letters to nature
weakly anisotropic upper mantle layer below 250 km depth isactively deforming by dislocation creep, and hence the top andbottom layers may be strongly coupled down to 400 km depth. Our
predictions (that weak seismic anisotropy will develop in olivinerich aggregates deforming by [001](hk0) slip in the deep upper
mantle) challenge the two traditional interpretations for regions in
the deep Earth of weak seismic anisotropy; (1) that they represent
zones of poor deformation coherence at the seismic length scale, or(2) that the dominant deformation mechanism (for example,
diffusion creep) in these regions does not produce CPO. Indeed,transition from dominant [100] to [001] slip at high pressure may
explain the variation with depth of the anisotropy patterns of P and
S waves, even if the entire upper mantle deforms coherently with adominant horizontal shearing component (as expected in a convective system with large-scale plates at the surface, like the Earth’s
mantle).
A
Received 11 August; accepted 30 November 2004; doi:10.1038/nature03266.
1. Montagner, J.-P. Seismic anisotropy of the Pacific Ocean inferred from long-period surface waves
dispersion. Phys. Earth Planet. Inter. 38, 28–50 (1985).
2. Cara, M. & Lévèque, J. L. Anisotropy of the asthenosphere: The higher mode data of the Pacificrevisited. Geophys. Res. Lett. 15, 205–208 (1988).
3. Nishimura, C. E. & Forsyth, D. W. The anisotropic structure of the upper mantle in the Pacific.
Geophys. J. 96, 203–230 (1989).
4. Montagner, J.-P. & Kennett, B. L. N. How to reconcile body-wave and normal-mode reference earthmodels. Geophys. J. Int. 125, 229–248 (1996).
5. Karato, S.-I. On the Lehman discontinuity. Geophys. Res. Lett. 19, 2255–2258 (1992).
6. Karato, S. I. & Rubie, D. C. Toward an experimental study of deep mantle rheology: a new multianvil
sample assembly for deformation studies under high pressures and temperatures. J. Geophys. Res. 102,
20111–20122 (1997).
7. Cordier, P. & Rubie, D. C. Plastic deformation of minerals under extreme pressure using a multi-anvil
apparatus. Mater. Sci. Eng. A 309–310, 38–43 (2001).
8. Durham, W. B., Weidner, D. J., Karato, S. I. & Wang, Y. in Plastic Deformation of Minerals and Rocks(eds Karato, S.-I. & Wenk, H.-R.) (American Mineralogical Society, Washington, 2002).
9. Couvy, H. et al. Shear deformation experiments of forsterite at 11 GPa – 1400 8C in the multianvil
apparatus. Eur. J. Mineral. (in the press).
10. Tommasi, A. Forward modeling of the development of seismic anisotropy in the upper mantle. Earth
Planet. Sci. Lett. 160, 1–13 (1998).
11. Silver, P. G. Seismic anisotropy beneath the continents: Probing the depths of geology. Annu. Rev.
Earth Planet. Sci. 24, 385–432 (1996).
12. Ekstrom, G. & Diewonski, A. M. The unique anisotropy of the Pacific upper mantle. Nature 394,
168–172 (1998).
13. Gung, Y., Panning, M. & Romanowicz, B. Global anisotropy and the thickness of continents. Nature
422, 707–711 (2003).
14. Lévêque, J. J., Debayle, E. & Maupin, V. Anisotropy in the Indian Ocean upper mantle from Rayleighand Love waveform inversion. Geophys. J. Int. 133, 529–540 (1998).
15. Gaherty, J. B., Jordan, T. H. & Gee, L. S. Seismic structure of the upper mantle in the central Pacificcorridor. J. Geophys. Res. 101, 22291–22309 (1996).
16. Revenaugh, J. & Jordan, T. Mantle layering from ScS reverberations 3. The upper mantle. J. Geophys.
Res. 96, 19781–19810 (1991).
17. Li, L., Raterron, P., Weidner, D., Chen, J. & Vaughan, M. T. Stress measurements of deforming olivine
at high pressure. Phys. Earth Planet. Inter. 143–144, 357–367 (2004).
18. Li, L., Raterron, P., Weidner, D. & Chen, J. Olivine flow mechanisms at 8 GPa. Phys. Earth Planet. Inter.
138, 113–129 (2003).
19. Ben Ismail, W. & Mainprice, D. An olivine fabric database: an overview of upper mantle fabrics andseismic anisotropy. Tectonophysics 296, 145–158 (1998).
20. Ben Ismail, W., Barruol, G. & Mainprice, D. The Kaapvaal craton seismic anisotropy: petrophysicalanalyses of upper mantle kimberlite nodules. Geophys. Res. Lett. 28, 2497–2500 (2001).
21. Vauchez, A., Dineur, F. & Rudnick, R. Microstructure, texture, and seismic anisotropy of thelithospheric mantle above a plume. Insights from the Labait volcano xenoliths. Earth Planet. Sci. Lett.
(in the press).
22. Lebensohn, R. A. & Tomé, C. N. A self-consistent anisotropic approach for the simulation of plastic
deformation and texture development of polycrystals: Application to zirconium alloys. Acta Metall.
Mater. 41, 2611–2624 (1993).
23. Wenk, H.-R., Bennet, K., Canova, G. R. & Molinari, A. Modelling plastic deformation of peridotite
with the self-consistent theory. J. Geophys. Res. 96, 8337–8349 (1991).
24. Tommasi, A., Mainprice, D., Canova, G. & Chastel, Y. Viscoplastic self-consistent and equilibriumbased modeling of olivine lattice preferred orientations. Implications for upper mantle seismic
anisotropy. J. Geophys. Res. 105, 7893–7908 (2000).
25. Abramson, E. H., Brown, M., Slutsky, L. J. & Zaug, J. The elastic constants of San Carlos olivine up to
17 GPa. J. Geophys. Res. 102, 12252–12263 (1997).
26. Chai, M., Brown, J. M. & Slutsky, L. J. The elastic constants of a pyrope-grossular-almandine garnet up
to 20 GPa. Geophys. Res. Lett. 24, 523–526 (1997).
27. Collins, M. D. & Brown, J. M. Elasticity of an upper mantle clinopyroxene. Phys. Chem. Miner. 26,
7–13 (1998).
28. Mainprice, D., Bascou, J., Cordier, P. & Tommasi, A. Crystal preferred orientations of garnet:
Comparison between numerical simulations and electron back-scattered diffraction (EBSD)
measurements in naturally deformed eclogites. J. Struct. Geol. 26, 2089–2102 (2004).
29. Bascou, J., Tommasi, A. & Mainprice, D. Plastic deformation and development of clinopyroxene
lattice preferred orientations in eclogites. J. Struct. Geol. 24, 1357–1368 (2002).

NATURE | VOL 433 | 17 FEBRUARY 2005 | www.nature.com/nature

30. Mainprice, D., Barruol, G. & Ben Ismaı̈l, W. in Earth’s Deep Interior: Mineral Physics and Tomography
from the Atomic to the Global Scale (eds Karato, S.-I., Forte, A. M., Liebermann, R. C., Masters, G. &Stixrude, L.) 237–264 (AGU, Washington DC, 2000).

Acknowledgements This Letter is dedicated to the memory of G. Canova, who introduced A.T.
and D.M. to VPSC modelling. H.C. was supported by the Deutsche Forschungsgemeinschaft.
Competing interests statement The authors declare that they have no competing financial
interests.
Correspondence and requests for materials should be addressed to D.M.
(David.Mainprice@dstu.univ-montp2.fr).

..............................................................

Stratigraphic placement andage of modern humans from
Kibish, Ethiopia
Ian McDougall1, Francis H. Brown2 & John G. Fleagle3
1
Research School of Earth Sciences, Australian National University, Canberra,
ACT 0200, Australia
2
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah
84112, USA
3
Department of Anatomical Science, Stony Brook University, Stony Brook,
New York 11794, USA.............................................................................................................................................................................

In 1967 the Kibish Formation in southern Ethiopia yieldedhominid cranial remains identified as early anatomically modern
humans, assigned to Homo sapiens1–4. However, the provenanceand age of the fossils have been much debated5,6. Here we confirm
that the Omo I and Omo II hominid fossils are from similar
stratigraphic levels in Member I of the Kibish Formation, despite
the view that Omo I is more modern in appearance than Omo
II1–3. 40Ar/39Ar ages on feldspar crystals from pumice clasts
within a tuff in Member I below the hominid levels place an older
limit of 198 6 14 kyr (weighted mean age 196 6 2 kyr) on thehominids. A younger age limit of 104 6 7 kyr is provided byfeldspars from pumice clasts in a Member III tuff. Geologicalevidence indicates rapid deposition of each member of the Kibish
Formation. Isotopic ages on the Kibish Formation correspond toages of Mediterranean sapropels, which reflect increased flow of
the Nile River, and necessarily increased flow of the Omo River.
Thus the 40Ar/39Ar age measurements, together with the sapropelcorrelations, indicate that the hominid fossils have an age close to
the older limit. Our preferred estimate of the age of the Kibish
hominids is 195 6 5 kyr, making them the earliest well-dated
anatomically modern humans yet described.
The principal outcrops of the Kibish Formation are along the
Omo River where it skirts the Nkalabong Range (Fig. 1), with thehighest outcrops close in elevation to that of the watershed between
the Omo and the Nile rivers7,8. Former hydrographic links areapparent from Nilotic fauna in the Turkana Basin sequence8–10.
The Kibish Formation (about 100 m thick) consists of flat-lying,tectonically undisturbed, unconsolidated sediments deposited
mainly in deltaic environments over brief periods. It comprises
the youngest exposed sedimentary sequence in the Omo Basin, andlies disconformably upon the Nkalabong Formation11,12 or on theunderlying Mursi Formation12. Strata are composed principally ofclaystone and siltstone, with subordinate fine sandstone, conglomerate and tuffs (Fig. 2).
Butzer et al.13 and Butzer14 divided the Kibish Formation into
Members I to IV on the basis of disconformities with up to 30 m
relief (Fig. 2). The members record discrete times of deposition© 2005 Nature Publishing Group

733

letters to naturewhen the northern margin of Lake Turkana and the Omo delta lay
about 100 km north of their current positions. As the higher lakelevels reflect significantly higher precipitation in the region, such
periods should be recognizable at least regionally, and perhaps
globally.
Member I (26 m thick; Fig. 2) was deposited disconformably on
the Nkalabong Formation in a deltaic environment. Small (less than
30 mm) rounded pumice clasts occur in an impure tuff in Member
I, 7 m below the base of Member II. This tuff lies near, but probably
slightly below, the levels from which Omo I and Omo II were
derived. Glass shards from the tuff are very similar in composition
to the glass from three pumice clasts enclosed in the tuff (Supplementary Table 1), indicating a common origin. Further, thiscomposition is distinct from the glass composition of any other tuff
in the Kibish Formation. Member II, about 28 m thick, wasdeposited on a topographic surface developed on Member I withat least 19 m of relief. Member II contains two discrete sequences,
separated by an internal disconformity, designated Members IIa andIIb on Fig. 2. Member II was incised by as much as 25 m before thedeposition of Member III11,14, which begins with thin siltstone andclaystone beds that drape topography. These beds, averaging
about 3 cm thick, fine upward internally, and represent annual
flooding. Thus, deposition of the lower part of Member III may
record less than 1 kyr. A tuff 3.5–12 m thick, 18 m above the baseof Member III, locally contains pumice clasts with alkali feldspar
phenocrysts. Glass of the pumices normally differs compositionally from the glass of the tuff (Supplementary Table 1). However,one pumice sample yielded two contrasting sets of analyses: one
corresponding to the tuff, the other to the dominant pumices.
The youngest unit of the Kibish Formation, Member IV, wasdeposited on the underlying sediments after up to 30 m ofdissection. Member IV comprises at most 21 m of strata deposited between about 9.5 and 3.3 kyr ago, on the basis of 14C datingof mollusc shell7,14,15.
Omo I was found at Kamoya’s Hominid Site (KHS; 58 24.15 0 N,
358 55.81 0 E), which was identified from contemporary photographs, from evidence of the 1967 excavations and by additional
hominid bone material conjoining the 1967 finds16. The hominid
fossils were recovered from a siltstone 2.4 m below the base of
Member II14. Thus, Omo I derives from near the top of Member
I. Butzer et al.13 reported a 230Th/234U date of 130 ^ 5 kyr on Etheria

Figure 1 Map showing the distribution of the Kibish Formation (shaded) in the lower Omo
Valley, southern Ethiopia, after Davidson29. Inset on lower left, locations of Omo I, Omo II,
measured sections, and dated samples.

Figure 2 Composite stratigraphy of the Kibish Formation. Member I was measured near
the type section at Makul; Member II was measured near Harpoon Hill, and Members III
and IV were measured along Camp Road (see Fig. 1 for locations).

734© 2005 Nature Publishing Group

NATURE | VOL 433 | 17 FEBRUARY 2005 | www.nature.com/natureletters to naturefrom essentially the same level as the hominid, but they and others
have questioned the reliability of the age5,6.
Omo II was found on the surface at PHS (Paul’s Hominid Site),
which Butzer14 mapped about 2.6 km northwest of KHS. A map
drawn at the time by Paul Abell (discoverer of Omo II), together
with contemporary photographs of the site supplied by K.W. Butzer,
have positively identified PHS at 58 24.55 0 N, 358 54.07 0 E, about
3.3 km west by north of KHS (Fig. 1). Although PHS was mislocatedon the published map, Butzer’s14 stratigraphic description at the site
is correct. The base of Member II lies about 3 m above theapproximate level from which Omo II was recovered. The basal
tuff of Member II is of unique composition, and correlates from
KHS to PHS (Supplementary Table 1).
Ages on alkali feldspars separated from pumice clasts from tuffs
in Members I and III reflect the time of their eruption, and provide
maximum and minimum ages for the hominid fossils, respectively.
40
Ar/39Ar results are summarized in Table 1, with details listed in
Supplementary Tables 2–4.
The five pumice clasts measured from Member I yielded threedifferent ages (Table 1), with at least the oldest age interpreted as
evidence for reworking of that particular pumice clast. Nine
analyses on 99-273A gave a mean age of 320 ^ 19 kyr. Seven
analyses on 99-273B yielded a mean age of 221 ^ 13 kyr after
rejecting one outlier (269 ^ 18 kyr). Single feldspar crystals from02-01B yielded a mean age of 195 ^ 11 kyr (n ¼ 14), forming aconcordant data set after one rejection (333 ^ 6 kyr). Multiple
crystals (n # 3) were analysed in 12 of 16 measurements for
02-01A, and in 8 of 14 analyses for 02-01C (n # 4). No ages were
rejected on 02-01A, whose mean age is 204 ^ 15 kyr, or on 02-01C,
whose mean age is 196 ^ 15 kyr. It is probable that these latter threepumice clasts are products of the same volcanic eruption, as shownby their concordant ages and similar mean K/Ca ratios. When
combined, all 44 analyses provide an arithmetic mean age of
198 ^ 14 kyr and a weighted mean age of 195.8 ^ 1.6 kyr. The
mean age calculated for pumice 99-273B is statistically older than
the ages determined on the three pumice clasts 02-01, so that 99273B might also be a reworked pumice clast. Clearly, the age ofdeposition of a tuff must be younger than the youngest igneouscomponent found within it. Thus, this tuffaceous level of Member Iof the Kibish Formation was deposited after 196 kyr ago, on
the basis of the mean age determined on the three 02-01 pumice
clasts.
Feldspars from four pumice clasts from the Member III tuff wereanalysed. Two were in situ (99-275A, C), and two (99-274A, B) had
weathered out of the same unit about 150 m farther east (Fig. 1).
Ages on 99-275A range from 98.0 ^ 7.7 kyr to 114.6 ^ 4.3 kyr. Five
single-crystal measurements have an arithmetic mean age of
105.5 ^ 7.0 kyr, identical to five measurements on groups of twoor three crystals (mean age 105.4 ^ 2.7 kyr), indicating a homo-

geneous population. The overall mean age is 105.4 ^ 5.0 kyr, with aweighted mean age of 106.0 ^ 1.6 kyr. Eleven analyses on sample
99-275C yield a mean age of 107.5 ^ 7.1 kyr and a weighted mean of
105.4 ^ 1.8 kyr, after rejection of two outliers (142.1 ^ 3.3 and128.8 ^ 8.3 kyr). Concordant results from 99-274A on seven single
crystals and six pairs of crystals gave an overall arithmetic mean ageof 98.1 ^ 5.3 kyr. The companion clast 99-274B gave a mean age of
105.0 ^ 8.3 kyr, after elimination of one outlier (Supplementary
Table 4). Again these results reflect a single age population, as shownby the similar mean ages of the individual pumices, and theoverlapping average K/Ca ratios. Combining all these results (except
outliers), the overall arithmetic mean age is 103.7 ^ 7.4 kyr(n ¼ 47) and the weighted mean age is 103.7 ^ 0.9 kyr. Thus, thedepositional age must be equal to or younger than 104 kyr, providing evidence that Members I and II of the Kibish Formation are
older than 104 kyr.
Each of the members of the Kibish Formation was deposited
during intervals when Lake Turkana was at a much higher level thanat present, and Member II has an internal disconformity. The upper
part of Member I was being deposited at or after 196 kyr ago, and theupper part of Member III was being deposited at or after 104 kyrago; 14C ages on Member IV correspond to deposition between 9.5
and 3.3 kyr ago. These ages are remarkably similar to ages ofMediterranean sapropels S7, S4 and S1. Sapropels S1–S7 have thefollowing estimated ages: 195 kyr (S7), 172 kyr (S6), 124 kyr (S5),102 kyr (S4), 81 kyr (S3), 55 kyr (S2) and 8 kyr (S1)17. In many cases
sapropels are related to a greatly increased flow of the Nile River into
the Mediterranean Sea as a consequence of intensification of theAfrican monsoon18, recorded in more negative d18O in planktonic
foraminifera. As the Omo River shares a divide with the Blue Nileand with tributaries of the White Nile, the Nile and the Omo must
be affected similarly. As noted, deposition of each of the members of
the Kibish Formation was probably very rapid. Thus, the close
correspondence between the ages of Member I (196 kyr) and ofsapropel S7 (195 kyr), of Member III (104 kyr) and of sapropel S4(102 kyr), and of Member IV (3.3–9.5 kyr) and of sapropel S1(8 kyr) is probably causally related. Sapropel S2 (55 kyr) is absent
from or poorly represented in many Mediterranean sedimentarycores and has a very small d18O residual; thus, it is not surprisingthat no deposits of this age have been identified in the Kibish
Formation. Sapropel S6, deposited during a European glacial
period19, might also be absent from the Kibish Formation, because
it too has a small d18O anomaly. The two parts of Member II may be
accommodated by sapropels S5 and S6, or they may correspond to
the two phases identified in sapropel S5 (119–124 kyr), which areseparated by 700–900 yr (ref. 20). This link between sapropel
formation in the Mediterranean and very high levels of Lake
Turkana is a particularly notable finding. In contrast, S3 is very
well represented in many Mediterranean sedimentary cores and isTable 1 Summary of 40Ar/39Ar alkali feldspar laser fusion ages from pumice clasts in tuffs of the Kibish area, Turkana Basin, Ethiopia
Sample no.

Tuff

Locality

Irradiationnn usedSimple meanage (kyr)

Weighted meanage (kyr)

Isochronage (kyr)

MSWD(40Ar/36Ar)i...................................................................................................................................................................................................................................................................................................................................................................

Kibish Formation, Member III
99-275A
Member III
99-275C
Member III
99-274A
Member III
99-274B
Member III

0.4 km SSE of KHS
0.4 km SSE of KHS
0.5 km SE of KHS
0.5 km SE of KHS

ANU58/L10
ANU58/L1
ANU58/L6
ANU58/L7

10
13
13
14

10
11
13
13

105.4 ^ 5.0
107.5 ^ 7.1
98.1 ^ 5.3
105.0 ^ 8.3

106.0 ^ 1.6
105.4 ^ 1.8
98.2 ^ 1.1
109.6 ^ 1.3

108.0 ^ 3.1
101.1 ^ 8.4
97.4 ^ 2.2
110.2 ^ 4.4

1.03
2.04
2.47
3.27

292.2 ^ 4.1
303.6 ^ 13.8
298.9 ^ 6.8
292.9 ^ 16.2

Kibish Formation, Member I, near Omo II site, just west of Omo River, Nakaa’kire, 58 24.6 0 N, 358 54.5 0 E
99-273A
Member I
Nakaa’kire
ANU58/L399319.8 ^ 18.6
99-273B
Member I
Nakaa’kire
ANU58/L48
7
220.8 ^ 13.3
02-01A
Member I
Nakaa’kire
ANU98/L916
16204.2 ^ 14.9
02-01B
Member I
Nakaa’kire
ANU98/L10
15
14
194.7 ^ 10.8
02-01C
Member I
Nakaa’kire
ANU98/L3
14
14
195.6 ^ 15.0

315.3 ^ 3.6
210.9 ^ 2.1
205.3 ^ 2.2
191.5 ^ 1.9
192.7 ^ 2.3

318.2 ^ 16.6
209.6 ^ 2.0
194.1 ^ 5.4
184.9 ^ 3.8
192.5 ^ 6.7

3.55
0.55
2.66
2.13
3.85

287.3 ^ 41.5
301.4 ^ 3.1
315.8 ^ 7.3
304.0 ^ 3.5
295.7 ^ 5.8...................................................................................................................................................................................................................................................................................................................................................................
40
K decay constant l ¼ 5.543 £ 10210 yr21. Fluence monitor: Fish Canyon Tuff sanidine 92-176 of reference age 28.1 Myr. Samples 99-275A and 99-275C (laboratory sample numbers
at ANU) ¼ KIB99-47 (field sample number); 99-274A and 99-274B ¼ KIB99-41; 99-273A and 99-273B ¼ KIB99-19. Results with errors are means ^ s.d. MSWD, mean square of weighted
deviates.

NATURE | VOL 433 | 17 FEBRUARY 2005 | www.nature.com/nature© 2005 Nature Publishing Group

735

letters to nature
therefore expected to be recorded in the Kibish Formation, but has
not been recognized. Given the large expanse of the plain in theregion underlain by the Kibish Formation, it is quite possible that
deposits correlative with sapropel S3 are present but are not exposedin the immediate Kibish region that we have studied.
Our palaeontological and stratigraphic studies support theoriginal report14 that Omo I and Omo II are derived from comparable stratigraphic levels within Member I of the Kibish Formation
despite their morphological differences1–3,21,22. Morphologicaldiversity among fossil hominids from the Middle and Late Pleistocene of Africa is of major importance in understanding the tempo
and mode of modern human origins21,23.
40
Ar/39Ar dating of feldspars from tuffs in Member I and Member
III of the Kibish Formation shows that its hominid fossils areyounger than 195.8 ^ 1.6 kyr and older than 103.7 ^ 0.9 kyr.
Direct isotopic dating of volcanic eruptions recorded in the Kibish
Formation does not enable us to place narrower limits on the age.
However, the suggested correlations of Member IIa and Member IIb
with either the two identified phases of sapropel S5 or sapropels S6and S5, respectively, indicate that deposition of Member I of theKibish Formation occurred earlier than about 125 kyr ago or earlier
than 172 kyr ago. The geological evidence for rapid deposition ofMember I and the remarkably close correspondence of the isotopic
ages on the youngest pumice clasts in the tuff of Member I at 196 kyrwith the estimated age of sapropel S7 is regarded as strongly
supporting the view that Member I was deposited close to
196 ^ 2 kyr ago. On this basis we suggest that hominid fossils
Omo I and Omo II are relatively securely dated to 195 ^ 5 kyr old,somewhat older than the age of between 154 and 160 kyr assigned
to the hominid fossils from Herto, Ethiopia24, making Omo I andOmo II the oldest anatomically modern human fossils yet
recovered.
A

Methods
Age measurements
Alkali feldspar crystals were separated from pumice clasts and cleaned ultrasonically in 7%
HF for 5–10 min to remove adhering volcanic glass and surface alteration. Althoughseparations were performed on the coarsest crystals present, in several cases crystals wereless than 1 mm, with masses less than 0.8 mg. In such cases several crystals were used foreach analysis (see Supplementary Tables 2–4).
Samples were irradiated in facilities X33 or X34 of the High Flux Australian Reactor(Lucas Heights, Sydney, Australia) for 6 h as described in ref. 25. Cadmium shielding
0.2 mm thick was used to reduce the (40Ar/39Ar)K correction factor, which was measured
by analysis of zero-aged synthetic potassium silicate glass co-irradiated with the
unknowns. This correction is particularly important because of the young age of thesamples, so interpolated values for each sample are given in the Supplementary Tables 2–4.
The fluence monitor employed was sanidine 92-176, separated from the Fish Canyon Tuff,with a reference age of 28.1 Myr (ref. 26).
After irradiation, feldspar crystals were loaded into wells in a copper sample tray,
installed in the vacuum system and baked overnight. Samples were fused with a focusedargon-ion laser beam with up to 10 W of power. After purification of the gases released
during fusion, the argon was analysed isotopically in a VG3600 mass spectrometer, using a
Daly collector. The overall sensitivity of the system was about 2.5 £ 10217 mol mV21. Massdiscrimination was monitored through regular measurements of atmospheric argon. The
irradiation parameter, J, for each unknown was derived by interpolation from themeasurements made on the fluence monitor crystals, with at least five analyses per level;
the precision generally was in the range 0.3–0.75%, standard deviation of the population.
Calcium correction factors27 used in all calculations were (36Ar/37Ar)Ca ¼ 3.49 £ 1024
and (39Ar/37Ar)Ca ¼ 7.86 £ 1024.

Data handlingIn calculating the arithmetic mean age for each pumice clast, any result more than two
standard deviations from the initial mean of each sample was rejected iteratively until nofurther outliers were identified. No more than two measurements were rejected in anygroup of analyses, and no results were rejected for about half the groups. The error quoted
in Table 1 is the standard deviation of the population, but because uncertainties on
individual ages are variable, a weighted mean age and error is also given, weighting eachage by the inverse of the variance. Differences in these mean ages are small (Table 1), but
the error of the weighted mean age is usually much lower. In addition, data from eachpumice clast, after exclusion of outliers, were plotted in an isotope correlation diagram(36Ar/40Ar versus 39Ar/40Ar), using the York28 procedure. The derived ages are quite close
to the mean ages (Table 1), and the calculated trapped argon composition generally has theatmospheric argon ratio of 295.5, within uncertainty.

736

Received 22 September; accepted 8 December 2004; doi:10.1038/nature03258.
1. Day, M. H. Omo human skeletal remains. Nature 222, 1135–1138 (1969).
2. Day, M. H. & Stringer, C. B. in Congrès International de Paléontologie Humaine I, Nice Vol. 2, 814–846(Colloque International du CNRS, 1982).
3. Day, M. H. & Stringer, C. B. Les restes crâniens d’Omo-Kibish et leur classification à l’intérieur dugenre Homo. Anthropologie 95, 573–594 (1991).
4. Day, M. H., Twist, M. H. C. & Ward, S. Les vestiges post-crâniens d’Omo I (Kibish). Anthropologie 95,
595–610 (1991).
5. Howell, F. C. in Evolution of African Mammals (eds Maglio, V. J. & Cooke, H. B. S.) 154–248 (Harvard
Univ. Press, Cambridge, Massachusetts, 1978).
6. Smith, F. H., Falsetti, A. B. & Donnelly, S. M. Modern human origins. Yb Phys. Anthropol. 32, 35–68(1989).
7. Butzer, K. W., Isaac, G. Ll., Richardson, J. L. & Washbourn-Kamau, C. Radiocarbon dating of East
African lake levels. Science 175, 1069–1076 (1972).
8. Fuchs, V. E. The geological history of the Lake Rudolf Basin, Kenya Colony. Phil. Trans. R. Soc. Lond. B
229, 219–274 (1939).
9. Arambourg, C. Mission Scientifique de l’Omo 1932–1933. Geologie–Anthropologie–Paleontologie(Muséum National d’Histoire Naturelle, Paris, 1935–1947).
10. Butzer, K. W. The Lower Omo Basin: Geology, fauna and hominids of Plio-Pleistocene formations.
Naturwissenschaften 58, 7–16 (1971).
11. Butzer, K. W. in Earliest Man and Environments in the Lake Rudolf Basin (eds Coppens, Y., Howell,
F. C., Isaac, G. Ll. & Leakey, R. E. F.) 12–23 (Univ. of Chicago Press, Chicago, 1976).
12. Butzer, K. W. & Thurber, D. L. Some late Cenozoic sedimentary formations of the Lower Omo Basin.
Nature 222, 1138–1143 (1969).
13. Butzer, K. W., Brown, F. H. & Thurber, D. L. Horizontal sediments of the lower Omo Valley: the Kibish
Formation. Quaternaria 11, 15–29 (1969).
14. Butzer, K. W. Geological interpretation of two Pleistocene hominid sites in the Lower Omo Basin.
Nature 222, 1133–1135 (1969).
15. Owen, R. B., Barthelme, J. W., Renaut, R. W. & Vincens, A. Palaeolimnology and archaeology of
Holocene deposits north-east of Lake Turkana, Kenya. Nature 298, 523–529 (1982).
16. Fleagle, J. et al. The Omo I partial skeleton from the Kibish Formation. Am. J. Phys. Anthropol. Suppl.
36, 95 (2003).
17. Lourens, L. J. et al. Evaluation of the Plio-Pleistocene astronomical timescale. Paleoceanography 11,
391–413 (1996).
18. Rossignol-Strick, M., Nesteroff, W., Olive, P. & Vergnaud-Grazzini, C. After the deluge:
Mediterranean stagnation and sapropel formation. Nature 295, 105–110 (1982).
19. Rossignol-Strick, M. & Paterne, M. A synthetic pollen record of the eastern Mediterrranean sapropelsof the last 1 Ma: implications for the time-scale and formation of sapropels. Mar. Geol. 153, 221–237(1999).
20. Rohling, E. J. et al. African monsoon variability during the previous interglacial maximum. Earth
Planet. Sci. Lett. 202, 61–75 (2002).
21. Haile-Selassie, Y., Asfaw, B. & White, T. D. Hominid cranial remains from Upper Pleistocene deposits
at Aduma, Middle Awash, Ethiopia. Am. J. Phys. Anthropol. 123, 1–10 (2004).
22. Rightmire, G. P. in The Origins of Modern Humans: A World Survey of the Fossil Evidence (eds Smith,F. H. & Spencer, F.) 295–325 (Alan R. Liss, New York, 1984).
23. Howell, F. C. in Origins of Anatomically Modern Humans (eds Nitecki, M. H. & Nitecki, D. V.) 253–319(Plenum, New York, 1994).
24. Clark, J. D. et al. Stratigraphic, chronological and behavioural contexts of Pleistocene Homo sapiens
from Middle Awash, Ethiopia. Nature 423, 747–752 (2003).
25. McDougall, I. & Feibel, C. S. Numerical age control for the Miocene–Pliocene succession at
Lothagam, a hominoid-bearing sequence in the northern Kenya Rift. J. Geol. Soc. Lond. 156, 731–745(1999).
26. Spell, T. L. & McDougall, I. Characterization and calibration of 40Ar/39Ar dating standards. Chem.
Geol. 198, 189–211 (2003).
27. Spell, T. L., McDougall, I. & Doulgeris, A. P. The Cerro Toledo Rhyolite, Jemez Volcanic Field, New
Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events. Bull. Geol. Soc.
Am. 108, 1549–1566 (1996).
28. York, D. Least squares fitting of a straight line with correlated errors. Earth Planet. Sci. Lett. 5, 320–324(1969).
29. Davidson, A. The Omo River Project (Bulletin 2, Ethiopian Institute of Geological Surveys, Addis
Ababa, 1983).

Supplementary Information accompanies the paper on www.nature.com/nature.
Acknowledgements We thank J. Mya, R. Maier and X. Zhang for technical support for thegeochronology; participants in the Kibish expeditions between 1999 and 2003, including
Z. Assefa, J. Shea, S. Yirga, J. Trapani and especially C. Feibel, B. Passey and C. Fuller for theirgeological contributions; and R. Leakey, K. Butzer and especially P. Abell for providing us withinformation and documents about the 1967 expedition to the Kibish area. We thank theGovernment of Ethiopia, the Ministry of Youth, Sports and Culture, the Authority for
Research and Conservation of Cultural Heritage, and the National Museum of Ethiopia for
permission to study the Kibish Formation. Support was provided by the National Science
Foundation, the Leakey Foundation, the National Geographic Society and the Australian
National University. Neutron irradiations were facilitated by the Australian Institute of
Nuclear Science and Engineering and the Australian Nuclear Science and Technology
Organization.
Competing interests statement The authors declare that they have no competing financial
interests.
Correspondence and requests for materials should be addressed to I.McD(ian.mcdougall@anu.edu.au).

© 2005 Nature Publishing Group

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