Ardipithecus is a very early hominin genus (subfamily Homininae). Because it shares several traits with the African great apes (genus Pan and genus Gorilla), it is considered by some to be on the chimpanzee rather than human branch, but most consider it a proto-human because of a likeness in teeth with Australopithecus. A. ramidus lived about 5.4 and 4.2 million years ago during the early Pliocene.

Two species have been described, Ardipithecus ramidus and Ardipithecus kadabba. The latter was initially described as a subspecies of A. ramidus, but on the basis of teeth recently discovered in Ethiopia has been raised to species rank. A. kadabba is dated to have lived between 5.8 million to 5.2 million years ago. The canine teeth show primitive features that distinguish them from those of more recent hominines. A. kadabba is believed to be the earliest organism yet identified that lies in the human line following its split from the lineage that gave rise to the two modern chimpanzee species. Remains from both species have been found in Middle Awash.

On the basis of bone sizes, Ardipithecus species are believed to have been about the size of a modern chimpanzee. The toe structure of A. ramidus suggests that the creature walked upright, and this poses problems for current theories of the origins of hominid bipedalism: Ardipithecus is believed to have lived in shady forests rather than on the savannah, where the faster running permitted by bipedalism would have been an advantage.

The forest lifestyle poses problems for the current theories regarding the development of bipedalism, most of which focus on the savanna. New thought will be necessary in order to reconcile these savanna theories with the current knowledge of early forest-dwelling hominids.

Further reading

• BBC News: Amazing hominid haul in Ethiopia
• New York Times article Two Splits Between Human and Chimp Lines Suggested published May 18, 2006. "The analysis, by David Reich, Nick Patterson and colleagues at the Broad Institute in Cambridge, Mass., sets up a serious conflict between the date of the split as indicated by fossil skulls, about 7 million years ago, and the much younger date implied by genetic analysis, as late as 5.4 million years ago. The conflict can be resolved, Dr. Reich's team suggests in an article published in today's Nature, if there were in fact two splits between the human and chimp lineages, with the first being followed by interbreeding between the two populations and then a second split."

early hominids | Pliocene

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