THE JAWS OF DESTINY

By MICHAEL D. LEMONICK

The arid badlands of northern Ethiopia's Hadar region have long been a fossil-hunter's paradise. It was there, two decades ago, that paleontologists unearthed the celebrated Lucy, a primitive ancestor of modern humans dating back more than 3 million years. On this trip, though, an international team of scientists was on the trail of something less ancient but much more mysterious. Somewhere between 2 million and 3 million years ago, experts believe, Lucy's descendants had evolved into true humans. But despite decades of searching, scientists had yet to find a single well-dated fossil that showed how, why or precisely when this fateful transition occurred.

Then two veteran fossil hunters from the local Afar tribe, exploring near a dry stream bed, spotted something out of place: two pieces of a prehistoric upper jaw that had eroded from a hillside. "The instant we fit the jaw together," says William Kimbel, science director of the Institute of Human Origins in Berkeley, California, and a leader of the expedition, "we knew we weren't dealing with an apelike Australopithecus [the scientific name for Lucy and her kin]."

In fact, as the researchers explain in a report that will appear in the December Journal of Human Evolution, the jaw belonged to the genus Homo, the line that includes modern Homo sapiens. The fossil has been dated at 2.33 million years old--arguably the oldest Homo fossil ever found, and right in the middle of the mystery zone. What's more, the bones were found near stone tools of the same age--the oldest combination of bones and artifacts ever discovered.

The find, made in 1994 but announced only last week, is important for a couple of reasons. First, the manufacture of stone tools is considered a key characteristic of the Homo line--something, along with brain size, skull shape and other anatomical features, that separates humans from less-than-humans. That has never been proved, though: the oldest known stone tools date back perhaps 2.5 million years, but they were found without any fossils around and could have belonged to anyone. And the oldest Homo fossils with universally accepted dates--until now, that is--date back only 1.9 million years. But the new find suggests that Homo's emergence and the invention of tools could have happened around the same time. And the fact that tools and bones were found together strongly implies, though it doesn't prove, that they are related.

Far more important, the discovery may help paleontologists finally figure out when and why the Australopithecus clan sprouted a new branch on the hominid tree. According to a leading theory, the trigger was a global cooling trend that began about 2.7 million years ago and transformed much of Africa's moist woodlands into dryer, more open savannah. Was the development of tools and a more upright stance an evolutionary strategy to cope with the rigors of the new environment? Perhaps. But until now nobody had found a Homo fossil that dated back anywhere near 2.7 million years.

Now they have. And there's plenty of evidence of climate change at the Hadar site. Near the jaw and tools, the team recovered fossils of grazing antelopes, indicative of an open, fairly grassy habitat, rather than forest-dwelling species like impala, which were so prevalent in Lucy's time.

But while the new discovery brings paleontologists closer to solving the mystery of Homo's origin, it still falls frustratingly short. Although the scientists found a nearly complete upper jaw, 10 teeth and a number of tooth fragments, they can't say for certain which species the fossil belongs to. By 1.9 million years ago, the Homo line had spawned at least two branches: Homo habilis and Homo rudolfensis. The new fossil resembles both species in some ways, but without a more nearly complete skull it's impossible to say more. "What we have now is a hypothetical human lineage with very little evidence on it," says anatomist Alan Walker of Pennsylvania State University. "This new fossil is another piece in the developing story."

Chances are that if the answer exists, it is waiting in Hadar. Kimbel and his colleagues realized several years ago that it was an ideal place to look for clues to humanity's immediate family because Lucy's bones didn't come from the most recent rocks in the area. By combing through younger rocks, the scientists hoped to uncover clues to what came later. The newly announced jawbone is among the first fruits of that labor, but it probably won't be the last.

--Reported by Andrea Dorfman/New York

With reporting by Andrea Dorfman/New York