Our Ancestor: Not Chimp, Not Human

Analyses of a partial skeleton of a female Ardipithecus ramidus nicknamed Ardi, suggest the early human would have stood at just under 4 feet (1.2 meters) tall with both primitive traits, such as a small brains size similar to living chimpanzees and those shared with later hominids, such as bipedal posture. Credit: © 2009, J.H. Matternes.

A petite female early human climbed along tree branches on all fours but spent time upright on the ground some 4.4 million years ago in her woodland home in what is now Ethiopia.

This ancestral image comes from a partial skeleton of a hominid nicknamed "Ardi," who weighed in at 110 pounds (50 kg) with a height of just under 4 feet (120 cm). Her remains along with others of her species, Ardipithecus ramidus, have come into the spotlight as an international team of scientists report on an extensive examination of 110 specimens in a special issue of the journal Science.

The species brings scientists closer than ever to the last common ancestor we shared with chimpanzees and gorillas, which lived about 6 million or more years ago just before early humans split off from the chimpanzees and bonobos. (Anatomically modern humans, Homo sapiens, didn't appear on the scene until about 200,000 years ago.)

The earliest, early humans identified to date, Australopithecus afarensis and Australopithecus anamensis, lived 3 million to 4 million years ago. (The most famous Australopithecus fossil belongs to the 3.2 million-year-old "Lucy," which was found in 1974 just north of where Ardi would later be discovered.)

With this gap between the chimp-human split and early human fossils, scientists could only speculate about this common ancestor. And partly because we share much of our genetic code with chimpanzees, bonobos and gorillas, researchers have presumed we evolved from a chimpanzee-like ancestor.

Under this view, the individual would have been adapted to swing and hang from tree branches and possibly get around on its knuckles while on the ground.

But the new research suggests otherwise. Ardipithecus had a mix of primitive traits it shared with the primates of the Miocene epoch (lasting 23.8 million to 5.3 million years ago) and features shared exclusively with later hominids.

"In Ardipithecus we have an unspecialized form that hasn't evolved very far in the direction of Australopithecus," said Tim White, of the University of California, Berkeley, who was one of the lead authors on research published in Science. "So when you go from head to toe, you're seeing a mosaic creature, that is neither chimpanzee, nor is it human. It is Ardipithecus."

The species likely lived in a woodland environment, where it climbed on all fours along tree branches (similar to primitive primates) and walked upright on two legs while on the ground. These hominids were likely not knuckle-walkers and didn't swing from tree branches, study researchers point out.

With these findings, the study scientists conclude that the African apes likely evolved extensively since we shared our last common ancestor. As such, living chimpanzees and gorillas are poor models for the last common ancestor and for understanding our own evolution since that time, the researchers say.

Read more about this discovery at the Washington Post. 

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