Event 32

6 to 4 million years ago

This is an illustration of the skeleton of Ardipithecus ramidus, an early hominid species that lived around 4.4mya. It is one of the earlier genera to display bipedal traits.
Credit: Illustration by Ori~, courtesy of Wikipedia.

One of the major features that separates humans from the apes is our mode of locomotion (getting around). On the ground, the modern African apes, all members of the taxonomic tribe Paninae walk on all fours (“knuckle walking”), while humans are of the tribe Homininae (“hominin”), and have evolved to walk upright on two legs. This way of getting around is called bipedality. This fundamental shift and icon of humanity can be seen in the fossil record starting around six million years ago. Two fossil localities in Africa have yielded specimens that appear to show hominin features.

At the site of Toros-Menalla in the Sahara of Chad, a skull assigned to the genus Sahelanthropus and estimated to be about 6 million years old shows dental features present in the human lineage and has a foramen magnum (the hole in the base of the skull where the spinal cord comes through to the brain) situated well under the skull suggesting upright posture and possible bipedality. From the site of Lukeino in the Baringo Basin of Kenya, fossils also dating to six million years ago include a proximal femur (upper thigh bone) that shows the morphology of a human or proto-human upright walker rather than a knuckle walking ape. Between 5.5 and 4.5 million years ago, hominin fossils of the genus Ardipithicus are known from the Ethiopian Rift Valley, including a partial skeleton. These primitive, presumably bipedal hominins represent a major departure from the 20-million year old ape condition, and appear to have lived in apparently closed woodland conditions, possibly spending an appreciable amount of time in the trees.


Ardipithecus skull
Reconstructed cranium of Ardipithecus.
Credit: Image by T. Michael Keesey, courtesy of Wikimedia.


Fossils from Chad and Kenya dating to approximately six million years ago show derived features shared only by the human lineage, including certain dental features and limb morphology consistent with upright walking. These tantalizing, fragmentary remains hint at a major threshold in human evolution. Bipedality freed the hands for much more manipulation of the environment, which would become a hallmark of the human condition.


These fossils very likely represent the early evolutionary experiments in upright walking by our ancestors. Every time you walk you are carrying on an evolutionary tradition that goes back millions of years. Exactly why our ancestors became bipeds is a hotly-debated subject. Theories include carrying to provision mates and offspring, long-distance travel, feeding from higher vegetation, and more intensive tool-use (of which there is no archaeological evidence).






The October 2, 2009 issue of Science showcased 11 articles about Ardipithecus ramidus.

A Science website containing a video of an interview with Tim White, Ann Gibbons, and Andrew Hill about the Ardipithecus fossils and associated fauna, flora, and stratigraphic material.

This is an article about Ardipithecus ramidus.

This is an article from American Scientists on bipedalism.



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