In an article in the Sept. 22 issue of Nature, the team describes fossils of the earliest-known hominids, discovered in the Middle Awash region near Aramis, Ethiopia.
The material is 4.4 million years old, almost a million years older than Australopithecus afarensis, hitherto the earliest-known human ancestor.
The fossils come from a new species, Australopithecus ramidus, the skull and teeth of which looked similar to a chimpanzee.
"This species is the oldest known link in the evolutionary chain that connected us to our common ancestor with the living African apes," said White.
"The discovery takes us one major step closer to this common ancestor. When you combine these Aramis fossils with the evidence from the molecules, clearly Darwin was right--humans evolved from an African ape," he said.
In a companion paper in Nature, Giday WoldeGabriel from the Los Alamos National Laboratory and colleagues show that A. ramidus lived in a forested environment, along with a diverse fauna of monkeys, antelopes, and other woodland species.
The new research, announced jointly by Nature and the University, offers a glimpse of the earliest roots of humanity.
The first hominid fossil was found by team member Suwa at Aramis on Dec. 17, 1992. As he walked across the barren outcrops under the midday desert sun, Suwa's eye was caught by the glint of a molar tooth among the hundreds of pebbles of the desert pavement.
"I knew immediately that it was a hominid," he said, "and because we had found other ancient animals that morning, I knew it was one of the oldest hominid teeth ever found."
The team worked at Aramis during the winters of 1992 and 1993 to recover additional remains from the same geological horizon.
Seventeen hominid individuals represented by different bones and teeth (including parts of the skull, the arm, and a partial lower jaw from a child) were found among over 600 fossil remains of other vertebrate animals.
After detailed comparisons with modern and fossil apes and other early hominids, White, Suwa, and Asfaw concluded that they had recovered a species whose existence had been the subject of speculation for many years.
White, who was a member of the team that discovered the famous partial skeleton, "Lucy," said, "The discovery of these ancient fossils and their context signals a major step in our understanding of human origins."
Dating reveals that the hominids died 80,000 generations before Lucy and 800,000 years before the earliest definitive representatives of afarensis.
The discoveries of Lucy and the Tanzanian footprint trails found by Mary Leaky about 20 years ago pushed the human line back to 3.6 million years ago.
More fossils belonging to Lucy's species, Australopithecus afarensis, were recovered from sites along the Awash River during the past three years, including a skull.
The search for the species ancestral to afarensis has been a central goal of researchers keen to push the record of human evolution even further back into the past.
The newest fossils were found in the remote Afar depression of Ethiopia, northeast of Addis Ababa, where Lucy was found. The fossils are now housed in the National Museum of Ethiopia in Addis Ababa.
The Middle Awash paleon-tological fields were first explored by Maurice Taieb, a French geologist who found Lucy's resting place. Since 1981, the Middle Awash research has been conducted under the direction of Desmond Clark and White, both of Berkeley, and Asfaw of Addis Ababa.
The new species differs from A. afarensis in having smaller premolars and molars, relatively larger canine teeth, thinner tooth enamel, a more primitive cranial base, and a chimpanzee-like deciduous first molar.
"The deciduous molar from the Aramis child's jaw was a complete surprise. Its shape is so different from homologous teeth in other human ancestors that this tooth alone would have been enough to recognize a new species," said White.
While Aramis teeth and skull fragments are more chimpanzee-like, Asfaw said the Aramis hominid was clearly not a chimpanzee. "The short cranial base and the hominid shapes of the canine and elbow show us that this species had already split from the apes. It had started to evolve toward human beings."
It was during the 1960s that two Berkeley scientists, the late Allan Wilson and Vincent Sarich, compared the molecular structure of modern African apes and humans and found a startling similarity.
They suggested that the evolutionary lines leading to the chimpanzees, gorillas, and humans had split only 4 to 6 million years ago, not some 20 million years ago, as had been believed.
The new discoveries show extremely ape-like cranial and dental anatomy in a creature living in a wooded habitat, said White. This suggests to him that the new species may be the last link in the chain to the still-elusive common ancestor.
"This discovery is important because it gives us our first good look at the biology of a very ancient ancestor that sits at the root of the family tree," said White.
He said the team is eager to return to the Middle Awash this winter. Studies on pollen, carbon isotopes, fossil wood, small mammals, and sediments should fill in the tantalizing picture of the ancient habitats of the earliest hominids.
Of course, the international team of more than 20 researchers hopes to find more hominid fossils. Asfaw said that project archaeologist Yonas Beyene will excavate one promising locality that has already yielded most of a hominid left arm. He hopes to find the rest of the skeleton nearby.
A key goal of future work at Aramis will be to refine the dating and geology, said White. "We have only begun to understand this ancestor and its environment. I am sure there are more surprises in store for us," he said.
White said he hopes the team will come up with a femur and an intact skull this winter, but he adds "given the great antiquity of this site, we are still amazed at what we have already found."