Dr. C. Owen Lovejoy

The term hominid describes humans, chimpanzees and gorillas, while hominin refers only to the direct human line of ancestry.

Ardipithecus ramidus, a hominin, existed 4.4 million years ago in present-day Ethiopia and represents the closest hominin to the last common ancestor of humans and apes that lived 6 million years ago.

The study does not alter the theory that apes and humans descended from a common ancestor. But it does provide evidence that apes might have diverged from our evolutionary line.

"People often think we evolved from apes, but no, apes in many ways evolved from us," Dr. Lovejoy said. "It has been a popular idea to think humans are modified chimpanzees. From studying Ardipithecus ramidus, or 'Ardi,' we learn that we cannot understand or model human evolution from chimps and gorillas."

In the special Oct. 2 edition of Science, an international team of 47 scientists thoroughly described Ardipithecus ramidus in 11 detailed papers that describe fossils that include a partial skeleton of a female, nicknamed "Ardi."

While Ardi doesn't represent the last common ancestor of apes and humans, she likely shared many of its characteristics.

For comparison sake, Ardipithecus is more than a million years older than "Lucy," the female partial skeleton of Australopithecus afarensis. Until the 1992 discovery of Ardipithecus remains, the fossil record contained scant evidence of hominins older than Lucy.

She was 110 pounds, stood 4 feet tall and had a brain about the size of a chimp's. Through analysis of Ardi's skull, teeth, pelvis, hands, feet and other bones, the research team determined that Ardi had a mix of "primitive" traits shared with her predecessors -- primates of the Miocene epoch -- and "derived" traits, which it shares exclusively with later hominins.

That's to say, Ardi pushes us closer to the still-elusive last common ancestor.

However, many of Ardi's traits do not appear in modern-day African apes. That bolsters conclusions that African apes have evolved extensively since the last common ancestor. That, in turn, suggests that living chimpanzees and gorillas are poor models for the last common ancestor and for understanding our own evolution since that time, Dr. Lovejoy said.

The last common ancestor, sometimes abbreviated as the LCA, is the creature to which both modern humans and modern chimpanzees can trace their ancestry.

The research on Ardi suggests that this ancestor didn't look nearly as much like a modern chimpanzee as had been previously suspected. Rather, the ancestor would have looked more like Ardipithecus.

This suggests that chimpanzees, far from being time machines for visiting the distant past, have themselves evolved significantly, including developing such skills as suspending from branches and knuckle-walking.

Dr. Lovejoy's focus was Ardi's skeletal remains below the neck, including her hands and feet, and social behavior. His work helped determine that Ardi could walk upright, although not as well as humans do. While she could climb trees, she was not as proficient a climber as chimpanzees or other great apes.

Those, among other results, place Ardi as an important transitional hominin in the evolutionary journey leading to humans.

"We've been waiting for this for 15 years," said Todd Disotell, an anthropologist at New York University's Center for the Study of Human Origins, who was not part of the study. "This is as important as Lucy. We've just extended that back by another third -- by another million years -- so we have a much better picture, which allows us to improve upon our estimate of the last common ancestor."

Before Ardi, anthropologists had to puzzle over 6 million years of ape evolution and 3 million years of human evolution. That meant extrapolating more than 9 million unknown years of evolution, Dr. Disotell said. Ardi brings us to within 1.6 million years of the last common ancestor.

Based on the Science package, Ardi lived in the forest, dispelling the notion that the first bipedal hominins lived in grasslands.

Lucy was so far advanced she did not provide many clues about human ancestry. But Ardi features transitional characteristics, including an upper pelvis geared toward climbing and a lower pelvis designed for walking, Dr. Lovejoy said.

While the lateral part of Ardi's foot is designed as a propulsive lever for walking, it also has a grasping toe for climbing. Ardi was able to walk but would not win any Olympic races, he said. When tired, she would climb and nest in a tree.

"If you saw her walk, you would probably think it was a little odd," Dr. Lovejoy said. "But over a lifetime she would be more subject to arthritis and injuries. She would be fatigued more quickly."

She could climb trees to reach fruit, but wouldn't have climbed into the tree's canopy, as great apes do.

Fossils also show that males were close in size to females, much like humans, and did not have long canine teeth, as do apes. That indicates that Ardipithecus males didn't fight other males for mating rights with females. Instead, the female chose a male adept at hunting for nuts, fruits and turtles, among other quality food. He shared food with the female in exchange for mating rights.

Because females only ovulated once every four or five years, it was to the male's advantage to develop a relationship with one female. Otherwise, Dr. Lovejoy said, the male's chance of finding an ovulating female was akin to finding a needle in a haystack.

The studies are destined to inspire a whole new round of scholarship, analysis and debate and advance the study of human origins.

"It's another step toward understanding human evolution and completing our understanding," Dr. Lovejoy said. "Ultimately it says something about our place in the world."

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