The Dawn Of Man

2001: A Space Odyssey Dawn Of Man

The region of Ethiopia called the Middle Awash, some 140 miles northeast of the capital of Addis Ababa, is a hot, harsh and inhospitable place--a rocky desert punctuated by tree-lined rivers, the occasional lake and patches of lava that are slowly being buried by sediments flushed out of the hills by the torrential rains that come along twice a year.

But between 5 million and 6 million years ago, the landscape here was very different. The same tectonic forces that racked the region with earthquakes and volcanic eruptions had also thrust the land up as much as a mile higher than it is today. As a result, the area was cooler and wetter and overgrown with trees, bushes and patches of grass. These fertile woodlands were rich in wildlife. Primitive elephants, giant bears, horses, rhinos, pigs, rats and monkeys lived here, along with dozens of other mammal species long since extinct.
And it was here too that nature indulged in what was perhaps her greatest evolutionary experiment. For it was in eastern Africa at about this time that a new type of primate arose--an animal not so different from its apelike ancestors except in one crucial respect: this creature stood on two legs instead of scurrying along chimplike on all fours. Its knuckle-walking cousins would stay low to the ground and never get much smarter. But while it wouldn't happen until millions of years in the future, this new primate's evolutionary descendants would eventually develop a large, complex brain. And from that would spring all of civilization, from Mesopotamia to Mozart to Who Wants to Be a Millionaire.
That's the broad outline, anyway. While this view of human evolution has generally been accepted by scientists for decades, no one has yet been able to say precisely when that first evolutionary step on the road to humanity happened, nor what might have triggered it.
But a discovery reported last week in the journal Nature has brought paleontologists tantalizingly close to answering both these questions. Working as part of an international team led by U.S. and Ethiopian scientists, a graduate student named Yohannes Haile-Selassie (no relation to the Emperor), enrolled at the University of California, Berkeley, has found the remains of what appears to be the most ancient human ancestor ever discovered. It's a chimp-size creature that lived in the Ethiopian forests between 5.8 million and 5.2 million years ago--nearly a million and a half years earlier than the previous record holder and very close to the time when humans and chimps first went their separate evolutionary ways.
"Having a fossil in this region of time, very near the divergence point, is really exciting," says anthropologist C. Owen Lovejoy of Ohio's Kent State University. "Going all the way back to Darwin, people have speculated how, when and why humans stood up on two legs. For paleontologists, this find is a dream come true."
As is often the case with discoveries like this, Haile-Selassie was not specifically looking for the things he found. He had set out to better understand how the ancient ecosystems worked and evolved. "I didn't even think about finding hominids," he says. "All I wanted to do was collect enough vertebrate bones so that I could write my dissertation." In December 1997, though, at a place called Alayla, he spotted a piece of jawbone lying on the rock-strewn ground. "I picked up the mandible less than five minutes after we got there," he recalls, "but didn't realize I had something really special until a year later, when we found some more bones and I started the serious analysis."
In all, the team eventually found 11 specimens--from at least five different individuals--in a cluster of sites, including Haile-Selassie's partial lower jaw with associated teeth, several hand and foot bones, and pieces of three arm bones and a collarbone. Luckily, the fossils were trapped in sediments that were sandwiched between layers of volcanic ash, whose age can be accurately gauged by a technique known as argon-argon dating. (This layering is still visible in places that have not been so heavily eroded, enabling the scientists to trace the area's geologic history.) The verdict, confirmed by a second dating method and by the other primitive animals found with the hominid remains: most of the fossils are between 5.6 million and 5.8 million years old, although one toe bone is a few hundred thousand years younger.
It was the detailed anatomy of these fragmentary fossils, especially the teeth, that convinced Haile-Selassie that he had discovered a new human ancestor. Although apelike, the lower canines and upper premolars, in particular, display certain traits found only in the teeth of later hominids--the term scientists use to describe ourselves and our non-ape ancestors. They also differ in shape from the teeth of all known fossil and modern apes. Even the way in which the teeth had been worn down was telling. Explains Haile-Selassie's thesis adviser, Berkeley paleontologist Tim White: "Apes all sharpen their upper canines as they chew. Hominids don't." The new creature's back teeth are larger than a chimp's too, while the front teeth are narrower, suggesting that its diet included a variety of fibrous foods, rather than the fruits and soft leaves that chimps prefer.