Orrorin

Credit: Pascal Goetgheluck.

*Orrorin* is an extinct genus of basal hominine that lived in Kenya during the Late Miocene.

## History

The type species, *O. tugenensis*, was discovered in 2000 by a team led by Brigitte Senut and Martin Pickford, and stored in the Kipsaraman village community museum, which has then since closed. According to Eustace Kitlonga, Community Museums of Kenya chairman, they were placed into a secret bank vault in Nairobi. In 2017, similarly dated footprints from Trachilos, Crete were found, and, if they can be assigned to an ape, may be of a related taxon, a type of basal Hominini or another convergently-evolved ape. It was named after the term "Orrorin" or "Orroriek" (plural), meaning "original man" in Tugen. The specific epithet is after the Tugen Hills, of which the type locality is found. The original describers envisioned the hominid lineage gradually inheriting megadonty, using the deep mandibular corpus, mosaic traits in the limbs and teeth and its earlier age to place it within the *Homo* lineage. Pickford et al. (2022) named a new species from the Mabaget Formation of the Tulu Borr Tuff, Tugen Hills, coining *O. praegens*. It was named from mandibular, phalange and humerus elements.

## Description

20 specimens from at least five individuals were discovered in 2007. Of these are the posterior of a mandible (in two fragments), a symphysis, isolate teeth, 3 femoral fragments, a partial humerus, a proximal phalanx and the distal thumb phalanx. Its teeth were proportionally small compared to the body size. Unlike *Australopithecus*, the teeth are smaller with mesiodistal measurements that are less elongate, and the enamel is thicker than *Ardipithecus*; differing from both, a mesial groove on the upper canines is present. The canines are apelike, but reduced, similar to the condition seen in Miocene apes and female chimpanzees. The postcanine dentition was microdont, unlike megadont australopithecines but like later *Homo*. However, it is difficult to determine whether these characters solidify *Orrorin* as a part of the human lineage. The femoral head is anteriorly rotated and spherical, with an elongated neck and medially-protruding lesser trochanter. Paired with an ovular cross-section, this may suggest bipedal locomotion paired with arboreal activity, which is indicated by the remaining postcranium. The proximal phalanx curves, unlike the distal pollical phalanx, which sits within human variation. This was interpreted as an adaptation for toolmaking but is more likely related to arboreality and tree-climbing. The holotypic specimen of *O. tugenensis* is interpreted as a young adult male, with a weight between 35-50 kilograms and a minimum height of 1.1-1.2 meters, the latter being noted as a conservative estimate. *O. praegens* was around the size of a chimpanzee and has tooth characters most similar to *Ardipithecus ramidus*. The description of *O. praegens* notes that sexual dimorphism and thus variability was probably present in this genus. As well, the distal extremity torsion and morphology of the proximal articular facet suggest that it was capable of at least some grasping. The former and the plantar aspect of the plantar tubercle also suggest bipedalism, with the second proximal phalanx in the foot indicating gasping while climbing.

## Classification

If it is a hominin, it being nearly 3 million years older than other taxa may suggest that *Praeanthropus* is a member of an offshoot lineage. The femur is supposedly closer to *Homo sapiens* than to Lucy (*Praeanthropus afarensis*), but the holotype femur is interpreted as male while the latter is interpreted as female. This has been cause for much dispute. Others find intermediateness with the internal anatomy of the femur between Miocene apes and australopithecines. The initial description postulated that *Samburupithecus* was the ancestral hominoid, diverging into a forking lineage: where *Ardipithecus* evolves directly into *Pan*, and another that further diverges into australopithecines and the *Praeanthropus*/*Homo* line. *O. praegens* is synonymous with *Homo antiquus praegens* or *H. praegens*, named from the Tabarin mandible, but the specific epithet proves valid if the holotypic specimen is a new taxon. Provisionally, the specimen was previously called a member of *Ardipithecus ramidus* (or cf. *A.* cf. *ramidus*), *"Ardipithecus" praegens* or *"Praeanthropus" praegens*. Enamel wear does not allow placement in any other hominid group, but having similarity with other early hominins and dissimilarity with australopithecines suggests that it is a member of *Orrorin*.

## Paleobiology

*O. tugenensis* show no derived features of hominoid ape relatives; rather, it shows several apomorphies with *Homo sapiens* and some australopithecines: an obturator extremus groove present, an elongate femoral neck, an anteriorly-twisted head, a femoral neck that is anteroposteriorly compressed, cortex in the femoral neck is asymmetrically distributed, the superior notch is shallow, a gluteal tuberosity that is well-developed and a gluteal tuberosity that posteriorly descents the femoral shaft. However, some traits are shared with Miocene apes, with some transitional of *Aegyptopithecus* and *Australopithecus*. Recent analyses suggest that *O. tugenensis* was a basal hominid with a form of bipedalism. The femoral head suggests some arboreal habits, likely foraging and shelter building. The femoral neck is elongate, as in australopiths and *Homo sapiens*. Initially, the femoral head was claimed to be larger than australopiths and closer to modern forms, but this has been challenged through the observation that male australopith femur heads are similar to *Orrorin*. Some have suggested that the male femur samples are of another taxon; regardless, bipedalism may have been present 6 million years ago. Like early hominins, the iliac blade is flared to resist body weight torque, like australopithecines. Pickford supports a greater connection to the human lineage. The phylogeny of *Orrorin* is still hotly debated.

## Paleoecology

The type species was discovered in the Lukeino Formation, which was a dry evergreen forest, unlike the idea that early hominins evolved in savannahs. The savannah hypothesis is further discredited through observation of chimpanzee bipedalism in Issa Valley. The Lukeino Formation preserves fossils from hyraxes, mustelids, *Hippopotamus*, various fish including cyprinids, felids, *Hipparion*, *Ceratotherium*, giraffes, bovids, pigs, various rodents, various proboscideans (such as *Stegotetrabelodon*, *Anancus*, *Primelephas* and *Deinotherium*), crocodiles, birds, testudines, varanids, aardvarks, bivalves and lagomorphs, among others. Sites from the Lukeino Formation preserve other hominids: *Praeanthropus afarensis* is known from ETE Locality 1098 and Hominidae indet. from ETE Locality 1099. The Mabaget Formation, where *O. praegens* is found, preserves fossils from ostracods, gastropods, bivalves, fish (such as cyprinids), testudines, crocodilians, birds, chiropterans, soricids, bears, mustelids, hyaenas, other primates (galagids, cercopithecids), lagomorphs, rodents, proboscideans (such as *Anancus*, ?*Primelephas* and *Deinotherium*), rhinoceros, chalicotheres, *Hipparion*, *Hippopotamus*, pigs, *Sivatherium* and mouse-deer.

## Synonyms

* *Homo antiquus*
* *Homo antiquus praegens*
* *"Ardipithecus" praegens*
* *"Praeanthropus" praegens*

## Notable Specimens

* BAR coll. "Millennium Man": A collection of elements belonging to around five individuals, coined the holo- and paratypes of *O. tugenensis*. The manual phalanx was found in the Kapcheberek locality, the proximal right femur in the Aragai locality and the remaining specimens from the Kapsomin locality.
	+ BAR 1002'00: A left femur, designated a paratype.
	+ BAR 1000'00: This specimen was given holotypic status; a mandible split in two: BAR 1000a'00 being a left mandible fragment with lower M2-3 teeth and BAR 1000b'00 being a right mandible fragment with a lower M3 tooth.
	+ BAR 1900'00: An isolated right upper M3 tooth, designated a paratype.
	+ BAR 1390'00: An isolated right lower M4 tooth, designated a paratype.
	+ BAR 1001'00: An isolated upper I1 tooth, designated a paratype.
	+ BAR 1425'00: An isolated right C, designated a paratype.
	+ BAR 1004'00: A right distal humerus, designated a paratype.
	+ BAR 1003'00: A proximal left femur, designated a paratype.
	+ BAR 349'00: A manual proximal phalanx, designated a paratype.
	+ BAR 1426'00: An isolated left upper M3 tooth, designated a paratype.
	+ BAR 1215'00: A fragmented right proximal femur, designated a paratype.
* KNM LU 335: A specimen found in 1974, a lower molar, from a different locality (Cheboit) and discovered by Martin Pickford. It was designated a paratype and identified as a member of *O. tugenensis* by Senut (2001).
* OCO BAR 150'10: A second pedal phalanx. It was assigned to *O. praegens*.
* OCO BAR 151’10: An isolated right D4 with medium wear. The roots are missing, and may have been resorbed before shedding or dead. It was assigned to *O. praegens*.
* OCO BAR 1046'11: A fragmentary right mandible with moderately-worn lower p4 and m1 teeth. It was assigned to *O. praegens*.
* OCO BAR 1'08: A right p3 tooth. It was assigned to *O. praegens*.
* OCO BAR 500'05: A left m3 tooth. It was assigned to *O. praegens*.
* KNM TH 13150: A fragmented mandible with m1 and m2 teeth. It was assigned to *O. praegens*.
* KNM BC 1745: A proximal humerus. It was assigned to *O. praegens*.

## References

1. ↑ 1.0 1.1 1.2 1.3 1.4 https://en.wikipedia.org/wiki/Orrorin
2. ↑ 2.0 2.1 2.2 2.3 https://ui.adsabs.harvard.edu/abs/2001CRASE.332..137S
3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 https://www.researchgate.net/publication/366490905\_New\_Pliocene\_hominid\_fossils\_from\_Baringo\_County\_Kenya
4. ↑ https://pubmed.ncbi.nlm.nih.gov/17318735/
5. ↑ https://phys.org/news/2022-12-early-humans-upright-trees.html
6. ↑ https://www.science.org/doi/10.1126/sciadv.add9752
7. ↑ https://paleobiodb.org/classic/displaySearchStrataResults?group\_formation\_member=Lukeino
8. ↑ https://www.discovermagazine.com/the-sciences/our-first-human-ancestor-appeared-around-6-million-years-ago