The first thorough examination of the oldest known skeleton in human lineage, by a team from the University of California at Berkeley, suggests that our human ancestors looked and acted much less like apes than has been widely believed.
The fossils indicate the gap between apes and humans was already wide four million years ago, and implies that apes may serve only as a limited stand-in for understanding human progenitors. The team, led by Tim D. White, a professor of integrative biology and director of the university's Human Evolution Research Center, published its findings in Science magazine on Friday.
The multiple papers are the result of 17 years of recovering and studying the fossilized remains from a species, named Ardipithecus ramidus, that lived 4.4-million years ago in what is now Ethiopia. Research team members and prominent authors of the papers include Ethiopian scientists, signaling more multinational involvement in a field that had been historically dominated by American and European researchers.
The most complete of the Ardipithecus skeletons, whose first fragments were found in 1992 in the Middle Awash region of Ethiopia and nicknamed "Ardi," is more than a million years older than "Lucy," the iconic Australopithecus afarensis specimen discovered in 1974 in Ethiopia that provided groundbreaking insights into the evolutionary history of humans.
The recovery and reconstruction of the Ardi skeleton, along with fossilized bone fragments from about 35 other individuals of the same species, is destined to redefine the understanding of human origins as much as Lucy did, the researchers said. .
A Different Evolutionary Path
A key conclusion from Mr. White and his team is that the last common ancestor between the human and ape lineages, a yet-undiscovered species that is believed to have lived about six million years ago, must have already acquired key human characteristics, blurring the common understanding that early man was essentially a version of a chimpanzee or other ape.
"With Lucy, you're only halfway back to the split" six million years ago, Mr. White said at a briefing here Thursday. "Now, with the illumination that Ardipithecus throws on this issue, one can see that not only has the human line been evolving dramatically over the last six million years; for the same six- to seven-million years, the chimpanzees have been evolving."
That means some of the behaviors seen in modern chimps and other apes—such as walking on the knuckles of their hands and climbing high in trees to feed on fruit—weren't necessarily part of the human evolutionary path, Mr. White said.
"We're no longer forced to rely on that modern, highly evolved reference species," he said of chimps and other apes, "when we have hard data from the deep past, and that's what Ardipithecus has done."
The Ardi skeleton, about 45 percent of which was recovered by Mr. White and his team by 1994, contains "just a treasure trove of surprises," said C. Owen Lovejoy, a professor of anthropology at Kent State University who assisted Mr. White in his analysis.
Evidence that the separation between humans and apes is deeper than previously understood include the greater similarity of the modern human hand to its ancient form than to the modern chimpanzee hand, Mr. Lovejoy said at the briefing. Also, the pelvis of the Ardi skeleton, a female, was evolved enough to allow upright walking, unlike the pelvises of chimpanzees, whose bodies are more adapted to climbing trees, he said.
The Ardi skeleton and other partial skeletons of that period also show a canine tooth that is relatively small, which Mr. Lovejoy described as evidence of a social structure in which males were living cooperatively with females to raise their young. The skeleton gives "a host of revelations about the earliest phases of human evolution that Lucy never provided us," he said.
The researcher who discovered Lucy, Donald C. Johanson, a paleoanthropologist at Arizona State University, said the find is "terribly important for all of our thinking" about human origins.
Mr. Johanson, founding director of the university's Institute of Human Origins, said Mr. White—who worked with Mr. Johanson on the Lucy analysis—and his group perform "exemplary" research. At the same time, Mr. Johanson said, he expected the team's initial interpretations "will undoubtedly generate widespread debate," perhaps even including the question of whether Ardi is actually a human ancestor.
Mr. Johanson said he was not among those who would raise that question. But, he said, "there must have been very rapid evolutionary change" for the human form to transform so quickly from Ardi to Lucy. While the Lucy skeleton itself dates to about 3.2-million years ago, her species dates to the range of 3.7-million to 3.8-million years, he said.
A Wish for More Sharing
Mr. White has faced criticism over the length of time he took to make his findings available to outside researchers. Scientific American magazine, in an editorial last month, cited his handling of Ardi as a prime example of the "problem of possessiveness in the field of human origins."
"Fossil hunters often block other scientists from studying their treasures, fearing assessments that could scoop or disagree with their own," the magazine said in its editorial. "In so doing, they are taking the science out of paleoanthropology."
A member of Mr. White's team, Giday WoldeGabriel, a geologist working at the Los Alamos National Laboratory, in New Mexico, said such critics don't seem to appreciate the amount of time it takes to carefully recover, clean, and assemble fossil remains, especially when using volunteers on part-time duty. "Fifteen years, to me, considering what it takes to do the work, is nothing," Mr. WoldeGabriel said. "Tim White and everybody else is very, very careful in what they do."
But the complaints don't concern the time it took to process Ardi, Mr. Johanson said, as much as the decision to keep information restricted during that period. "That upset some folks," he said. "There was a frustration." Mr. White said it is common practice for a research team to keep its materials to itself until it publishes its initial findings.
Mr. White's approach also has garnered praise from Ethiopians for his conscious efforts to bring natives of the country, such as Mr. WoldeGabriel, into key roles on his team and help them gain their own expertise in a field that is bringing international recognition to their homeland.
Mr. White's team made its announcement on Thursday simultaneously in Washington and in Addis Ababa, the capital of Ethiopia, where the Ardi fossils remain. The Ethiopian ambassador to the United States, Samuel Assefa, attended the Washington event to give thanks.
"More and more, we see our own people being part of it, leading it," Mr. Assefa said of the archaeological expeditions in his country. "We are very well represented in the scientific community—we are not just a site."
Mr. White is a leader in what has become a growing trend toward incorporating local students and professionals into overseas research, said Dennis H. O'Rourke, a professor of anthropology at the University of Utah who is president of the American Association of Physical Anthropologists.
"It is certainly something that many of us are interested in doing more of in the future," Mr. O'Rourke said. The practice helps American researchers operate in foreign countries, and it helps American universities broaden and improve their student bodies, he said.
Mr. White said he recognized as a graduate student in the 1970s that there were far too few Africans participating in a field that largely involved discoveries on their continent. "I just felt that that was wrong," he said. Current practices of involving Africans, he said, give "a new look and a great new look to paleoanthropology."