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In a special issue of Science, an international team of scientists has for the first time thoroughly described Ardipithecus ramidus, a hominid species that lived 4.4 million years ago in what is now Ethiopia. This research, in the form of 11 detailed papers and more general summaries, was published today in the journal's 2 October 2009 issue. Science is published by AAAS, the nonprofit science society.
The package of research offers the first comprehensive, peer-reviewed description of the Ardipithecus fossils, which include a partial skeleton of a female, nicknamed “Ardi.” Publication of the new research was the subject of simultaneous news conferences today in the Ethiopian capital of Addis Ababa, and at AAAS/Science headquarters in Washington, D.C., with major international news media quickly conveying the story to a worldwide audience.
“What we celebrate here today are the results of a scientific mission to the very deep past,” said Tim White of the University of California Berkeley, one of the lead authors of the research, at the AAAS news conference.
The discovery and publication of the research is “an extraordinary event,” Samuel Assefa, Ethiopian ambassador to the United States, said at AAAS. “The deeper point for all of us is a deeper sense of our interconnectedness.”
Alan I. Leshner, chief executive officer of AAAS and executive publisher of Science, called the publication “truly a landmark event in our understanding of human origins.”
The last common ancestor shared by humans and chimpanzees is thought to have lived six million or more years ago. Though Ardipithecus is not itself this last common ancestor, it likely shared many of this ancestor's characteristics. For comparison, Ardipithecus is more than a million years older than “Lucy,” the partial female 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.
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,” said White, a professor at the Human Evolution Research Center and the Department of Integrative Biology at the
University of California at Berkeley. “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.”
“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,” Gen Suwa of the University of Tokyo, the project paleoanthropologist and also a lead Science author, said in an interview.
C. Owen Lovejoy, a professor of anthropology at Kent State University in Ohio, reconstructed the “Lucy” skeleton and was a lead researcher and author on the “Ardi” project. He called the “Ardi” skeleton “a treasure-trove of surprises” and “one of the most revealing hominid fossils I ever could have imagined.”
Brooks Hanson, deputy editor for physical sciences at Science, hailed the importance of the work, calling it “one of those special and wonderful moments in science.”
“These articles contain an enormous amount of data collected and analyzed through a major international research effort,” Hanson said. “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.
“Science is delighted to be publishing this wealth of new information, which gives us important new insights into the roots of hominid evolution and into what makes humans unique among primates,” said Hanson.
The special collection of Science articles is published 150 years after the publication of Charles Darwin's “On the Origin of Species.” The special issue 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 (110 pounds) and stood about 120 centimeters (just under 4 feet) 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 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,” he added. “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.”
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, 47 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,” WoldeGabriel, project co-director and geologist, said before his appearance at AAAS.
“Ardi's” bones will be quartered at the Ethiopian National Museum, where there's already a significant “Lucy” exhibit. WoldeGabriel reminded reporters at AAAS of the extensive collections of hominid fossils discovered in his nation, some dated to 6 million years ago. “Ethiopia can rightly claim to be the cradle of mankind,” he said.
The 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.
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