Ancient skeleton gives clues to human evolution

02 October 2009

A 4.4 million-year-old fossil skeleton of an ancient human-like creature uncovered in Ethiopia is revealed today in the journal Science.

Named Ardipithecus ramidus, or Ardi, this female specimen reveals important clues about early human evolution because she lived close to the time when the human evolutionary branch separated from the common ancestor we share with chimpanzees. 

The specimen is the most complete hominid skeleton from this time and allows scientists to understand more about the possible nature of this common ancestor.

17 years of research

An international team, led by Ethiopian and American scientists, has spent 17 years studying Ardi and thousands of other fossils at the site in the Afar Rift, Ethiopia, and they have published their research today in a total of 11 scientific papers.

Few fossils

Very few fossils of the earliest human relatives have been discovered up until now. Other than a few fossil fragments, the previous oldest known near-complete skeleton of an early human relative belonged to the 3.2 million-year-old 'Lucy', Australopithecus afarensis, uncovered in 1974, which revolutionised thinking on human origins.

Chris Stringer, human origins expert at Natural History Museum explains, 'The newly-published research on the 4.4 million-year-old Ardipithecus ramidus skeleton is as important for studies of early human evolution as “Lucy’s” were, some 30 years ago'.

'The “Ardi” skeleton shows human-like features in its small canine teeth and short face, but in other respects, she is surprisingly primitive for a fossil only 400,000 years older than the first representatives of Lucy’s group.'

Ardi's appearance

Ardi had a brain and body the size of a chimpanzee. Although she spent most of her time in the trees, the researchers say her hip bone showed she walked upright and not in the chimp-like way using her knuckles. 

However, this upright walking was not the same as modern humans' but was more of an early stage of upright walking. 

Chris adds, 'Ardi's hip bone and legs were not strongly adapted for upright-walking, and her big toes still had the ability to grasp.'

So where does Ardi sit in the human evolutionary tree?

Ardi was not human and was not a chimp. She had some human-like features such as the small teeth and flexible hands and wrists.

However, Ardi did not have some chimp features that many scientists had expected to be found in a creature that lived so soon after the human-chimp common ancestor. This suggests chimpanzees may have evolved some of their chimp features long afterwards.

It is possible that Ardi could have been a direct ancestor of humans. However the more primitive features in her skeleton, such as the opposable big toes, pose a problem. 

Ardi is very different from Australopithecus, the ancient group that may have led to modern humans. And there is also a small length of time, in evolutionary terms, of less than half a million years between the two. 

Stringer explains, 'Such primitive features suggest that either Ardipithecus evolved very rapidly into Australopithecus (Lucy’s group) if she is an ancestor of that group.'

'Or, Ardi is a remnant of a more ancient stage of human evolution, closer in many ways to the ancestor we shared with our closest relatives, the chimpanzees, more than 6 million years ago.'

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