Anthropologists took the wraps off the oldest known human ancestor on Thursday - a 4.4-million-year-old Ethiopian skeleton named Ardi, which challenges many long-held assumptions about how humans and apes evolved.
"It's not a chimp. It's not a human. It shows us what we used to be," said paleoanthropologist Tim White of the University of California-Berkeley, co-director of the research group that discovered and analysed more than 110 specimens of the 4.4-million-year-old species Ardipithecus ramidus.White was speaking at a Washington press conference a day before a series of 47 articles on the find are published in the American journal Science.
Ardi is the most complete skeleton among the specimens and is more than a million years older than the famous Lucy skeleton uncovered in the 1970s. She was found in 1992 in Ethiopia's harsh Afar desert at a site called Aramis, just 74 km from where Lucy's species, Australopithecus afarensis, was found in 1974.
An initial paper was published in 1994, but the fossils were so fragile it took a further 15 years to reconstruct Ardi's skeleton and analyse it.
The findings provide scientists with information on what a common ancestor for humans and apes may have been like, with a mix of traits from earlier species and later species.
But Ardi and her species were less like modern apes than scientists expected - indicating that apes likely evolved extensively after scientists say the apes and humans diverged. This challenges the long-standing scientific belief that apes give a good look at what an early ancestor of humans may have looked like.
Small-brained, 120 cm tall and weighing about 50 kg, Ardipithecus ramidus lived in a wooded environment, climbing on all fours in the trees and walking on two feet on the ground, not walking on their knuckles like gorillas or swinging from the trees like chimps.
"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," White said.
"Darwin said we have to be really careful. The only way we're really going to know what this last common ancestor looked like is to go and find it.
"Well, at 4.4 million years ago we found something pretty close to it. And, just like Darwin appreciated, evolution of the ape lineages and the human lineage has been going on independently since the time those lines split, since that last common ancestor we shared," White said.