At 4.4 million years, Ethiopian fossil clarifies human–chimp relationships.
Published online 1 October 2009 | Nature | doi:10.1038/news.2009.966
In a far-reaching reordering of human evolution, researchers report today the discovery of the oldest hominid skeleton, a fairly complete 4.4-million-year-old female from Ethiopia1.
The discovery shows that humans did not evolve from ancient knuckle-walking chimpanzees, as has long been believed. The reports, in Science, illuminate how early phases of humans evolved along a separate lineage from the last common ancestor shared by early hominids and extinct apes.
The new fossils of Ardipithecus ramidus — known as ‘Ardi’ — offer the first substantial view of the biology of a species close to the time of the last common ancestor, estimated to be at least 6 million years ago. Like modern humans, Ardi could walk upright and didn’t use her arms for walking, as chimps do. Still, she retains a primitive big toe that could grasp a tree like an ape.
“This spectacular specimen shows why fossils really matter,” says Andrew Hill, head of anthropology at Yale University in New Haven, Connecticut.
Previously, the oldest near-complete skeleton of a human ancestor was the 3.2-million-year-old Australopithecus afarensis skeleton known as Lucy, also from Ethiopia. Because Lucy had many traits in common with modern humans, she didn’t provide much of a picture of the earlier lineage between apes and humans, says Alan Walker, a biological anthropologist at Pennsylvania State University in University Park. The new A. ramidus does.
“This specimen is so much more important — and strange,” says Walker, adding that it will prompt “considerable rethinking of not only our evolutionary past, but also that of our living relatives, the great apes”.
In a publishing tour de force, 11 Science papers include descriptions of the remains and the geology and palaeoenvironment of the discovery site, in the Afar desert 230 kilometres northeast of Addis Ababa. The papers are the culmination of 17 years of study by 70 investigators collaborating as the Middle Awash Project. Forty-seven of them are authors.
“The great thing about these fossils is that they illuminate a black hole in evolution 4.5 million years ago,” says Tim White, a paleoanthropologist at the University of California, Berkeley, and a project co-director.
The earliest known Ardipithecus — A. ramidus kadabba — lived around 5.8 million years ago in Ethiopia2. The other oldest known hominids are Orrorin tugenensis, from about 6 million years ago in Kenya3, and Sahelanthropus tchadensis, from at least 6 million years ago in Chad4.
In 1992, Middle Awash team member Gen Suwa found the first specimen of an A. ramidus species near the Ethiopian village of Aramis. “You crawl around on a ridge of bones looking for fossils,” says Suwa, from the University of Tokyo. “I saw the root of a tooth sticking up from a clump of matrix. But we had no idea of its importance — we didn’t believe it was a rich area.”
Yet within two years, enough fossils had been found to produce the first article that named and sketchily described the animal, from a total of 17 fossils5. What followed was one of the most intensive anthropological investigations ever undertaken. Roughly 6,000 vertebrate specimens from the site have been catalogued for the Ethiopian National Museum in Addis Ababa.
The work was done under intense secrecy, prompting some to dub it ‘the Manhattan Project of anthropology’. Some competing researchers have complained about the time it has taken to publish work about the fossils.
“We weren’t interested in how many papers we could publish,” says Berhane Asfaw, a co-director of the Middle Awash Project at the Rift Valley Research Service in Addis Ababa. “Our interest was in the full chain of information; that produces the power of the work.”
From more than 135,000 vertebrate bone or tooth pieces, the team identified 110 A. ramidus specimens, representing a minimum of 36 individuals. Ardi’s skeleton comprises 125 pieces.
Such a wealth of anatomical specimens is unheard of for these periods. O. tugenesis is based on two femurs, some teeth and a few other broken bones; S. tchadensis is named from a skull, two mandibles and some teeth. Lucy’s skeleton is missing key diagnostic bones from her hands and feet.
The fossils come from a sediment layer sandwiched between two layers of volcanic rock known as tuff — each dated to 4.4 million years ago, thereby locking in the dates for the specimens, says a team led by Giday WoldeGabriel, of Los Alamos National Laboratory in New Mexico. Fossils in the sediments include plants, pollen, invertebrates and birds, which helped to pinpoint the woodland environment where Ardi lived.
Years of field work uncovered Ardi’s skull, teeth, arms, hands, pelvis, legs and feet — all of which had to be painstakingly prepared. Ardi’s skull was recovered crushed in more than 60 pieces that were broken and scattered about. The bone was poorly fossilized — so soft that each piece had to be moulded in a silicon rubber cast then digitized by computed tomography scans. “This is an exciting technology,” says Hill. “There was no way to describe this skull 15 years ago.”
Her hands and wrists don’t show several distinctive chimp characteristics, such as some larger bones and a tendon ‘shock absorber’ system in the hand and wrist to withstand bodyweight, says team member Owen Lovejoy of Kent State University in Ohio. The foot, with its big toe sticking out sideways, would have allowed Ardi to clamber in trees, walking along limbs on her palms. And her teeth show no tusk-like upper canines, which most apes have for weapons or display during conflict. “This is a major feature showing that Ardi is not in the lineage of modern chimps,” Suwa says.
A big question now is when our last common ancestor with apes actually lived. “I believe it was 8-10 million years ago,” says Lovejoy.
All White would say was: “Find the fossils.”
1. White, T. D. et al. Science 326, 75-86 (2009).
2. Haile-Selassie, Y. Nature 412, 178-181 (2001).
3. Senut, B. et al. C. R. Acad. Sci. Paris Ser. IIa 332, 137-144 (2001).
4. Brunet, M. et al. Nature 418, 145-151 (2002).
5. White, T. D. , Suwa, G. , Asfaw, B. Nature 371, 306-312 (1994).