David DeGusta in the National Museum of Ethiopia, Addis Ababa, holding a fossil hominid tooth his team recovered in Weyto, Ethiopia.

Courtesy of David DeGusta

When David DeGusta was a graduate student sorting monkey fossils in an Ethiopian desert in 1995, he had no idea what payoff – if any – his work would have.

But after spending much of his time over the next five years in what seemed like the middle of nowhere, and another six years as an assistant professor of anthropology at Stanford analyzing the African fossils he collected, DeGusta finds himself part of a major discovery that sheds new light on one of the earliest phases of human evolution.

DeGusta is a co-author on two of 11 papers that appear in a special issue of the journal Science on Friday, Oct. 2, showing how hominids – the earliest human ancestors – made their way from living in trees to walking on two legs.

Much of the evidence is contained in fossils of three dozen 4.4-million-year-old hominids dubbed Ardipithecus ramidus (or "Ardi," for short). The Ardi fossils, which include a partial skeleton, are more than a million years older than the "Lucy" skeleton of Australopithecus afarensis, which was discovered in 1974.

Nearly 10,000 animal fossils, such as the monkey, antelope and lizard remains that DeGusta analyzed at Stanford, help give a picture of what Ardi's habitat looked like.

For decades, most scientists thought early humans went through an evolutionary period of knuckle-walking, like chimpanzees. And previous research pointed to open grasslands as the place where hominids learned to walk. But DeGusta and his colleagues – led by Tim White, a paleoanthropologist at UC Berkeley – draw on almost two decades of research to paint a much different portrait of early human evolution and environment.

By studying the Ardi and animal fossils, the research team determined that hominids began splitting their time between trees and the ground in closed woodlands and small patches of forest. But when on the ground, Ardi walked upright on two legs rather than knuckle-walking on all fours.

"These new fossils are the earliest good evidence for human evolution, and they tell us a lot about the initial steps toward what we look like now," DeGusta said. "People often use modern chimpanzees as a model for what the common ancestor of humans and chimpanzees looked like. Ardi shows that the common ancestor was quite different than living chimps." 

DeGusta's current research picks up where Ardi leaves off. He'll be spending the next two months in the Gobaad Basin of Djibouti, about 100 miles from where Ardi was found, followed by another month in the Weyto area of Ethiopia looking for more fossils and stone tools he hopes will illuminate later time periods in human evolution.

The work he did on the Ardi project serves as a model for his upcoming fieldwork, and proves the importance of doing long-term multidisciplinary research into human evolution.

"The full picture of human evolution is like a jigsaw puzzle," DeGusta said. "These current findings are a big piece of that puzzle. But the puzzle itself is massive and you don't get to choose what pieces you find – you just hope you get some important ones."