The Miocene Hominoids and the Earliest Putative Hominids

Abstract

For many years molecular studies suggested that the hominid family emerged during the Pliocene. But today we have good evidence of hominids in African Upper Miocene strata. Reconstructing our earliest history is a difficult task as the Miocene data is scanty and fragmentary. Furthermore, the tendency for anthropologists to consider the modern chimpanzee as a good model for the last common ancestor of African apes and hominids has obscured our understanding of hominid evolution because the supposed distinctive apelike features are defined on the basis of a modern animal and not on those of Miocene hominoids. Taking into account detailed studies of Miocene apes and modern hominoids, it appears that bipedalism is probably the most reliable feature for defining hominids. Of the new hominoid taxa discovered in the Upper Miocene, only Orrorin tugenensis exhibits clear evidence of adaptation to bipedalism. Bipedalism in Sahelanthropus tchadensis and Ardipithecus kadabba still needs to be demonstrated. A long-lasting idea in hominoid evolution was that hominids emerged in dry, savannah-like environments; but the data obtained from the Upper Miocene levels in Baringo (Kenya) demonstrate that the environment of the earliest hominids was more forested and humid than expected. Finally, recent discoveries of modern-looking apes made in 12.5 Ma strata at Ngorora (Kenya), 10.5 Ma strata at Nakali (Kenya), 10 Ma strata at Chorora (Ethiopia), 11–5.5 Ma strata in Niger, and 6 Ma deposits at Kapsomin and Cheboit indicate that the dichotomy between African apes and humans could be much older than generally thought. Alternatively, one or more of these hominoids could represent early stages in the lineages of modern African apes.