The palaeobiological record of 12 million to 7 million years ago (Ma) is crucial to the elucidation of African ape and human origins, but few fossil assemblages of this period have been reported from sub-Saharan Africa. Since the 1970s, the Chorora Formation, Ethiopia, has been widely considered to contain ~10.5 million year (Myr) old mammalian fossils1,2,3,4,5,6,7. More recently, Chororapithecus abyssinicus, a probable primitive member of the gorilla clade6, was discovered from the formation. Here we report new field observations and geochemical, magnetostratigraphic and radioisotopic results that securely place the Chorora Formation sediments to between ~9 and ~7 Ma. The C. abyssinicus fossils are ~8.0 Myr old, forming a revised age constraint of the human–gorilla split. Other Chorora fossils range in age from ~8.5 to 7 Ma and comprise the first sub-Saharan mammalian assemblage that spans this period. These fossils suggest indigenous African evolution of multiple mammalian lineages/groups between 10 and 7 Ma, including a possible ancestral-descendent relationship between the ~9.8 Myr old Nakalipithecus nakayamai8 and C. abyssinicus. The new chronology and fossils suggest that faunal provinciality between eastern Africa and Eurasia had intensified by ~9 Ma, with decreased faunal interchange thereafter9,10,11,12. The Chorora evidence supports the hypothesis of in situ African evolution of the Gorilla–Pan–human clade, and is concordant with the deeper divergence estimates of humans and great apes based on lower mutation rates of ~0.5 × 10−9 per site per year (refs 13, 14, 15).
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We thank the Authority for Research and Conservation of Cultural Heritage, Ministry of Culture and Tourism of Ethiopia, for permissions and facilitation; we thank the Western Hararge Chiro Zone Culture and Tourism Office and the Mieso Woreda for fieldwork support; we thank all participants in the fieldwork, especially the Gololcha people, who were essential to the success of the project. Neutron irradiation for 40Ar–39Ar dating was done under the Visiting Researchers Program at the Kyoto University Research Reactor Institute. We thank J. Morton for assistance with major and trace element analyses, and the Janet and Elliott Banes Professorship and National Science Foundation EAR-1028789 for support to W.H. We thank H. Ishiguro for assistance with sampling enamel for the isotope analysis, and M. Nakatsukasa and Y. Kunimatsu for comparative materials. We thank D. Geraads for providing field information about previous geochronological sampling localities. This project was supported primarily by the Japan Society for the Promotion of Science (Kakenhi grant numbers 21255005 and 24000015).
The authors declare no competing financial interests.
The Chorora Formation vertebrate fossils have been deposited in the Paleontology and Paleoanthropology laboratories of the Authority for Research and Conservation of Cultural Heritage, Ministry of Culture and Tourism, Addis Ababa, Ethiopia.
Extended data figures and tables
C. abyssinicus is known from only nine teeth (or fragments)6 and N. nakayamai from a mandibular corpus with M1–M3, and ten other isolated teeth (excluding a possible antimere)8. The only informative dental elements shared by the two reported samples are the lower M3 and the lower M1 (the latter damaged in C. abyssinicus and considerably worn in N. nakayamai). This fragmentary evidence makes cladistic evaluations difficult. C. abyssinicus was considered to share the following derived combination of features with the modern gorilla: large postcanine size, upper molars buccolingually narrow and mesiodistally elongate, with a relatively long and mesiobuccally extending mesial protocone crest, reduced protoconule, and lower molars with a prominent anterior transverse crest. Following ref. 6, we consider Chororapithecus to be a basal member of the gorilla clade. When compared with the Middle Miocene examples of Kenyapthecini and Equatorini, C. abyssinicus and N. nakayamai share reduced cingula in both upper and lower molars, although ref. 8 pointed out that the lower M3 cingulum appears slightly better developed in N. nakayamai. The available N. nakayamai teeth include elements (not reported in C. abyssinicus) that are morphologically more derived than in Kenyapthecini/Equatorini, such as upper premolars that are relatively elongate mesiodistally (narrow buccolingually) and a lower P3 that is not as obliquely elongate and transversly compressed as in the Middle Miocene forms. On the basis of these observations, ref. 8 preferred a stem modern African ape–human cladistic position of N. nakayamai (option 1). However, N. nakayamai also shares two possibly derived features with C. abyssinicus: large postcanine size (although slightly less so) and prominent transverse crest in the lower molars. The latter is inferred from the characteristic wear pattern of the holotype mandible lower M1, the dentine exposures of the lingual cusps that are buccolingually linear (figure 1 of ref. 8). These observations suggest that N. nakayamai may also be a stem member of the gorilla clade but more primitive than C. abyssinicus (option 2).
a, View north-northwest from the southeastern margin of the type locality. Yellow and red arrows show the rhyolitic ignimbrite units considered by previous workers to overlie the Chorora Formation sediments1,2,3. The ignimbrite unit (yellow arrows) adjacent to the sediment exposures has been interpreted to directly overlie the Chorora Formation sediments including the gravel unit that forms the resistant ledge along the western margin of the whitish lacustrine exposures. A second ridge top unit (red arrows) was considered to cap the entire sequence. RhyUn, RhyOv1 and RhyOv2 indicate sampling localities of the present study for K–Ar dating. The RhyUn unit dips westwards and stratigraphically underlies the Chorora Formation sediments. The horizontally bedded light brown sediments are Middle Pleistocene in age and unconformably cap both RhyUn and Chorora Formation sediments. The yellow and red dotted lines indicate approximate positions of the step faults that resulted in uplifted RhyOv1 and RhyOv2 exposures. b, View northeast at the RhyOv1 sampling spot. Yellow arrows point to the RhyOv1 rhyolitic ignimbrite unit forming a planar terrace. c, View west of the b RhyOv1 section (yellow arrows). Note that the rhyolitic ignimbrite unit terminates abruptly (dotted line) in fault contact with sediments previously considered to underlie the ignimbrite2,3.
Results of individual crystals with stable ages (indicated by arrows) are shown. Errors are 1σ. Samples 10TC-1 and 10TC-58 are from the same consolidated tuff unit collected <5 m apart. Samples 10TC-52 and 10TC-53 are from the same pumiceous tuff collected ~30 m apart. The weighted means of these are shown in Fig. 3 and Extended Data Table 1.
Extended Data Figure 5 Chorora Formation stratigraphic columns of the type locality, Beticha and nearby localities.
The sections were taken at exposures adjacent to or continuous with the sampling locations of the dated volcanic samples (coordinates tabulated in Extended Data Table 1). Radioisotopic dates (K–Ar ages) and samples correspond to those shown in Fig. 3 and Extended Data Table 1. Eight tephra units are considered to occur at multiple localities, and shown as marker tuffs CT-1 to CT-8. The analytical details of the tuffs are presented in Supplementary Tables 1 and 2. CT-6 is identified at Adadi from lithologic, petrographic, glass morphology and refractive index analyses. Results of the remanent magnetism analysis are given in Supplementary Table 4. See Extended Data Fig. 6 for explanation of the lithological codes.
The sections were taken at exposures adjacent to or continuous with the sampling locations of the dated volcanic samples (coordinates tabulated in Extended Data Table 1). Radioisotopic dates (K–Ar ages) and samples correspond to those shown in Fig. 3 and Extended Data Table 1. Tephra unit CT-4 occurs at multiple localities. The analytical details of the tuffs are presented in Supplementary Tables 1 and 2. Analytical results of the remanent magnetism analysis are given in Supplementary Table 4.
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Katoh, S., Beyene, Y., Itaya, T. et al. New geological and palaeontological age constraint for the gorilla–human lineage split. Nature 530, 215–218 (2016). https://doi.org/10.1038/nature16510
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