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.
When Haile-Selassie compared the newly discovered bones and teeth with those of Ardipithecus ramidus, a 4.4 million-year-old hominid found in the Middle Awash in the early 1990s that was the previous record holder, he realized that the two creatures were very similar. But the older one's teeth, while different from an ape's, do have a number of characteristics that are decidedly more apelike than those of the younger hominid.
On the basis of these minor but distinctive differences, Haile-Selassie decided to classify the new human ancestor as a subspecies, or variant, of ramidus and has given it the name Ardipithecus ramidus kadabba. (The name is derived from the local Afar language. Ardi means ground or floor; ramid means root; and kadabba means basal family ancestor. In accordance with the sometimes bizarre nomenclature of science, the younger creature now gets renamed Ardipithecus ramidus .)
Haile-Selassie and his colleagues haven't collected enough bones yet to reconstruct with great precision what kadabba looked like. But they do know it was about the size of modern common chimpanzees, which when standing average about 4 ft. tall. That makes it roughly the same size as its close relative A. ramidus ramidus and about 20% taller than Lucy, the famous 3.2 million-year-old human ancestor discovered about 50 miles away in 1974 that is even further along the evolutionary track. The size of kadabba's brain and the relative proportions of its arms and legs were probably chimplike as well.
But unlike a chimp or any of the other modern apes that amble along on four limbs, kadabba almost certainly walked upright much of the time. The inch-long toe bone makes that clear. Two-legged primates (modern humans included) propel themselves forward by leaving the front part of their foot on the ground and lifting the heel. This movement, referred to as toeing off, causes the bones in the middle of the foot to take on a distinctive shape-a shape that is readily apparent in the ancient toe bone. "If you compare a chimp's foot bones with its hand bones, they look the same because they're used for the same thing"-that is, for grasping-Haile-Selassie explains. "Hominid fingers and toes don't look alike at all."
Exactly how this hominid walked is still something of a mystery, though with a different skeletal structure, its gait would have been unlike ours. Details of kadabba's lifestyle remain speculative too, but many of its behaviors undoubtedly resembled those of chimpanzees today. It probably still spent some time in trees. It probably lived in large social groups that would include both sexes. And rather than competing with one another for mates, the males may well have banded together to defend the troop against predators, forage for food and even hunt for game.
But that kadabba walked upright at all is hugely significant. Paleontologists have suspected for nearly 200 years that bipedalism was probably the key evolutionary transition that split the human line off from the apes, and fossil discoveries as far back as Java Man in the 1890s supported that notion. The astonishingly complete skeleton of Lucy, with its clearly apelike skull but upright posture, cemented the idea a quarter-century ago.
What's been much tougher to pin down is just why two-leggedness arose. The conventional wisdom has long focused on the fact that eastern Africa became significantly dryer about the time that humans first evolved. The change would have tended to favor grasslands over forests, and, so went the theory, our ancestors changed to take advantage of the new conditions. We learned to walk upright so that we could see over the tall grasses to spot predators coming; an upright posture, moreover, would offer a much smaller target for the oppressive heat of the grassland sun, and a larger target for cooling breezes.
The only trouble with this theory is that it's wrong. The earliest humans, it turns out, didn't live in grasslands. Dry climate or not, a companion paper published last week in Nature shows on the basis of the other fossilized flora and fauna, as well as the chemistry of the ancient soil, that Ardipithecus ramidus kadabba lived in a well-forested environment. That's also the case with other extremely ancient hominids found during the past several years, including Ardipithecus ramidus and a species called Orrorin tugenensis , announced last December by French and Kenyan researchers. And while the ability to walk on two legs probably started out as an increasingly frequent behavior, evolution demands an explanation for why it persisted. On first blush, bipedalism just doesn't make much sense. For our earliest ancestors, it would have been slower than walking on all fours, while requiring the same amount of energy. Says Lovejoy bluntly: "It's unnatural. It's bizarre."
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