Abstract

Until 2000, several scientists considered Ardipithecus ramidus from Ethiopia, at 4,5 Ma old (million years) to be a very early ancestor of later hominids, australopithecines and hominines. Based on molecular clocks, the dichotomy between apes and humans was supposed to be situated around 6 Ma, and this discovery was viewed as supporting the molecular data. But, at the time, the hominoid fossil record between 9,5 and 4,5 Ma was very poor (only a few fragments of bones, maxillae and mandibles were known – all from Kenya) and new material was clearly needed. In 2000, field work by the Kenya Palaeontology Expedition (a Franco-Kenyan cooperative project) led to the discovery of Orrorin tugenensis in the Tugen Hills (Kenya), which dates to between 6,0 and 5,7 Ma. The species was originally represented by eleven specimens: a mandible in two pieces, several isolated teeth and postcranial bones (three fragmentary femora, a distal humerus and a proximal manual phalanx). Since 2000, the number of specimens has doubled, including more teeth and postcranials. The teeth show a complex mixture of primitive ape-like features (such as the presence of a low distal shoulder on the upper canine crown, a mesiodistally elongated upper canine) and derived hominid features (the molars – small, squarish with thick enamel and with almost vertical lingual walls – recall those of hominids, a lower canine with a distal tubercle and a mesial marginal ridge, and the absence of a C-P3 diastema). The femoral features show clear evidence of adaptation to bipedalism (presence of an obturator externus groove, an elongated femoral neck, an anteriorly twisted head, and the pattern of the cortical distribution in the femoral neck among others). However, Orrorin exhibits some differences from the australopithecines in the morphology of the femoral neck, the position of the lesser trochanter and projection of the femoral head; but also shares some features with them (asymmetrical cortical distribution in the neck, length of the neck, presence of the m. obturator externus groove, among others). The humerus and phalanx imply arboreality in Orrorin. Dentally, Orrorin appears to be a very early hominid, a conclusion reinforced by the adaptation to bipedalism.