New Species Of Human Ancestor

Washington, DC -- Two-and-a-half-million-year-old cranial and tooth remains found in Ethiopia belong to a previously unknown hominid that may have been the immediate predecessor of humans, according to a team of Ethiopian, American, and Japanese researchers. Team members also describe other fossils from the same geological layer showing that hominids--though not necessarily the newly identified species--were walking on humanly proportioned legs and using stone tools to strip meat and scrape marrow from the bones of antelopes and horses. This constitutes the earliest example yet of tool-assisted meat-eating. Two reports describe the finds in the 23 April 1999 issue of Science.

Researchers came across the fossils outside the small village of Bouri, a hard two-day drive northeast of Addis Ababa. The site is located in a harsh, desert region of Ethiopia called the Middle Awash, already famous for other major discoveries such as the oldest known hominid, found by the same team of researchers. The new hominid--dubbed Australopithecus garhi, after the local people's word for "surprise"--possesses features that place it at the forefront of one of the hottest debates in paleoanthropology: from what evolutionary branch did the first humans appear?

"No one predicted garhi," said University of California-Berkeley biologist Tim White, who co-led the team with Berhane Asfaw of Ethiopia's Rift Valley Research Service. Instead, researchers have been looking in eastern Africa for A. africanus, a smallish, upright-walking hominid known to have roamed southern Africa two to three million years ago and thought by many to be the best candidate for humanity's immediate forebear. But in Science, the research team presents anatomical analyses and measurements from the A. garhi fossils that they say sharply distinguish the new species from A. africanus and from the other hominid species known to be alive around the same time, including two robust species that eventually died out. If anything, said White, A. garhi's big teeth and projecting face best resemble an older East African species known as A. afarensis, whose most famous representative goes by the name of Lucy.

In addition to the cranial and tooth remains of A. garhi, the research team found in the same geological layer arm and leg bones from several other hominid individuals. Without associated dentition, these individuals can't reliably be assigned to a species. Intriguing, though, is the distinctive way in which their relative limb proportions are intermediate between that of apes and humans. That is, while "Lucy" (3.2 million years ago) had upper arms that were long relative to her legs, and H. erectus (1.7 million years ago) had the shortened forearms and longer femurs of modern humans, the unidentified Bouri hominids were smack in the middle--showing that the femur lengthened at least one million years before the forearm shortened. What this may imply about hominid locomotion and other behaviors, and what pressures this might have put on the subsequent direction of human evolution, remains to be determined.

While walking around on human-like legs, the unidentified Bouri hominids were apparently also using stone tools to fillet meat and pull marrow from the bones of large animals that thrived in the open, grassy plains once surrounding an ancient lake. Researchers found one of the hominid legs buried next to catfish and antelope bones, the latter of which showed definitive cut marks from stone tools. Scattered elsewhere throughout the same geological layer were other antelope and horse bones with similar tell-tale signs of butchery--for example, a lower jaw whose tongue presumably had been cut out and leg bones purposely fractured at both ends, indicative of marrow extraction.

Such unprecedented and unique access to high-fat meat and marrow would have constituted a "dietary revolution," says White, one that "would have opened up a whole new world of food" and possibly fueled humanity's eventual migration out of Africa.

The tools themselves, however, have proved frustratingly elusive. The researchers found only a few isolated tools strewn about the surface of the site and none during excavation. This left them unable to determine the tools' ages or whether the tools belonged with the butchered bones. In their report, the researchers suggest that the area around Bouri 2.5 million years ago lacked the natural features (such as large rushing streams with cobbles or rock-outcroppings) that would have served as source material for stone tools, and that therefore Bouri hominids had been forced to carry in whatever tools they needed to exploit animal life at the lake margins. Still, the site at Bouri begs comparison to another 2.5-million-year-old site at nearby Gona, Ethiopia, where in 1997 nearly 3,000 stone tools were found--the oldest stone tools yet discovered. In contrast to Bouri, the Gona site lacked evidence of what the tools were used for or who might have made them. The researchers argue that the Bouri hominids, including A. garhi, must now be considered strong candidates for the Gona tool-makers.

The newly identified hominid fossils provide much-needed information about what may have been happening in Africa two to three million years ago, a crucial juncture in human evolutionary history. "You go into this period with, in essence, bipedal big-toothed chimps and come out with meat-eating large-brained hominids," said White. "That's a big change in a relatively short time. We'd really like to know more about what happened there."