simply a\nmore neutral way of characterizing the enduring Oldowan technology? The overrepresentation of crypto-crystalline silica is\nof great interest. SM Fig. 11 shows that this pattern increases over time: does this raw material fit with distinctive morpho-\nfunctional attributes on flakes, e.g. in terms of cutting-edge extension? Given the role of raw material in this study, the\ninclusion of a detailed table showing the proportions of each raw material type across the sites would be beneficial (see also\nbelow).\nThe multi-proxy environmental studies indicate increased aridification between 2.7 and 2.2 Ma, a trend which is supported\nby a wealth of highly relevant data. This trend confirms what has already been extensively documented in other parts of the\nTurkana Basin. In the discussion, the authors jump from the local Koobi Fora scale to a global scale, which seems out of the\nscope of the article. It would make more sense, especially given the title of the article, to extend the scope to the Turkana\nBasin, where there is a wealth of paleontological, environmental and archaeological data available for this time window.\nIntegrating the data from the neighboring Nachukui and Shungura formations would indeed show that, under the same\nenvironmental conditions, the earliest Oldowan occurrences only date to ca. 2.3 Ma in these contexts. Looking further south\nin the modern Lake Victoria margin, the Oldowan is documented since ca. 3.0-2.9 Ma at Nyayanga, well before the onset of\nclimate aridification. In addition to the fact that the climate shift documented from ca. 2.8 Ma onwards appears to be more of a\ngradual process over hundreds of thousands years, its causal effect on the emergence of the Oldowan ultimately appears to\nbe the less robust element of the discussion. This seems to be at least an oversimplification, while a more complex scenario,\nsuch as climatic aridification affecting resource availability differently in different parts of the Turkana Basin, could be\ndiscussed. In any case, more detail on the potential role of local raw material availability will strengthen the argument. This\nwould once again require more detail on the drainage system and on site location within this system.\nBelow are a number of more targeted corrections and additions:\n- The literature is not always appropriately cited. 1. The omission of Gona in the list of the ‘earliest’ early Oldowan sites, in\nthe introduction, as well as in the discussion and conclusion, has to corrected. Age estimates for Ledi Geraru and Gona\nlargely overlap and there is no reason to consider one and not the other (Gona), which is furthermore the best documented\nPlio-Pleistocene site complex so far for the 3 to 2.5 Ma time range. 2. The references for the source data used in the main\ntext (Fig. 5) and in the Supplementary Material (e.g. SM Figs 11 and 17) need to be cited, for instance in SM Table 8, even\nthough these data are replicated from tables previously published by the authors. It is absolutely required for reproducibility\npurposes, especially since a quick check shows that the data from SM Table 8, that are used in the statistics from Fig.5,\ndo not match, or only partly, with the published primary data, in particular for Gona, Nachukui and Shungura occurrences (we\ndid not check the other site complexes). 3. The position of the references needs sometimes to be more appropriate, eg.\n‘These changes were instrumental in shaping the evolution and adaptations of both flora and fauna in eastern Africa as we\nshow locally in this study 17,61,64,69,70’: as obviously the references cited here do not proceed from this study, they have to be\ndisplaced after ‘Africa’. 4. As a general rule, please cite first-hand data rather than second-hand ones.\n- The authors’ raw dataset that have been used for their quantitative analyses, including rock types, size and technological\nattributes, for the three assemblages from Area 40, should be compiled in an .xls file, as it is done for the paleomagnetic and\npaleoenvironmental datasets, again for reproducibility purposes.\n- Figure 2 and SM Figures 2 & 3: the meaning of the abbreviations used in the stratigraphic columns, e.g. C1, C2, PO, P1,\nL1, etc. has to be detailed in the caption.\n- Figure 4: the right panel is aesthetically interesting but confusing to read, with too many overlapping colors. The legend for\nproduct types and raw materials requires improvement. Its placement within the figure creates confusion regarding which\nelements correspond to each part.\n- Figure 5: please enlarge the legend below the right panel\n- Figure 6: the right side of the figure should be edited. The legend in Fig.6-K is cropped. The dates are also cropped in Fig6-\nJ.\n- There are different notations for \"STP_601\", referred to as STP601 in some instances and as STP_601 in others. This\nshould be homogenized.\n- SM Figure 1: including the location of the sites alongside the geological sections would be beneficial.\n- SM Figure 6: It would be helpful to specify the type of bone or clarify whether it is unidentified, rather than broadly referring\nto it as a ‘portion of a size 3 suid’. The caption mentions four bones exhibiting evidence of butchery marks. For those not\nshown here, to which species do they belong, and what types are bones are they?\n- SM Figure 19 is not mentioned in the main text.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": "Detailed response to reviews comments for ““Early Oldowan Technology Thrived During\nPliocene Environmental Change in the Turkana Basin, Kenya”\nDavid R. Braun1,*, Dan V. Palcu2,3,4,*, Eldert L. L. Advokaat5, Will Archer6,7, Niguss G. Baraki8,9, Maryse Biernat10, Ella\nBeaudoin11, Anna K. Behrensmeyer12, René Bobe13,14,15, Katherine Elmes16, Frances Forrest17, Ashley S. Hammond18, Luigi\nJovane4, Rahab N. Kinyanjui19,20, Ana P. de Martini4, Paul Mason5, Amanda McGrosky21, Joanne Munga8, Emmanuel K.\nNdiema19, David B. Patterson22, Jonathan S. Reeves1, Diana C. Roman23, Mark J. Sier5,24, Priyeshu Srivastava4,25, Kristen\nTuosto8,26, Kevin Uno27, Amelia Villaseñor28, Jonathan G. Wynn29, John W. K. Harris30, Susana Carvalho13,14,15,31\nReviewer #1 (Remarks to the Author):\nComment 1.1 (Reviewer 1) It is unclear whether the authors are arguing for or against\ntechnological innovation being linked to paleoenvironmetal change, or why in some scenarios,\nfor example at the end of the Pliocene, innovative technology is linked to paleoenvironmetal\nchange, whereas during the early Pleistocene there is a stasis in stone tool technology. The\nauthors suggest that paleorivers are related to stone tool technology, and that once rivers replaced\na lake on the landscape, stone tool technology persisted. This is explained well. However, it\nwould be helpful to explain and expand on how local changes in vegetation, as evidenced from\nthe well-described carbonates, plant wax biomarkers, phytoliths, and faunal remains, are related\nto the stasis in stone tool technology described by the authors.\n_Response to reviewer comment:_ We appreciate your observation regarding the relationship\nbetween technological innovation and paleoenvironments, particularly the contrast between the\ninnovation observed at the end of the Pliocene and the stasis during the early Pleistocene. To\nclarify, we argue that the emergence and persistence of the Oldowan tool technology were\nclosely tied to specific local and regional environmental changes. Our revised text emphasizes\nthe nuanced interaction between geological factors (availability of large cobbles and paleoriver\nsystems) and paleoecological conditions (changes in the availability of resources). By presenting\nevidence from the Namorotukunan area (FwJj 52), we demonstrate how relatively dry climatic\nepisodes facilitated artifact production and burial. Regarding the connection between vegetation\nshifts and the stasis in stone tool technology, we expanded our discussion to highlight the role of\nvegetation and its influence on hominin foraging strategies and tool use. Using data from\ncarbonates, plant wax biomarkers, phytoliths, and faunal remains, we propose that the relative\ndrying trend in vegetation observed locally during the early Pleistocene may have contributed to\nthe appearance of stone tool technology in this area at this time due to paleoecological and\npaleogeographical shifts. These changes in the ancient ecosystems likely provided consistent\nresources, which, coupled with the availability of raw materials and depositional conditions,\nsupported a continuity in Oldowan tool use .[Main text line 1268-1277]\nComment 1.2 (Reviewer 1) In order to strengthen the paper, the authors should include section\nphotographs with in-situ artifacts. Wall sections with point provenience are shown in Figure 4,\nbut photographs of artifacts in situ should also be included. If figure space is an issue, then\nperhaps section photographs with artifacts can be added to the supplementary material. It is\nessential in a paper describing some of the earliest excavated Oldowan technology.\n_Response to reviewer comment:_ We have now provided a supplementary figure\n(Supplementary Figure 6) that provides images of artifacts in situ showing their position at the\ntime of their discovery and also provides images of these artifacts in detail.[Supplementary\nMaterials]\nComment 1.3 (Reviewer 1) The authors refer to controversial stone tools from Lomekwi and\ncite replies that question the context of those artifacts. Section photographs of FwJj 52 with in\nsitu artifacts should be included here. It would also be helpful to report the distance between the\nthree archaeological sites.\n_Response to reviewer comment:_ We now provide details of the size of the area around the\narchaeological sites. We also provide exact distances between sites. These are also provided in\nFigure 2 (see green lines between sections with distances). In addition, the distances between\nsites are visible in Supplementary Figure 1, which provides an overview map of the region. We\nalso provide images of artifacts in situ showing the relationship between artifacts and the\nstratigraphy indicating that the majority of artifacts are over a meter below the modern surface\n[Supplementary Materials and Figure 2]\nComment 1.4 (Reviewer 1) Figure 1a should have a scale bar for the Turkana Basin.\n_Response to reviewer comment:_ We have modified Figure 1a to include a scale bar for the\nTurkana Basin and we have updated this figure to reflect the shifts in the names of the\nexcavations as requested by Reviewer 2.[Line 232-243]\nComment 1.5 (Reviewer 1) Figure 6j is cropped at the base, Figure 6k the legend and label “k”\nare cropped and not visible. What do the boxes and crosses represent on Figure 6k? It is not\nindicated on the legend. In addition, for Figure 6k, the legend says this is showing change, but it\nlooks like this figure is showing a snapshot of modeled climate, not change from time slice 1 to\ntime slice 2. It is unclear if we are seeing modeled climate or which areas become more arid\nfrom the Pliocene to the Pleistocene. Please revise this figure as it is unclear from both the figure\nitself and the legend.\n_Response to reviewer comment:_ This figure has been completely re-organized based on\nmultiple reviewer suggestions. We have now included details that are related to specific parts of\nthe Turkana Basin and we have modified Figure 6 to make it clearer and more focused on local\nand regional shifts.[Line 731-740]\nComment 1.6 (Reviewer 1) Supplementary Figure 6 a scale bar is needed on the microscope\nimages, and the lowermost inset image is rectangular while the box is square on the lower color\nphotograph. It looks like the second inset image is not indicated on the correct area on the color\nphotograph, since we can see the edge of the bone on the inset image, but the box is not on the\nedge of the bone in the color photograph.\n_Response to reviewer comment:_ This figure [now Supplementary Figure 8]has been revised\nso that the shape and orientation of the boxes that are placed on the large-scale images of the\nbone are the same as the microscope images. These microscope images are made with a\nspinning disc Nanofocus Microscope. As a result, the “edge” of the microscope image\nrepresents the edge of the 3D model that the microscope creates and not the actual edge of the\nbone. This is also the reason that scale bars cannot be accurately placed because the image\nreflects an oblique image of the surface of the bone which provides the ability to see internal\nstriations on the marks (a hallmark of cut marks).\nComment 1.7 (Reviewer 1) Minor comments: Supplementary Table 7 has spelling errors in the\nnames of the fauna. Is “Aepyceotiini” referring to Aepycerotini? Some “sp.” are italicized while\nothers are not.\nResponse to reviewer comment: We have modified this table to correct these\nmistakes.[Supplementary Materials]\nReviewer #2 (Remarks to the Author):\nComment 2.8 (Reviewer 2) FwJj52 is an important archaeological site and will be cited by\nmany colleagues in the future, so probably deserves a name easy to remember. So, if the name of\nthis site has not been published before I suggest a simpler name, easier to remember and place on\na map, if possible with a geographical reference, for example, the name of the closest town,\nIleret52?, local topographic name.\nResponse to reviewer comment: Thank you for your thoughtful comment regarding the naming\nof FwJj52 and its importance as a future reference point for colleagues. We appreciate the\nsuggestion to adopt a simpler, geographically meaningful name for the site.\nIn response, we have revised the site name to \"Namorotukunan,\" which is a local topographic\nname closely associated with the area. Additionally, we have redefined the names of the\nindividual archaeological excavations within the site to ensure clarity and consistency. These\nare now named as follows:\nSite Name: Namorotukunan (we retain the FwJj52 moniker as an official name because it is the\nname that the National Museums of Kenya uses to classify the site.)\nArchaeological Excavations:\n- Namorotukunan1 (NMT1 – STP601 - the oldest excavation)\n- Namorotukunan2 (NMT2 – KT_016-05 - the second oldest excavation)\n- Namorotukunan3 (NMT3 – KT_016-04 - the youngest excavation)\nThese changes have been incorporated into the manuscript to align with the suggestion for a\nsimpler and more memorable naming convention, while maintaining ties to the geographical and\ncultural context of the site.\nWe hope this addresses your concern and enhances the clarity and accessibility of our work for\nfuture reference.[These changes can be found throughout the manuscript]\nComment 2.8 (Reviewer 2) : It is not clear why you correlated NZ2 with the Feni 208 sub-chron\n(Reunion excursion) and not with Olduvai.\nEspecially when considering the second scenario with Burgi Unconformity at the C7/L1 lithologic boundary,\nas previously suggested by Kidney23 and Baldes et al.26. This would solve two problems, a) sedimentation\nrate would be more reasonable for this interval, and B) you can correlate the upper lacustrine unit with\nLorenyang lake that occurs at Olduvai and not with pre-Lorenyang lake.\nThis does not necessarily change the age of your sites since an erosional surface exists at this boundary.\nPlease explain better your reasons for this correlation and maybe include this possibility in your model that\nincludes the Burgi Unconformity .\nResponse to reviewer comment: Thank you for your insightful comment regarding the correlation of NZ2\nwith the Feni 208 sub-chron (Reunion excursion) and your suggestion to consider its correlation with the Olduvai\nsub-chron. We appreciate your perspective and recognize the importance of addressing alternative interpretations\nto enhance the robustness of our analysis.\nIn response to your feedback, we have incorporated the following changes into the manuscript:\nWe have added a detailed discussion of the alternative scenario that correlates NZ2 with the Olduvai sub-chron.\nSpecifically, we considered the implications of placing the Burgi Unconformity at the C7/L1 lithologic boundary. We\nsuggest that absence of the KBS Tuff in the Area 40 sequence ( a widespread marker that is also visible directly to\nthe south across the Gele fault - See Kidney 2012) makes it likely that this upper normal magnetozone is not the\nOlduvai sub-chron. However, we mention in the text that it is not possible to rule this option out. [Line334- 341]\nTo reflect this alternative interpretation, we have updated the relevant figure to include the Burgi Unconformity\nscenario [Figure 3]. The revised figure now clearly depicts both the original correlation with the Feni 208 sub-chron\nand the alternative correlation with the Olduvai sub-chron. Annotations and explanations have been added to\nensure clarity and to facilitate comparison between the two models.\nWe believe these additions enhance the clarity and scientific rigor of the manuscript by presenting a more\ncomprehensive analysis of the possible correlations. We hope this addresses your concerns effectively and\ndemonstrates our commitment to thoroughly evaluating alternative hypotheses.\nComment 2.10 (Reviewer 2): Methods: please specify from the 160 paleomagnetic samples you study,\nyou should indicate the number of samples that correspond to a single strata (sampling station), and how\nmany stratigraphic levels (sampling stations) you sample.\nResponse to reviewer comment: Thank you for your valuable feedback and for highlighting the need for\nadditional clarity in the methods section regarding the paleomagnetic samples.\nIn response, we have updated the text to provide a more detailed explanation of the sampling strategy (in the\nSupplementary Materials pgs. 1-3). Specifically, we have clarified that the 160 paleomagnetic samples correspond\nto distinct stratigraphic levels (sampling stations). Each sample represents a unique stratigraphic level, ensuring\nthat the dataset provides a comprehensive representation of the stratigraphic sequence studied. We have added\nmore information to clarify that the 160 samples correspond to 147 distinct stratigraphic levels with 13 doubles\n(two destroyed during measurements). Additionally, we have indicated the number of stratigraphic levels sampled\nto further enhance transparency in the methods.\nComment 2.11 (Reviewer 2): Also, consider placing, supplementary material Figure 3 in the main text,\nsince this is a very important figure.\nResponse to reviewer comment: We have now modified Figure 2 so that it incorporates the details of\nSupplementary Figure 3. We have maintained Supplementary Figure 3 as a reference. We expect readers to use\nSupplementary Figure 3 to focus exclusively on the paleomagnetic results. However, most of the details provided in\nSupplementary Figure 3 can now be found in Figure 2.\nComment 2.12 (Reviewer 2): In supplementary info: figures 18 and 19, cite the software you use to\nprepare the plots. In pmag supplementary tables: use only English language in the text\nResponse to reviewer comment: Thank you for your comments and suggestions regarding the\nsupplementary information and tables. We have updated the supplementary information to cite the software\n(FORCinel and paleomagnetism.org) used to prepare Supplementary Figures 18 and 19 (now Supplementary Figure\n20 and 21). Specifically, we clarified that the plots were created using an online Java-based application,\npaleomagnetism.org, and included additional details about the software and its features to ensure transparency.\nAdditionally, the text in the paleomagnetic supplementary tables has been reviewed and revised to ensure that all\nentries are in English. These corrections improve the clarity and accessibility of the supplementary materials, and\nwe appreciate your feedback in helping us enhance the quality of the manuscript.\nComment 2.13 (Reviewer 2): Line 210: Mammoth and Kaena are periods with persistent reverse\npolarity during 84ka and 123ka, so you should use the term “subchron” or “subchronozone” not “excursion”\nfor Mammoth and Kaena.\nResponse to reviewer comment: Thank you for this suggestion. We have now modified the text and Figures\nto use the term subchron for references to the Mammoth and Kaena subchrons.[line 329-330]\nComment 2.14 (Reviewer 2): Line 293: is the shift towards arid conditions compatible with the lake\nphases between 2.2 and 2.4Ma shown in Fig 3 ? explain better\nResponse to reviewer comment: We cannot find the specific text reference indicated in this comment within\nthe manuscript. We have now modified the text to describe the paleoecology of the region in a way that describes\nvarious events as the result of local and regional events. The changes in the local ecosystem associated with the\nonset of lake phases at the top of the sequence may be associated with major lake phases described previously (e.g.\nNutz et al., Boes et al.) however as these shifts occur after the archaeological horizons they do not influence our\ndiscussion of the interaction between ecosystems and the appearance of the Oldowan record. [Line 1282-1306]\nComment 2.15 (Reviewer 2): Line 427: since “Lorenyang lake” occurs in the Turkana lake system, I\nwonder why is not considered it just as Lake Turkana highstand? (Lorenyang highstand),\nResponse to reviewer comment: Thank you for your thoughtful comment regarding the terminology used to\ndescribe the Lorenyang Lake in the Turkana lake system. We acknowledge that referring to it as the \"Lake Turkana\nhighstand\" (or \"Lorenyang highstand\") is a valid alternative and part of a broader, ongoing discussion.\nIn response, we have opted to retain the term \"Paleo-Lake Lorenyang transgression in the Turkana basin\" as it is\nclearer and we found Lorenyang is commonly employed in the literature (Boes et al. 2018). However, we have\nadded clarifications in the text to acknowledge this ongoing debate and to provide context for our choice. We hope\nthis additional clarification addresses your concerns and provides a balanced perspective.[Line 537]\nComment 2.16 (Reviewer 2): Line: 434 “During the late Pliocene, the EARS witnessed significant\nlandscape and climate transformations such as lake regression, decreased woody cover, and aridification (Fig\n6; e.g., 17,53,62–67) that significantly transformed vegetation communities, expanded grasslands, reduced\nwoody cover “\nResponse to reviewer comment: Thank you for this astute correction, we have removed the term woody\ncover in the second instance of this term in this sentence. [Line 1230]\nComment 2.17 (Reviewer 2): Line: 446 A pivotal event during this period was the closure of the Panama\nSeaway roughly between 2.8 to 2.5 million years ago, which had profound implications for Earth's climate\nand ecosystems 73\nThe reference indicated (73) suggests a late Miocene closure of the Panama isthmus (not Pliocene) based on\nU/Th zircons from Panama present in the North Andes, so rephrase please or add additional references.\nResponse to reviewer comment: Thank you for bringing this to our attention. We have revised the text to\nclarify the timing and implications of the closure of the Panama Seaway. Initially, we referenced several sources to\nprovide a broader context for this event, but in the updated text, we have refined our focus to the specific closure\nevent around 2.75 million years ago. The references now align exclusively with this timeframe to ensure accuracy\nand consistency with the evidence. We appreciate your careful review and believe this revision resolves the issue\neffectively.[Line 1290]\nComment 2.18 (Reviewer 2): Line 579 Using a 2.5 cm-diameter diamond-coated bit powered by a hand\npump. This sounds strange, usually battery or gasoline-powered drills are used to collect pmag samples, hand\npump should refer to the water pump used to lubricate the bit.\nResponse to reviewer comment: Thank you for pointing this out. We appreciate the opportunity to clarify\nour methodology. In this case, the hand pump referred to was used to air-cool the samples during drilling, as the\nunconsolidated nature of the samples prevented the use of liquid for cooling the drill bit. This approach was chosen\nto minimize the risk of altering or damaging the samples due to heat generated during the drilling process. We\nhave revised the text to make this distinction clear and to avoid any potential confusion.[Line 1471-1474]\nComment 2.19 (Reviewer 2): Figure 2: This is a nice figure, I suggest placing the stratigraphic sections in\nchronological order (A, B, C, D etc) and below the geological cross-section to read better the stratigraphy. I\nsuggest placing the edges of the diagram vertically to improve the perspective.\nResponse to reviewer comment: Thank you for your suggestions regarding Figure 2. We have made the\nrecommended changes. The stratigraphic sections are now arranged in chronological order (A, B, C, D, etc.). They\nare placed below the geological cross-section to improve readability. We have also adjusted the edges of the\ndiagram to be vertical for better perspective. Additionally, we have completely revised the figure to enhance clarity\nand visual presentation. We appreciate your feedback and believe the updated figure addresses your concerns\neffectively.[Figure 2]\nComment 2.20 (Reviewer 2): Figure 3: the letters in the column are too small (ex TB).\nResponse to reviewer comment: As described above this figure was completely redesigned which included\nincreasing text size for readability.[Figure 3]\nComment 2.21 (Reviewer 2): Figure 4: I suggest indicating in the figure the % for tools of specific\nlithology, (ex. basalt 65%)\nResponse to reviewer comment: Thank you for your suggestion regarding Figure 4. We appreciate your\nattention to detail. The figure originally included a graphical representation of the percentages of each raw\nmaterial. We have now added text that explicitly describes the percentages for specific lithologies, such as basalt\n(e.g., XX%). This is now incorporated into the visual representation. However, we have ensured that these\npercentages are written out and easy to interpret for the reader.[Figure 4]\nComment 2.22 (Reviewer 2): Figure 6: I suggest including the stratigraphic units at the base of the\nmagnetic susceptibility plot, to see the relations of changes in different proxies with lithology.\nIn the plot “Water deficit vs age”, the age numbers are cut. In the figure of “Plio-Pleistocene modeled\nclimate”, the legend can’t be read well.\nResponse to reviewer comment: Thank you for this suggestion. We have added a plot of lithology to\nSupplementary Figure 17 to address this point. This plot highlights the stratigraphic units and provides clarity on\nthe relationship between magnetic susceptibility changes and lithological variations. We have also included\nadditional explanations in the figure caption and the main text to link the observed magnetic susceptibility changes\nwith specific lithological changes. Figure 6 has also been completely revised to make it easier to read.\nComment 2.23 (Reviewer 2): This figure has too much information, I suggest reducing it (50%) for\nexample by leaving the data produced in this study (two columns with 3 plots each), and placing the other\nplots as Supplementary information.\nResponse to reviewer comment: Thank you for your feedback regarding the complexity of this figure. We\nunderstand your concerns and have addressed them by reorganizing and correcting the figure. Specifically, we now\ninclude data exclusively focused on the Turkana Basin to reduce the amount of information presented at once. We\nincreased the font size to facilitate readability. These changes should make the figure clearer and more accessible\nwhile retaining the critical information. Additional plots not directly produced in this study have been moved to a\nSupplementary Figure to streamline the main figure.[Figure 6]\nComment 3.24 (Reviewer 3): This article represents a huge amount of multidisciplinary analytical effort,\nfor a new site complex that will definitely enrich our understanding of the early stages of the Oldowan\ntechnology. Our recommendations aim to better highlight the potential of these new sites for discussing\npatterns of hominin technological adaptation, based on the local environmental settings and associated\nresources at Koobi Fora and more broadly in the Turkana Basin, where discrete trajectories are documented.\nThis would bring more originality to the article, which, despite presenting important new archaeological\nfindings, tends to stick to long-debated ideas without providing groundbreaking evidence.\nResponse to reviewer comment: We have now modified aspects of the manuscript (in response to multiple\nreviewer comments) to focus more exclusively on the local and regional changes in the Namorotukunan region and\nthe impact on the appearance of stone artifacts in this part of the record. We highlight contextual variables within\nthis region and its influence on tool use. [These changes occur in many places within the manuscript but specifically\nin Lines:128-155; 481-485; 1275-1285]\nComment 3.25 (Reviewer 3): One of the main challenges posed by this article is the dating issue, as the\nmarker layers that bracket the archaeological horizons define a particularly long time windows, ranging from\n3.44 Ma to 2.2 Ma.\nErosional phases and unconformities occurred within this time range, although it is not very clear how and\nwhere in the sequence they have been identified. The authors point out in p.5 that there is “a gap between\n3.0 -2.5 Ma represented by the Burgi Unconformity” in Area 40, a statement which, unless explained, does\nnot conform with what follows. Clarification about the Burgi Unconformity in this area is absolutely required\nto avoid confusion and to provide a solid basis for chronostratigraphic studies.\nResponse to reviewer comment: Thank you for your insightful comment regarding the Burgi Unconformity\nand its implications for the dating of the archaeological horizons. We agree that identifying and clarifying the Burgi\nUnconformity is critical for ensuring a solid basis for chronostratigraphic studies.\nIn response, we confirm that we do indeed identify the Burgi Unconformity in the sequence, and this is explicitly\nillustrated in Figure 3 of the manuscript (as indicated previously by Baldes et al. 2024) in the Namorutukanan\nregion. We also recognize the importance of this unconformity in interpreting the stratigraphy and its potential to\nrepresent large time gaps. However, as noted by Reviewer 1, even with an alternative explanation that the Burgi\nUnconformity represents a significant period of time (e.g., 3.0–2.5 Ma), this does not affect the ages of the\narchaeological sites. All sites are situated stratigraphically below the unconformity, meaning that their placement\nin the sequence is unaffected by the presence or interpretation of this erosional surface. The confirmation of the\nGauss-Matuyama boundary below the Burgi Unconformity as well as the well dated Tulu Bor Tuff at the base of the\nsection provides upper and lower boundaries to create a chronostratigraphic model of these sediments.\nWe have revised the text to clarify the location and implications of the Burgi Unconformity in Area 40, ensuring that\nits identification and significance are clearly explained. We hope these adjustments address your concerns and\nimprove the clarity and robustness of the manuscript. [Line 225-228;334-341]\nComment 3.26 (Reviewer 3): While we are not qualified to discuss the details of the paleomagnetic\ninvestigations, some precautions must be taken in the text, e.g. the age model, as robust as it appears, is\nbased on age estimates, except for the Tulu Bor Tuff at the bottom of the sequence, and not on radiometric\ndates.\nFor this reason, it should be more appropriate to use the term ‘age estimates’ rather than ‘dates’ or ‘dated’\n(see for instance the Introduction). Similarly, the three chronological points inferred from the paleomagnetic\ndata cannot be taken as local age markers equivalent to the Tulu Bor Tuff, but rather as derived or inferred\nbenchmark ages.[3]\nResponse to reviewer comment: We appreciate the reviewer’s thoughtful comments regarding the\nterminology used to describe chronological markers and age estimates. Specifically, we acknowledge that\npaleomagnetic shifts provide robust chronological markers and understand the importance of referring to them as\nrelative age markers rather than absolute dates. We have made the changes in the text as well as figures. We have\nreplaced references to 'dates' or 'dated' with 'age estimates' where appropriate, and we emphasize that the age\nmodel is indeed based on inferred age estimates rather than radiometric dates, except for the Tulu Bor Tuff at the\nbottom of the sequence [Line 93 and 353].\nComment 3.27 (Reviewer 3): At a more local scale, precision regarding the depositional environment of\nthe archaeological sites is limited to their location in coarse sands within the gravel and paleosol package.\nPrecisions regarding their position within the drainage system, and which drainage system (e.g. meandering,\nbraided river?), could however be highly relevant for assessing the proximity of the sites to cobble-rich\nconglomerates that could have served as raw material supply areas.\nThese data would enrich the arguments about raw material availability, as put forward in the discussion.\nThey would also allow for more consistent insights into site taphonomy. We can infer from the\nsedimentological data that the archaeological remains are in secondary position.\nResponse to reviewer comment: We appreciate the reviewer’s suggestion to provide greater precision\nregarding the depositional environment of the archaeological sites and their relationship to the\nsurrounding drainage systems. In response, we have expanded our description to include detailed\ninformation about the types of river systems associated with the sites. Specifically, we now identify\nwhether the archaeological horizons are associated with braided river systems, as well as the geomorphic\ncontext of the sites within these systems.\nAdditionally, we have clarified that these river systems are directly adjacent to the archaeological\nhorizons. This revised discussion highlights the proximity of the sites to cobble-rich conglomerates that\ncould have served as raw material sources, providing further context to support our arguments about raw\nmaterial availability in the discussion [Line 479-486].\nWe agree that these data also have important implications for understanding site taphonomy. As\nindicated by our site formation analysis, the sites have been impacted by higher energy deposition.\nHowever, this was not extensive enough to remove the smaller fraction of the flaked materials\n(Supplementary Figure 4) or orient the artifacts that were deposited (Supplementary Figure 5).\nComment 3.28 (Reviewer 3): Demonstrating how the drainage system affected depositional and\nredepositional processes would therefore be highly relevant. For example, what explains the strong vertical\ndispersion of artifacts as seen in Fig.4, particularly in the older and younger sites?\nIn this context, how can the artifacts be considered as deposited on a flat surface (SM Fig5)? What about\nassemblage integrity and artifact alteration, e.g. degree of abrasion? What might explain limited size sorting\nand the absence or limited proportions of preferentially oriented artifacts in coarse sands (SM Fig5)?\nResponse to reviewer comment: Thank you for your detailed comment regarding the role of the drainage\nsystem and its impact on depositional and post-depositional processes. We appreciate the opportunity to clarify\nthese points and address your concerns.\nThe vertical dispersion of artifacts, as seen in Figure 4, is representative of the extent of coarse-grained deposits at\neach locality. Specifically, the vertical dispersion at all three sites reflects the distribution and thickness of these\ncoarse-grained depositional units. Importantly, at all of the localities, artifacts are present in the stratigraphic\nsequence when rocks of a suitable size (to make stone artifacts) are also deposited in these sequences. This\nsuggests a strong association between artifact deposition and the availability of suitable sedimentary contexts.\nArtifacts are considered to have been deposited on a relatively flat surface, as indicated by the orientation analysis\n(Supplementary Material Figure 5). While artifact orientations align with the dominant river direction, their flat-\nlying positions suggest deposition occurred on multiple aggrading surfaces rather than erosional (or subsequent\nredeposited) surfaces. These aggrading surfaces facilitated the rapid burial of artifacts by coarse-grained deposits,\npreserving them in situ and maintaining assemblage integrity. Some size sorting is indicated by the absence of the\nsmallest fraction seen in the NMT3 assemblage (Supplementary Figure 4). This interpretation is further supported\nby the absence of clear erosional features associated with these depositional surfaces.\nRegarding assemblage integrity and artifact alteration, we observe limited evidence of abrasion, which supports\nthe interpretation that artifacts were rapidly buried after deposition. Similarly, the limited size sorting and absence\n(or minimal proportions) of preferentially oriented artifacts within coarse sands reflect the dynamic but localized\ndepositional environment of these aggrading surfaces. Together, these findings indicate that the artifacts were not\nsignificantly reworked after deposition, preserving the integrity of the assemblages. [Lines 362-426]\nWe have incorporated these clarifications into the manuscript to address these points and better illustrate the\ndepositional and post-depositional processes at play. [Lines 362-426]\nComment 3.29 (Reviewer 3): The attribution of the assemblages to the early Oldowan seems well\nestablished, although pictures or drawings of artifacts illustrating a sample of flakes and cores are lacking to\nfully support this statement.\nThe artifacts shown in Figure 4 lack descriptive captions and they are presented at too small a scale to be\nreally informative.\nResponse to reviewer comment: Thank you for the suggestion of increasing the number of artifacts that can\nbe displayed. We have now added a subsequence Supplementary Figure 7. This provides more images of the\nartifacts at a larger scale.[Supplementary Figure 7]\nComment 3.30 (Reviewer 3): The relevance of data used to statistically discriminate Oldowan,\nLomekwian and modern nonhuman primates must be justified, at least in Supplementary Material.\nResponse to reviewer comment: Thank you for your comment regarding the justification of the data used to\nstatistically discriminate between Oldowan, Lomekwian, and modern non-human primate artifacts. We appreciate\nthe opportunity to clarify this important aspect of the study.\nThe data used to discriminate between Lomekwian and Oldowan artifacts are based on variables that have been\nextensively described and utilized in previous studies (Braun et al. 2019; Plummer et al. 2023). These variables are\ngrounded in the original publications that first characterized the Lomekwian (Harmand et al. 2015) and Oldowan\n(Leakey 1973) technologies, reflecting key morphological and technological patterns distinguishing these\nassemblages. Similarly, the differences between non-human primate artifacts and Oldowan tools follow the\npatterns described in the original publications (Proffitt et al. 2023) that documented and analyzed non-human\nprimate artifacts.\nOur study builds on this foundational work by providing a statistical framework to quantitatively distinguish these\nassemblages. The measurements we used are those explicitly described in the original studies (Harmand et al.\n2015; Braun et al. 2019; Plummer et al. 2023) and were chosen specifically because they have been demonstrated\nto distinguish between these groups. We have added clarifications in the Supplementary Material to ensure that\nthe basis for these measurements is clear and explicitly tied to the existing literature. We have also provided a new\ntable (Supplementary Table 9) which provides detailed references for each measurement.\nGiven the robust statistical differences demonstrated between the groups using these measurements, we are\nconfident that the data are both relevant and appropriate for the purpose of this analysis. If there are additional\nconcerns or specific aspects that reviewers feel need further justification, we would be happy to address them in\nmore detail.\nWe further thank the reviewer for noting possible discrepancies in the Nachukui assemblages. We note that we\nfollowed the protocol described by Braun et al. 2019. That protocol describes an imputation process so that\nportions of the dataset that were not available in the literature were replaced via imputation based on the\nsurrounding data. Given that some of the data from the Lokalalei 2C assemblage was imputed we have chosen to\nremove this assemblage from the analysis. The resulting visualization shows similar patterns to the analysis\npreviously presented [Figure 5].\nComment 3.31 (Reviewer 3): Beyond their affiliation with the early Oldowan, what patterns of\ncontinuity can be observed between the three sites? Adding more detail on local chronological trends, or lack\nof thereof, will provide more insight into the notion of continuity beyond the long duration of the early\nOldowan technology, which is already well established on a broader eastern African scale. Interpreting the\ndata from KF Area 40 as indicative of continuity echoes what has previously been presented as Oldowan\n‘stasis’ by Semaw and colleagues.\nResponse to reviewer comment: We appreciate the reviewer’s insightful comments on the patterns of\ncontinuity and local chronological trends across the sites. In our revised manuscript, we emphasize that our\nanalyses of technological attributes reveal no statistically significant differences in artifact morphology between\nthe different horizons at Namorutukanan. This finding highlights that the materials from the three excavations\ncannot be distinguished based on the technological attributes analyzed in this study.\nIn response to the comment regarding local chronological trends. We do not explicitly refer to the concept of stasis\nwithin the Oldowan but rather describe the technological patterns through time. A broader study of change\nthroughout the Oldowan would require data from across numerous Oldowan sites, while controlling for raw\nmaterial availability and quality. This data is not currently available. While the early Oldowan’s long duration is well\nestablished on an eastern African scale, we interpret the data from Namorotukunan as providing evidence of local\ncontinuity [Figure 5; Supplementary Figure 9, 11, 18]. Here we only emphasize that these assemblages are most\nsimilar to the Early Oldowan assemblages. We have integrated this perspective into our discussion to provide a\nmore nuanced interpretation of local technological trends within our dataset [Line 1300-1319; 1421-1432.\nComment 3.32 (Reviewer 3): Does the notion of continuity rather than stasis represent a paradigm\nshift, and on what basis, or is it simply a more neutral way of characterizing the enduring Oldowan\ntechnology? The overrepresentation of crypto-crystalline silica is of great interest. SM Fig. 11 shows that this\npattern increases over time: does this raw material fit with distinctive morpho-functional attributes on flakes,\ne.g. in terms of cutting-edge extension?\nResponse to reviewer comment: We appreciate the reviewer’s thought-provoking question regarding\nwhether the notion of continuity rather than stasis represents a paradigm shift or simply a more neutral\ncharacterization of the enduring Oldowan technology. In our manuscript, we do not aim to propose a paradigm\nshift from previous interpretations of technological competence in the Oldowan. Rather, we describe the patterns\nas they appear in the archaeological record. Our analyses indicate no technological differences between the\nhorizons, supporting a consistent pattern of technological behavior over time.\nRegarding the overrepresentation of crypto-crystalline silica and its increase over time, we agree that this is a\ncompelling observation. We have examined whether this raw material correlates with distinctive technological\nattributes on flakes through the sequence. Our analysis, however, finds no evidence of such a correlation within the\ndataset. This suggests that the preference for crypto-crystalline silica was not tied to specific technological\ncharacteristics of the tools. All of the chalcedony flakes in the 3 layers appear to be similar in shape and size. A\nmore detailed analysis of the raw material properties and their relationship to specific morpho-functional attributes\nis beyond the scope of this study.\nComment 3.33 (Reviewer 3): Given the role of raw material in this study, the inclusion of a detailed\ntable showing the proportions of each raw material type across the sites would be beneficial (see also\nbelow).\nResponse to reviewer comment: Thank you for your helpful comment regarding the inclusion of detailed raw\nmaterial data. In response, we have included this information in the revised version of Figure 5, which now provides\na clear visual representation of the proportions of each raw material type across the sites. Additionally, we have\nattached spreadsheets containing the raw material attributions for all artifacts analyzed in this study. These\nadditions ensure transparency and allow readers to fully engage with the raw material data presented in our\nanalysis.\nComment 3.34 (Reviewer 3): The multi-proxy environmental studies indicate increased aridification\nbetween 2.7 and 2.2 Ma, a trend which is supported by a wealth of highly relevant data.\nThis trend confirms what has already been extensively documented in other parts of the Turkana Basin.\nIn the discussion, the authors jump from the local Koobi Fora scale to a global scale, which seems out of the\nscope of the article. It would make more sense, especially given the title of the article, to extend the scope to\nthe Turkana Basin, where there is a wealth of paleontological, environmental and archaeological data\navailable for this time window.\nResponse to reviewer comment: Thank you for your detailed feedback and constructive suggestions. We\nappreciate your emphasis on the importance of contextualizing our study within the broader framework of the\nTurkana Basin.\nIn response, we have modified Figure 6 to include information from the entire Turkana Basin, encompassing data\nfrom the Nachukui Formation and other key components of the Omo Group. This revised figure provides a more\ncomprehensive representation of the multi-proxy environmental studies across the Basin and underscores the\nbroader trend of increased aridification between 2.7 and 2.2 Ma, supported by extensive paleontological,\nenvironmental, and archaeological data.\nTo address your concern about the scope of the discussion, we have revised this section to focus more explicitly on\nthe Turkana Basin rather than making broad global-scale inferences. This adjustment aligns with the title of the\narticle and highlights the wealth of data from the Turkana Basin that substantiates the observed aridification trend.\nBy doing so, we aim to strengthen the connection between our findings and the broader regional context while\nmaintaining the article's central focus[Line 736 – 1319].\nComment 3.35 (Reviewer 3): Integrating the data from the neighboring Nachukui and Shungura\nformations would indeed show that, under the same environmental conditions, the earliest Oldowan\noccurrences only date to ca. 2.3 Ma in these contexts.\nResponse to reviewer comment: See our response regarding Figure 6 and our focus on Figure 6 and the\nassociated details of the Omo Group.\nComment 3.36 (Reviewer 3): Looking further south in the modern Lake Victoria margin, the Oldowan is\ndocumented since ca. 3.0-2.9 Ma at Nyayanga, well before the onset of climate aridification. In addition to\nthe fact that the climate shift documented from ca. 2.8 Ma onwards appears to be more of a gradual process\nover hundreds of thousands years, its causal effect on the emergence of the Oldowan ultimately appears to\nbe the less robust element of the discussion.\nResponse to reviewer comment: Thank you for your insightful comment regarding the timing of the\nOldowan's emergence in relation to global climate shifts. We have addressed this in our revised text by\nincorporating the following points:\nFirst, the Nyayanga material from the modern Lake Victoria margin is dated to between 2.6 and 3.0 Ma. Plummer\net al 2023 acknowledge that the large error bars of the radiometric dates only allow an estimation of “early within\nthe temporal range of the C2An.1n Subchron” (Plummer et al. 2023: 563) which currently dates to 2.581 - 3.032\nMa. This time range overlaps with the timing of the 2.75 Ma shift in global climates. It is not possible to definitively\nconfirm whether the Nyayanga artifacts were created at the time of major climatic shifts or not. Thus the influence\nof this global climate transition on the appearance of stone artifacts in eastern Africa cannot be investigated using\nthe Nyayanga materials.\nSecond, our revised text emphasizes that the appearance of artifacts at Namorotukunan is associated with a local\nshift in ecosystems rather than being directly tied to global trends. While we discuss the potential influence of\nglobal climate changes, we stress that these shifts are more likely related to regional dynamics within the basin.\nSpecifically, we highlight that local environmental changes, combined with the availability of suitable raw materials\nfor tool production, played a key role in the emergence of stone artifacts in this area. [Line 1310-1319;1421-1432]\nBy focusing on these local factors and providing explanations for how regional environmental changes may reflect\nbroader climatic processes, we aim to present a balanced and nuanced perspective. We appreciate your feedback,\nwhich has helped us refine our discussion and clarify these important points.\nComment 3.37 (Reviewer 3): This seems to be at least an oversimplification, while a more complex\nscenario, such as climatic aridification affecting resource availability differently in different parts of the\nTurkana Basin, could be discussed. In any case, more detail on the potential role of local raw material\navailability will strengthen the argument. This would once again require more detail on the drainage system\nand on site location within this system.\nResponse to reviewer comment: We appreciate the reviewer’s insightful comment regarding the complexity\nof environmental factors and the role of local raw material availability. In our revised manuscript, we have provided\nadditional detail about the drainage systems and their relationship to raw material availability at the sites.\nSpecifically, we describe the geomorphic context of the sites within these systems and discuss how proximity to raw\nmaterial sources may have influenced technological behaviors.\nWe also emphasize the importance of local paleogeographic shifts. While our data highlight patterns specific to this\nportion of the Turkana Basin, we recognize that these dynamics could vary significantly across different regions of\nthe basin or other parts of Africa. This variability underscores the need to consider both local and regional factors\nwhen interpreting patterns in the archaeological record.\nBy integrating these points, we aim to present a more nuanced understanding of how local raw material\navailability and environmental changes may have shaped early Oldowan technological behaviors.\nComment 3.38 (Reviewer 3): The omission of Gona in the list of the ‘earliest’ early Oldowan sites, in\nthe introduction, as well as in the discussion and conclusion, has to corrected. Age estimates for Ledi Geraru and\nGona largely overlap and there is no reason to consider one and not the other (Gona), which is furthermore\nthe best documented Plio-Pleistocene site complex so far for the 3 to 2.5 Ma time range.\nResponse to reviewer comment: We have now revised the manuscript to include references to Gona in all\nmentions of the earliest Oldowan assemblages. This ensures that the discussion acknowledges the importance of\nGona as a key site in understanding the origins and early development of Oldowan technology. We appreciate your\nfeedback, which has allowed us to improve the completeness and accuracy of our references [Line464-486; Line\n764-1278].\nComment 3.39 (Reviewer 3): 2. The references for the source data used in the main text (Fig. 5) and in\nthe Supplementary Material (e.g. SM Figs 11 and 17) need to be cited, for instance in SM Table 8, even\nthough these data are replicated from tables previously published by the authors. It is absolutely required for\nreproducibility purposes, especially since a quick check shows that the data from SM Table 8, that are used in\nthe statistics from Fig.5, do not match, or only partly, with the published primary data, in particular for Gona,\nNachukui and Shungura occurrences (we did not check the other site complexes).\nResponse to reviewer comment: We appreciate the reviewer’s detailed feedback regarding the references\nand reproducibility of the data used in the manuscript, particularly in relation to Table 8, Fig. 5, and the\nSupplementary Material. To address these concerns, we have added citations for all source data used in the\nanalysis. These are outlined in Supplementary Table 9. This ensures full transparency and reproducibility by clearly\nidentifying the studies from which the variables were derived.\nThe values in Table 8 have been cross-checked against those reported in the original publications, and we believe\nthey accurately reflect the primary data. However, if there are any remaining discrepancies, we are happy to revise\nthe table to incorporate updated or corrected data as new publications become available.\nRegarding Lokalalei 2C, we have excluded this site from the canonical analysis because imputing the large number\nof missing variables would have compromised the robustness of the analysis.\nThese revisions aim to ensure that the data are accurately represented, reproducible, and aligned with the original\nsources. We thank the reviewer for highlighting these points and helping us improve the clarity and reliability of the\nmanuscript. [Supplementary Table 9, 10]\nComment 3.40 (Reviewer 3): 3. The position of the references needs sometimes to be more\nappropriate, e.g. ‘These changes were instrumental in shaping the evolution and adaptations of both flora\nand fauna in eastern Africa as we show locally in this study 17,61,64,69,70’: as obviously the references cited\nhere do not proceed from this study, they have to be displaced after ‘Africa’. 4. As a general rule, please cite\nfirst-hand data rather than second-hand ones.\nResponse to reviewer comment: We have modified the references to conform with standard procedures. We\nappreciate the reviewer identifying these concerns [Supplementary Table 9,10].\nComment 3.41 (Reviewer 3): - The authors’ raw dataset that have been used for their quantitative\nanalyses, including rock types, size and technological attributes, for the three assemblages from Area 40,\nshould be compiled in an .xls file, as it is done for the paleomagnetic and paleoenvironmental datasets,\nagain for reproducibility purposes.\nResponse to reviewer comment: Thank you for your comment regarding the inclusion of raw datasets for the\nartifact assemblages. We have provided an .csv file containing artifact numbers along with basic technological\ninformation, including rock types, and tool type, for the three assemblages from Namorotukunan. This level of\ndetail surpasses the data that has been previously made available for other Oldowan sites. We believe this dataset\nenhances transparency and reproducibility and is a much higher standard of data sharing than has been provided\nfor other stone artifact assemblages (Supplementary Table 6].\nComment 3.42 (Reviewer 3): - Figure 2 and SM Figures 2 & 3: the meaning of the abbreviations used in\nthe stratigraphic columns, e.g. C1, C2, PO, P1, L1, etc. has to be detailed in the caption.\nResponse to reviewer comment: Thank you for pointing out the need for clarity regarding the abbreviations\nused in Figure 2 and Supplementary Figures 2 and 3. We have revised the captions to include these details. We also\nmake reference to Supplementary Table 2, where all the abbreviations (e.g., C1, C2, PO, P1, L1, etc.) are defined\nand their descriptions are detailed. This ensures that readers have access to the necessary information to interpret\nthe stratigraphic columns accurately. We appreciate the reviewer highlighting this and have taken steps to make\nthe figures and their captions more transparent and user-friendly.\nComment 3.43 (Reviewer 3): - Figure 4: the right panel is aesthetically interesting but confusing to read,\nwith too many overlapping colors. The legend for product types and raw materials requires improvement.\nIts placement within the figure creates confusion regarding which elements correspond to each part.\nResponse to reviewer comment: Thank you for your thoughtful feedback on Figure 4. The right panel is a\nSankey plot, a type of visualization routinely used in social science data to represent complex relationships between\nmultiple groups. We have now added details to ensure clarity, including providing the raw data used to generate\nthe plot. Readers can calculate the actual values from the raw data included in the supplementary materials.\nAdditionally, we have improved the legend for product types and raw materials by reformatting its placement to\nminimize confusion [Figure 4].\nComment 3.44 (Reviewer 3): - Figure 5: please enlarge the legend below the right panel\nResponse to reviewer comment: Thank you for your comment regarding Figure 5. We have made the\nrequested changes by enlarging the legend below the right panel to improve its readability [Figure 5].\nComment 3.45 (Reviewer 3): - Figure 6: the right side of the figure should be edited. The legend in\nFig.6-K is cropped. The dates are also cropped in Fig6-J.\nResponse to reviewer comment: This change has been noted by other reviewers and this figure has been\ncompletely revised in the resubmitted version [Figure 6].\nComment 3.46 (Reviewer 3): - There are different notations for \"STP_601\", referred to as STP601 in some instances and as STP_601 in others. This\nshould be homogenized.\nResponse to reviewer comment: The multiple different uses of the terms for excavation is now negated\nbecause of the revised names for all excavations as noted in our previous response to reviewer 1’s comment about\nthe names of the site and the excavations [Multiple changes throughout the text].\nComment 3.47 (Reviewer 3): - SM Figure 1: including the location of the sites alongside the geological\nsections would be beneficial.\nResponse to reviewer comment: Supplementary Figure 1 has been revised to incorporate the new names\nused in the entire manuscript as per the revision suggested by Reviewer 1.\nComment 3.48 (Reviewer 3): - SM Figure 6: It would be helpful to specify the type of bone or clarify\nwhether it is unidentified, rather than broadly referring to it as a ‘portion of a size 3 suid’. The caption\nmentions four bones exhibiting evidence of butchery marks. For those not shown here, to which species do\nthey belong, and what types are bones are they?\nResponse to reviewer comment: Thank you for your comment regarding the identification of bone types in\nthe figure. The type of bone is already mentioned in the caption, but we recognize the need for additional clarity. In\nresponse, we have modified the caption to include descriptions of the types of other bones with modifications that\nwere recovered during the excavations. This added detail ensures a more comprehensive and transparent\npresentation of the data [Supplementary Data].\nComment 3.49 (Reviewer 3): - SM Figure 19 is not mentioned in the main text.\nResponse to reviewer comment: Supplementary Figure 19 (now supplementary Figure 21) has now been\nincorporated into the text.\nResponse to the Editor and Reviewers\nWe thank the editor and reviewers for their detailed feedback, which helped improve\nthe clarity and robustness of our chronostratigraphic framework. We have carefully\nrevised the manuscript to better align our interpretation of the geological and\nmagnetic polarity data with current regional stratigraphic models, particularly as they\nrelate to the KBS Tuff and the Upper Burgi Unconformity (UBU).\nReviewer 1 Comment: Reviewer 1 provided encouragement and support that our\nrevised manuscript provides a clearer and more robust understanding of the context\nand location of artifacts at the NMT sites.\nReviewer 2 Comment: The second reviewer made some minor suggestions about\ntypographical errors and incomplete references. We have made the adjustments in\nthe revised text.\nReviewer 2 also mentions that after reviewing the paleomagnetic and stratigraphic\ninformation, they believe that the chrono-stratigraphy we present is well supported by\nthe data. Reviewer 2 also reviews the current description of the Burgi Unconformity in\nthe Koobi Fora Fm. They indicate that based on current photographic evidence in all\ncurrent publications there is no convincing evidence for and actual angular\nunconformity. The reviewer confirms that our age estimate is a conservative estimate\nand that other photographic evidence does not provide adequate information\nnecessary to support and a major unconformity. The reviewer suggests that current\nevidence does not support the presence of a significant unconformity. They suggest\nthat we indicate this sedimentary sequence as conformable given the gradual\ntransition over to lacustrine sediments above the G/M boundary.\nWe have opted to provide an age model that includes multiple possible scenarios (as\nindicated in Figure 3). Importantly, the age of NMT1, and NMT 2 are not affected by\nthis because they sit adjacent to or below the G/M boundary. Therefore, as the\npotential Burgi Unconformity only influences the age of the youngest archaeological\nsite we opted to provide the conservative age estimate. As indicated in our response\nto Reviewer 3, we believe we have provided the most conservative age model.\nFurthermore providing multiple possible scenarios allows for the opportunity for\nmultiple interpretations of the sedimentary sequence.\nReviewer 3 Comment: Stratigraphic correlation and absence of KBS Tuff\nWe acknowledge the reviewer’s concern about the correlation of the Namorotukunan\nsequence to the broader Koobi Fora stratigraphy, especially regarding the presence\nof the KBS Tuff and the UBU. In response, we have provided a more detailed\ndiscussion of the geological, lithostratigraphic, and paleomagnetic context in the\nrevised manuscript.\nThe KBS Tuff is an important and regionally extensive stratigraphic marker, and its\nabsence at Namorotukunan plays a key role in our interpretation. We now emphasize\nthat the KBS Tuff has been described ~600 meters west of Namorotukunan, across\nthe North Gele Fault (previously mapped and samples by Kidney 2012), where it\noverlies a thick succession of lacustrine clays. This lithologic package differs\nmarkedly from the Namorotukunan stratigraphy, which ends in a thinner lake clay\nsequence, lacking both the KBS Tuff and the associated lithologies. This strongly\nsuggests that the Namorotukunan sequence does not extend into the KBS Member.\nReviewer 3 Comment: Interpretation of the Burgi Unconformity\nWe have expanded our discussion of the Burgi Unconformity (UBU) in light of its\nexpression—or lack thereof—in Area 40. Although we acknowledge that may of the\nvolcanic ashes in the Turkana Basin are variably expressed (lateral variability in the\nthickness of tuffs is noted throughout the Turkana Basin). However, the fact that the\nKBS tuff is expressed as a highly tuffaceous unit (>2m thick) directly south of the\nTulu Bor and Burgi Member deposits in Area 40. As Kidney states in his 2012\ndescription of the area: “The KBS member must have been deposited in the study\narea, however after faulting occurred, subsequent erosion removed the KBS member\nstrata east of the North Gele fault.”(page 108). The UBU has been described as\nmarking a significant depositional hiatus in other parts of the Koobi Fora Formation.\nHowever, in Area 40, the transition from fluvial to lacustrine deposits is conformable,\nand we find no sedimentological evidence for an erosional surface at this contact. We\ntherefore present two scenarios in the revised manuscript:\n Scenario A: Continuous sedimentation across the transition.\n Scenario B: A brief and localized hiatus, potentially represented by a subtle\ndisconformity not apparent in the field.\nBoth scenarios are compatible with the archaeological and paleoenvironmental\nevidence and do not alter the placement or interpretation of the artifact-bearing\nhorizons. We have also clarified that if the UBU is present, it must be restricted to a\nshort interval within RZ1, implying a much briefer gap than that documented further\nsouth in Koobi Fora.\nOur data indicate that sedimentation in Namorotukunan resumed during an interval\nwhen other parts of the basin were still within the unconformity stage. This indicates\nthat, if an unconformity occurred here, it ended earlier and was shorter than at other\nsites. In the B scenario, we have used the published 2.5 Ma age for the onset of the\nunconformity; however, if the upper bound differs, it is equally reasonable to question\nwhether the lower bound is the same. Without direct constraints on its true duration,\nany age model incorporating this unconformity would rely on arbitrary assumptions.\nFor this reason, scenario A, the linear interpolation model remains the most stable\nand defensible option, and its use does not affect the paper’s key findings.\nReviewer 3 Comment: Correlation of magnetic polarity zones\nWe have revised the magnetostratigraphic interpretation to make clear that R1\ncorresponds to the Matuyama Chron, and that NZ2 is more parsimoniously\ncorrelated with the Feni excursion (~2.12–2.14 Ma) rather than the Olduvai subchron\n(~1.95–1.78 Ma). This interpretation is based on several lines of evidence:\n The absence of the KBS Tuff in the lacustrine sediments associated with NZ2.\n The stratigraphic position of NZ2 above the Gauss–Matuyama boundary.\n The lithological mismatch between the Namorotukunan sequence and the\nlacustrine clay package associated with the KBS Member immediately to the\nwest of the Gele fault.\nWhile we acknowledge the possibility that NZ2 could correspond to the Olduvai\nsubchron, we find this less likely given the local sedimentological context and the\nlack of tephrostratigraphic markers supporting such a correlation.\nSummary of Key Revisions in the Manuscript\n We now clearly situate the KBS Tuff ~600 m west of Namorotukunan and contrast\nthe stratigraphy across the North Gele Fault.\n We provide a two-scenario model for the expression (or absence) of the UBU in\nArea 40.\n We offer a reasoned argument favoring correlation of NZ2 with the Feni\nexcursion, consistent with the paleomagnetic, stratigraphic, and\ntephrostratigraphic data.\n We explicitly state that both scenarios result in consistent placement of the\narchaeological sites between ~2.75 and 2.44 Ma, during the late Pliocene and\nearly Pleistocene.\nWe trust these clarifications address the reviewer’s concerns and help support the\ninternal consistency of our geochronological model.\nSmaller editorial revisions. The reviewers indicate that the x-axis was missing in\nfigure 6 e, f, g. However, the x-axis on this figure is time. We have aligned all\nsections of this figure to align with time. We have provided the age on two of the\nfigures, but we feel as though repeating the same axis 5 times would be repetitive.\nThe reviewers note that the name of the Order Perissodactyla has been spelled\nincorrectly in Supplementary Table 8. We have edited this, and we have also edited\nthe misspelling of names in Supplementary Table 10.\nReviewer #2 cross‐comments to Reviewer #3\nReviewer #3\nFirst of all, I would like to reiterate my appreciation for the substantial collective analytical effort\npresented in this article and to highlight the significant interest of the newly (re)investigated\nArea 40, located in the northern part of the Koobi Fora Formation. There is no doubt that the\npaleoenvironmental, stratigraphic, and archaeological data compiled in this study represent an\nimportant contribution to our understanding of the Early Pleistocene bio-environmental\ndynamics.\n-I agree with this\nAlthough various minor aspects could still be discussed, my second review will focus exclusively\non a single critical point: the chronological framework. This issue is pivotal, especially when\naddressing the earliest stages of human evolution. The challenge is particularly pronounced in\nthis context due to the absence of radioisotopic dates that directly bracket the archaeological\nrecord combined with a discontinuous sedimentary sequence. Despite improvements in the\nmanuscript and figures—especially Figures 2 and 3—and the inclusion of some details in both\nthe main text and supplementary materials, key information is still missing.\n-After the review of all cited references, I have not found a specific paper about the chronology\nof area 40, so I think the new data improve the chronology of this area in Koobi Fora.\nIn the Koobi Fora Formation, a major disconformity known as the Upper Burgi Unconformity has\nlong been recognized by numerous researchers (e.g., Brown, Feibel, Kidney, Gathogo).\nAccording to the recent study by Baldes et al. (2023), this unconformity spans a time gap\nbetween approximately 2.70 and 2.08 Ma.\nThis study is acknowledged briefly in the article (lines 141–142):“In Area 40, where\nNamorotukunan FwJj 52 is located, sedimentary strata are estimated to range roughly between\n4.3 to 1.6 Ma, with a gap between 3.0 and 2.5 Ma represented by the Burgi Unconformity.”\nHowever, this important stratigraphic feature is not further discussed in detail, despite an\napparent inconsistency with the age estimates proposed by Braun et al. (2.75, 2.58, and 2.44\nMa), which place the three archaeological sites squarely within the time gap identified by Baldes\net al. for the Upper Burgi Unconformity (Area 116). A thorough discussion explaining this\ndivergence in the proposed chronological framework is essential to substantiate the authors’\nargument for “an enduring technological adaptation in the hominin lineage throughout the late\nPliocene and earliest Pleistocene” (lines 128–129).\n-Baldes et al 2023 do not study the area 40 (Namorotukunan area), their data comes mainly\nfrom area 116, which is >40km away. The field pictures presented in the article show\nconformable strata, no unconformable strata. In case of clear evidence of sedimentary hiatus,\nthis boundary should be named a paraconformity to be more concise.\n-Different from the submitted paper that supply the position of Tulu Bor tuff and\npaleomagnetic boundaries, Baldes et al 2023 use only the lithological contact with units of\ndifferent color to map the cited unconformity, but no chronological data are supplied below\nand above this plano-parallel contact to justify a sedimentary hiatus (see published fig.6).\n“Light Tulu Bor Member deposits (blue) contrast with dark brown Upper Burgi clays (green).\nPurple line marks the contact used to map the UBU (fig 6)” .\n[FIGURE REDACTED]\nI have seen many vertical changes of facies (lake transgressions) like this in similar Pliocene-\nPleistocene deposits from Ethiopia and Kenya triggered by climatic processes, not by\ntectonics, that do not imply any relevant sedimentary hiatus.\nFig. 6 from Baldes et al 2023 in area 116 40km away from Namorotukunan sites.\nMore specifically, the geochronological model presented by Braun et al. implies two potential\nscenarios:\n• Scenario 1: the Upper Burgi Unconformity is absent in Area 40, which is the scenario used in\nthe paper.\n• Scenario 2: the unconformity is present, marked by L1 bed, corresponding to the Lorenyang\nlake transgression as proposed by previous studies; the implication of this scenario, which is not\ndiscussed in the article, would be a time gap of ca. 0.3 Ma in Area 40, based on the 2.2 Ma age\nproposed by Braun et al. for the Lorenyang lake transgression (Lines 242-243). This differs\nsignificantly from Baldes et al. who place this boundary at 2.08 Ma in other parts of the Koobi\nFora Fm, for a 0.62 Ma estimated time gap.\nI don’t see that Braun et al propose a 2.2 Ma age for the Lorenyang lake transgression in Lines\n242-243. In scenario B the authors place the lacustrine claystones during Olduvai times, which\nis a time younger than 2Ma, see fig.3.\no This second scenario opens the way for two options with regard to paleomagnetic correlations\nwith lake transgression phases, implying a correlation of L1 with either the Pre-Lorenyang lake\n(option 1) or with Lorenyang lake (option 2), the first one being chosen because of the absence\nof the KBS tuff in the Area 40 sequence, which also differs from Baldes et al. who correlate L1\nwith the Lorenyang lake transgression.\nThis raises several important questions:\n• If the Upper Burgi Unconformity is absent in Area 40 (Scenario 1), contra Baldes et al. (among\nothers) who correlate it with the bottom of L1 bed, which lake transgression could L1 bed\ncorrespond to?\n-Baldes et al 2023 do not study the area 40 (Namorotukunan area), their data comes mainly\nfrom area 116 >40km away.\n• What geomorphological or tectonic processes could account for continuous sedimentation in\nArea 40, in contrast to conclusions drawn by previous studies?\nAfter this new work, area 40 has more magnetostratigraphic data, and the previous\ninterpretation should be reviewed, I have not worked in the area, but the first option to\nexplore to explain this lake transgression would be wetter paleoclimate.\n• Why is Scenario 1 (absence of unconformity) favored? Is the rationale that it “provides more\nconservative (younger) age estimates” (line 272) truly convincing or relevant in this context?\nI think there are different reasons to favor scenario 1, the most important is that there is no\nfield evidence of an unconformity in the studied section. The second is the absence of KBS tuff,\nwhich occurs in the area in fluvial (not lacustrine) deposits.\n• If the unconformity is present (Scenario 2), how can you explain that the Upper Burgi\nUnconformity would be significantly shorter in Area 40 compared to other parts of the Koobi\nFora Formation, such as Area 116?\nThis should be studied; many explanations are possible\n• In a discontinuous sedimentary sequence, due to lake transgression phases, is it relevant to\nuse the absence of chronological markers (e.g., KBS tuff) for establishing chrono-stratigraphic\ncorrelations?\nThe absence of a chronostatigraphic marker in lacustrine claystones is informative data,\nespecially when this marker is widely distributed and is present in area 40 in fluvial sediments\n(not in lacustrine), (Gathogo and Brown 2006).\n• In the absence of an uppermost tephra layer, why not consider a third scenario—i.e., a longer-\nduration unconformity placing the new sites higher in the stratigraphic sequence, perhaps\nwithin the KBS Member?\nThe new sites are below the unconformity, near the Gauss/Matuyama boundary, so the\nduration of this possible hiatus located above the sites, it is not relevant for their chronology.\nUltimately, regardless of the scenario selected, the chronostratigraphic model proposed in this\narticle diverges significantly from previously established frameworks.\nIn my opinion, the chronoststratigraphy of the Namorotukunan area was not studied in detail,\nand the newly available data solves the age, especially for the lower part of the stratigraphy\nThis alternative model must be explicitly compared and contrasted with earlier work. With the\nevidence currently available, non-specialists in Koobi Fora chrono-stratigraphy are not in a\nposition to confidently evaluate the robustness of the proposed age model. Given the\nsignificance of the archaeological sites and associated paleoenvironmental data,\nstrengthening the chronological framework should be a top priority before drawing\nbehavioral or paleoenvironmental inferences.\nI’m a non-specialist in Koobi Fora chrono-stratigraphy, but after reviewing the available\npublished information presented in the manuscript and references, I have a quite clear idea\nabout the chrono-stratigraphy of Namorotukunan. Of course, future work can supply new\ndata and refine this new chronostratigraphic frame."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have read through the revised manuscript and replies to reviewers. I think that the authors have done an impressive and\ndetailed job addressing the concerns of the reviewers. Thank you for adding Supplementary Figure 6, showing in situ\nartifacts, and addressing all other comments I raised in a previous review. Figure 6 is a great improvement, although the x-\naxis labels are missing on Figures 6c, 6e, and 6f. Also, what does the vertical gray shading on Figures 6e, 6f, and 6g\nindicate? I think that the results of this study are noteworthy, and the claims are supported by the data presented.\nI recommend this manuscript for publication.", "reviewer_2": "Dear Editor,\nThe present version of the manuscript has covered all suggestions and questions presented in my previous review. The\nauthors have modified the manuscript, making the paper stronger and easier to read. As I said before, the report of three new\nsuperposed archaeological levels at the Namorotukunan site dated between 2.7 and 2.4Ma is an important contribution,\nrepresenting the earliest evidence of Oldowan technology within the Koobi Fora Formation and one of the oldest in the\nworld. In addition, the article supplies important chronological and paleoenvironmental data for the studied period that place\nthe finds in a paleoenvironmental context. For these reasons, I think this article deserves to be published in Nature\nCommunications.\nIn my initial review, I was concerned about the age model, but the authors clarified my questions quite well in their answers\nand improved figures/text. I read carefully again the new version of the paper, and the cited references about the “Burgi\nUnconformity” (Kidney 2012, Baldes et al 2023 and Gathogo and Brown 2006). This has allowed me to have my personal\nopinion about the age and continuity of sedimentation in the studied area, which is a key question. By the end of my review, I\nsuggest an important change in one paragraph that should be made to give consistency to the conclusions(see below).\nApart of that, I only found two minor typos in the Supplementary material easy to correct:\n-Supplementary table 8: Perrisodactyla should be Perissodactyla, and this word appears separated in two parts as\nCetartiodactyla.\n-Supplementary table 9: check the references there are some typos, for example “de al Torre et al. 2003” should be “de la\nTorre et al. 2003”\nReview of the Namorotukunan chrono-stratigraphy:\n1-The presence of the Tulu Bor tuff dates the bottom of the section at 3.44Ma, this tuff is associated with normal magnetic\npolarity of NZ1, which after the Tulu Bor radiometric dates (3.44 Ma) corresponds to Gauss magnetozone. This normal zone\nis followed by a long reverse magnetic zone (RZ1) that should be Matuyama, so the Normal/Reverse polarity change\nidentified in the lower part of the stratigraphy corresponds to the Gauss/Matuyama magnetic boundary (2.6Ma).\n2-Three superposed archaeological sites appear below and above the Gauss/Matuyama boundary, allowing accuracy in the\nchronology of the three superposed archaeological sites. So, the main conclusion: “The site comprises three distinct\narchaeological horizons spanning approximately 300,000 years (2.75 - 2.44 Ma) within a persistent fluvial setting ” is well\nsupported by data.\n3-The presence or absence of the Burgi Unconformity would affect only the age of strata in the uppermost part of the\nstratigraphy, above the archaeological sites, having a minimum effect on the chronology of the sites. But would not affect the\ncontinuity in the sedimentation around the sites all placed below the supposed unconformity.\n4-The Burgi unconformity in the studied area:\nThe information about this unconformity in the study area (Namorotukunan, area 40) is not well documented. The\nunconformity is not mapped in Kidney 2012 map, where the contacts between Tulu Bor and upper Burgi Mbs are shown as\nconformable in his map (see below). In the studied area, the contact between Tulu Bor Mb and Burgi Mb is not erosional or\nangular, it is a plano-parallel contact. This plano-parallel contact is visible in the pictures presented by the authors of this\nstudy as well in other areas, >40 km south (Baldes et al 2023 of area 116). However, Gathogo and Brown mention a 20º\nangular unconformity between these members in the Illeret area. Unfortunately, Gathogo and Brown 2006 do not show any\nfield picture of this angular unconformity. From my field experience, faults and unconformities are very important structures\nthat, if they exist, should be well documented because of their implication in the geological history of an area.\nAccording to the same authors, the sedimentary hiatus in area 40 represented at Burgi unconformity is close to 2Ma. “In\nAreas 40 and 41, the temporal gap is even larger, approaching 2 Ma, where the upper Burgi Member is in contact with the\nundifferentiated Moiti and Lokochot Members.” (Gathogo and Brown 2006). We know after the present work that this is not\ncorrect because Tulu Bor tuff is in the section.\nWith the available information, I do not see any field evidence for the presence of an unconformity in the upper part of the\nstudied section in Namorotukunan area. In contrast, the transition from alluvial to lacustrine units seems to be gradual\naccording to figure 2, with an initial lacustrine incursion. The contact between Lake clays and underlaying sediments is\nplano-parallel and does not show angular or erosional features. If someone claims this unconformity, here should name this\nboundary as a paraconformity and present evidence of a correlation with an angular/erosional unconformity elsewhere.\n5-Age models\nThe authors propose two age models, one considers the presence of this unconformity and a sedimentary hiatus, and the\nother shows continuous sedimentation. If the unconformity is considered, the chronology of the two upper archaeological\nsites would be a bit older. The authors prefer the model without unconformity, because they have not seen the widely\ndistributed KBS tuff (dated at 1,87 Ma and included in Olduvai magnetic subchron).\n“In this paper, we use the first scenario, which provides more conservative (younger) age estimates. It is important to note\nthat these scenarios do not affect the age estimation of the oldest archaeological levels presented in this paper”\nAccording to Gathogo and Brown 2006 “In area 40 immediately north of Area 10, the KBS Tuff (K03-077; Table 2) is overlain\nby medium sandstones that fill channels oriented SSE–NNW” so this tuff seems to be associated in area 40 to fluvial\ndeposits while in the studied area, the possible equivalent strata in the presence of unconformity, are lacustrine clays. The\nabsence in these lacustrine deposits of the widely exposed KBS tuff (that occurs in Olduvai), supports the correlation of the\nupper normal event with Feni subchron.\nWith all the presented data, I agree with the preferred age model supporting a continuous sedimentation rate. However, a\nparagraph on line 135 introduces confusion, and it should be corrected.\nPage 135:\n“In Area 40, where Namorotukunan is located, sedimentary strata are estimated to range roughly between 4.3 to 1.6\nMa24,25, with a gap between 3.0 -2.5 Ma represented by the Burgi Unconformity24,26,27. Above this unconformity, a series\nof lake clays indicate a local change in tectono-sedimentary patterns and the Paleo-Lake Lorenyang transgression in the\nTurkana basin24,27.\n6-Comment:\nThis paragraph is included in the “Results/ Geological context” chapter and accepts the presence of the unconformity in area\n40, covering in part the age of the new sites, which contradicts the selected age model. It is important to notice that from the\nreferences cited, the only one that directly includes the study of Namorotukunan area is Kidney 2012. But, although I did not\nhave access to the full document (Phd), his map of the area shows a conformable contact between units, but not an\nunconformity (marked in geological maps with a dashed line). The other references do not include the Namorotukunan area.\nThis sentence should be rewritten to avoid confusion. Part of it should be included in the introduction/geological setting,\njust to refer to the cited unconformity in the region. In the results chapter should be mentioned that no evidence of the Burgi\nUnconformity is found in the Namorotukunan section, which gives consistency with the selected age model.\nI said before, the transition from alluvial to lacustrine units seems to be gradual in area 40, and can be related to wetter\npaleoclimate without involving tectonics and unconformity.\n---------------------------------------------------------------------------------------------------\nSee Kidney 2012 map at\nhttps://cdn.prod.websitefiles.com/5cc7bd30669af234434d1dca/5d266093b3094d49b7acf7f4_Kidney.pdf\nMap published by Kidney 2012 showing a conformable contact between Tulu Bor (Tstb) and Upper Burgi (Tsub) Mbs\naround the Namorotukunan hill. In contrast, see unconformable contacts (dashed lines) for units Qs (Holocene) and T5ln\n(early Pliocene).", "reviewer_3": "First of all, I would like to reiterate my appreciation for the substantial collective analytical effort presented in this article and\nto highlight the significant interest of the newly (re)investigated Area 40, located in the northern part of the Koobi Fora\nFormation. There is no doubt that the paleoenvironmental, stratigraphic, and archaeological data compiled in this study\nrepresent an important contribution to our understanding of the Early Pleistocene bio-environmental dynamics.\nAlthough various minor aspects could still be discussed, my second review will focus exclusively on a single critical point:\nthe chronological framework. This issue is pivotal, especially when addressing the earliest stages of human evolution. The\nchallenge is particularly pronounced in this context due to the absence of radioisotopic dates that directly bracket the\narchaeological record combined with a discontinuous sedimentary sequence. Despite improvements in the manuscript and\nfigures—especially Figures 2 and 3—and the inclusion of some details in both the main text and supplementary materials,\nkey information is still missing.\nIn the Koobi Fora Formation, a major disconformity known as the Upper Burgi Unconformity has long been recognized by\nnumerous researchers (e.g., Brown, Feibel, Kidney, Gathogo). According to the recent study by Baldes et al. (2023), this\nunconformity spans a time gap between approximately 2.70 and 2.08 Ma. This study is acknowledged briefly in the article\n(lines 141–142):“In Area 40, where Namorotukunan FwJj 52 is located, sedimentary strata are estimated to range roughly\nbetween 4.3 to 1.6 Ma, with a gap between 3.0 and 2.5 Ma represented by the Burgi Unconformity.” However, this important\nstratigraphic feature is not further discussed in detail, despite an apparent inconsistency with the age estimates proposed by\nBraun et al. (2.75, 2.58, and 2.44 Ma), which place the three archaeological sites squarely within the time gap identified by\nBaldes et al. for the Upper Burgi Unconformity (Area 116). A thorough discussion explaining this divergence in the proposed\nchronological framework is essential to substantiate the authors’ argument for “an enduring technological adaptation in the\nhominin lineage throughout the late Pliocene and earliest Pleistocene” (lines 128–129).\nMore specifically, the geochronological model presented by Braun et al. implies two potential scenarios:\n• Scenario 1: the Upper Burgi Unconformity is absent in Area 40, which is the scenario used in the paper.\n• Scenario 2: the unconformity is present, marked by L1 bed, corresponding to the Lorenyang lake transgression as\nproposed by previous studies; the implication of this scenario, which is not discussed in the article, would be a time gap of\nca. 0.3 Ma in Area 40, based on the 2.2 Ma age proposed by Braun et al. for the Lorenyang lake transgression (Lines 242-\n243). This differs significantly from Baldes et al. who place this boundary at 2.08 Ma in other parts of the Koobi Fora Fm, for a\n0.62 Ma estimated time gap.\no This second scenario opens the way for two options with regard to paleomagnetic correlations with lake transgression\nphases, implying a correlation of L1 with either the Pre-Lorenyang lake (option 1) or with Lorenyang lake (option 2), the first\none being chosen because of the absence of the KBS tuff in the Area 40 sequence, which also differs from Baldes et al. who\ncorrelate L1 with the Lorenyang lake transgression.\nThis raises several important questions:\n• If the Upper Burgi Unconformity is absent in Area 40 (Scenario 1), contra Baldes et al. (among others) who correlate it with\nthe bottom of L1 bed, which lake transgression could L1 bed correspond to?\n• What geomorphological or tectonic processes could account for continuous sedimentation in Area 40, in contrast to\nconclusions drawn by previous studies?\n• Why is Scenario 1 (absence of unconformity) favored? Is the rationale that it “provides more conservative (younger) age\nestimates” (line 272) truly convincing or relevant in this context?\n• If the unconformity is present (Scenario 2), how can you explain that the Upper Burgi Unconformity would be significantly\nshorter in Area 40 compared to other parts of the Koobi Fora Formation, such as Area 116?\n• In a discontinuous sedimentary sequence, due to lake transgression phases, is it relevant to use the absence of\nchronological markers (e.g., KBS tuff) for establishing chrono-stratigraphic correlations?\n• In the absence of an uppermost tephra layer, why not consider a third scenario—i.e., a longer-duration unconformity\nplacing the new sites higher in the stratigraphic sequence, perhaps within the KBS Member?\nUltimately, regardless of the scenario selected, the chronostratigraphic model proposed in this article diverges significantly\nfrom previously established frameworks. This alternative model must be explicitly compared and contrasted with earlier\nwork. With the evidence currently available, non-specialists in Koobi Fora chrono-stratigraphy are not in a position to\nconfidently evaluate the robustness of the proposed age model. Given the significance of the archaeological sites and\nassociated paleoenvironmental data, strengthening the chronological framework should be a top priority before drawing\nbehavioral or paleoenvironmental inferences.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": ""}]

Decision letter