Ardipithecus ramidus

A new analysis for an ancient hominid

Ardipithecus ramidus: Something like an ancestor of chimps and humans?

Location of Ethiopia Credit: Rei-Artur

The package of articles offers the first comprehensive description of the Ardipithecus fossils, which include the famous partial skeleton, nicknamed "Ardi," originally discovered in 1992-1993 by an expedition led by Tim White of the University of California, Berkeley.

The special issue includes an overview article, three articles describing the habitat of A. ramidus, five that analyze specific parts of its anatomy, and two that discuss what this new body of scientific information may imply for human evolution.

"These articles contain an enormous amount of data collected and analyzed through a major international research effort. They throw open a window into a period of human evolution we have known little about, when early hominids were establishing themselves in Africa, soon after diverging from the last ancestor they shared with the African apes," said Brooks Hanson, one of Science's deputy editors.

In the overview article, White and his coauthors introduce their discovery of over 110 bones and bone fragments including a partial skeleton that includes portions of the skull, hands, feet, limbs and pelvis. This individual, "Ardi," was a female who weighed about 50 kilograms and stood about 120 centimeters tall.

Many biologists believe a common ancestor of humans and chimpanzees existed six or more million years ago. In this package of articles no claim is made that Ardipithecus, which is known only from 4.4 mya, is in fact this ancestor, but the assertion is made that ardipithecines had many traits that would be like those of the common ancestor if it were ever discovered.

Ardipithecus is more than a million years older than the partial skeleton of Australopithecus afarensis, known as Lucy. But it is not as old as such forms as Sahelanthropus tchadensis and Orrorin tugenensis (view human evolution timeline).

After analyzing available skeletal material, the researchers concluded that Ardipithecus shares certain traits with its predecessors, the primates of the Miocene epoch, and others exclusively with later hominids.

Because of its antiquity, they argue that Ardipithecus is closer to a common ancestor of humans and apes than any other known hominid. Many of its characteristics, however, are not found in modern-day African apes. Their conclusion, then, is that not only humans, but also the African apes have evolved extensively during the last 6-7 million years. In the past, many scientists have pictured a common ancestor of apes and humans that was more like a chimpanzee than a human being (e.g., one that knuckle-walked or was suited for swinging and hanging from tree branches).

However, these ardipithecines seem to have lived in a woodland environment, where they climbed on all fours along tree branches -- as some of the Miocene primates did -- and walked, upright, on two legs, while on the ground. They do not appear to have been knuckle-walkers, like gorillas and chimpanzees, or to have spent much time swinging and hanging from tree branches, as chimps do.

"In Ardipithecus we have an unspecialized form that hasn't evolved very far in the direction of Australopithecus. 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," says White.

"The only way we're really going to know what this last common ancestor looked like is to go and find it," White says, "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."