Late Miocene hominids from the Middle Awash, Ethiopia

Molecular studies suggest that the lineages leading to humans and chimpanze es diverged approximately 6.5-5.5 million years (Myr) ago, in the Late Mioc ene(1-3). Hominid fossils from this interval, however, are fragmentary and of uncertain phylogenetic status, age, or both(4-6). Here I report new homi nid specimens from the Middle Awash area of Ethiopia that date to 5.2-5.8 M yr and are associated with a wooded palaeoenvironment(7). These Late Miocen e fossils are assigned to the hominid genus Ardipithecus and represent the earliest definitive evidence of the hominid clade. Derived dental character s are shared exclusively with all younger hominids. This indicates that the fossils probably represent a hominid taxon that postdated the divergence o f lineages leading to modern chimpanzees and humans. However, the persisten ce of primitive dental and postcranial characters in these new fossils indi cates that Ardipithecus was phylogenetically close to the common ancestor o f chimpanzees and humans. These new findings raise additional questions abo ut the claimed hominid status of Orrorin tugenensis(8), recently described from Kenya and dated to similar to6 Myr(9).