Researchers writing in Science have described Ardipithecus ramidus, a hominid species that lived 4.4 million years ago in what is now Ethiopia.  

The last common ancestor shared by humans and chimpanzees is thought to have lived six or more million years ago and Ardipithecus likely shared many of this ancestor's characteristics. In context, Ardipithecus is more than a million years older than the famous "Lucy" female partial skeleton of Australopithecus afarensis. Until the discovery of the new Ardipithecus remains, the fossil record contained scant evidence of other hominids older than Australopithecus.

Through an analysis of the skull, teeth, pelvis, hands, feet and other bones, the researchers have determined that Ardipithecus had a mix of "primitive" traits, shared with its predecessors, the primates of the Miocene epoch, and "derived" traits, which it shares exclusively with later hominids.

The research in Science offers the first peer-reviewed description of the Ardipithecus fossils, which include a partial skeleton of a female, nicknamed "Ardi."

Ardipithecus ramidus pelvis partial skeleton picture
Original and reconstructed pelvis of the “Ardi” partial skeleton. This image relates to an article in the 02 October 2009 issue of Science. The study, by Dr. C. Owen Lovejoy of Kent State University and colleagues, is titled, "The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking."  Image courtesy of Science/AAAS

Because of its antiquity, Ardipithecus takes us closer to the still-elusive last common ancestor. However, many of its traits do not appear in modern-day African apes. One surprising conclusion, therefore, is that it is likely that the African apes have evolved extensively since we shared that last common ancestor, which thus makes living chimpanzees and gorillas poor models for the last common ancestor and for understanding our own evolution since that time.

"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," said Tim White of the University of California Berkeley, who is one of the lead authors of the research. 

Ardipithecus ramidus digitally rendered composite foot
Digitally rendered composite foot of the “Ardi” partial skeleton. This image relates to an article in the 02 October 2009 issue of Science. The study, by Dr. C. Owen Lovejoy of Kent State University and colleagues, is titled, “Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus.”  Image courtesy of Science/AAAS

"With such a complete skeleton, and with so many other individuals of the same species at the same time horizon, we can really understand the biology of this hominid," said Gen Suwa of the University of Tokyo, Project paleoanthropologist and also a lead Science author.

The special collection of Science articles begins with an overview paper that summarizes the main findings of this research effort. In this article, White and his coauthors introduce their discovery of over 110 Ardipithecus specimens including a partial skeleton with much 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. 

Until now, researchers have generally assumed that chimpanzees, gorillas and other modern African apes have retained many of the traits of the last ancestor they shared with humans – in other words, this presumed ancestor was thought to be much more chimpanzee-like than human-like. For example, it would have been adapted for swinging and hanging from tree branches, and perhaps walked on its knuckles while on the ground. 

Ardipithecus ramidus digitally rendered composite hand
Digitally rendered composite hand of the “Ardi” partial skeleton. This image relates to an article in the 02 October 2009 issue of Science. The study, by Dr. C. Owen Lovejoy of Kent State University and colleagues, is titled, “Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive.”  Image courtesy of Science/AAAS

Ardipithecus challenges these assumptions, however. These hominids appear 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, or to have spent much time swinging and hanging from tree-branches, especially as chimps do. Overall, the findings suggest that hominids and African apes have each followed different evolutionary pathways, and we can no longer consider chimps as "proxies" for our last common ancestor. 

"Darwin was very wise on this matter," said White.

"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.

The special issue of Science includes an overview article, three articles that describe the environment Ardipithecus inhabited, five that analyze specific parts of Ardipithecus' anatomy, and two that discuss what this new body of scientific information may imply for human evolution.

Altogether, forty-seven different authors from around the world contributed to the total study of Ardipithecus and its environment. The primary authors are Tim White of the University of California, Berkeley, Berhane Asfaw of Rift Valley Research Service in Addis Ababa, Giday WoldeGabriel of Los Alamos National Laboratory, Gen Suwa of the University of Tokyo, and C. Owen Lovejoy of Kent State University. 

"These are the results of a mission to our deep African past," said WoldeGabriel, who is also Project co-Director and geologist. 

This research was funded by the National Science Foundation, the Institute of Geophysics and Planetary Physics of the University of California at Los Alamos National Laboratory (LANL), the Japan Society for the Promotion of Science, and others.