ON ITS OWN TWO FEET

By Christine Gorman

It is one of the fundamental mysteries of human evolution. When did the first apelike creatures begin to walk upright? Scientists believe this transformation probably occurred between 4 million and 6 million years ago, but until recently they had no fossils to back up their hypothesis. In 1994 researchers reported that they had found teeth and other fragments of a 4.4 million-year-old hominid in Ethiopia. But without key bones from below the waist, they could not say for sure how the animal moved.

Now, less than a year later, another team of paleontologists, led by Meave Leakey from the National Museums of Kenya and Alan Walker of Pennsylvania State University, has announced the best evidence so far that a previously unknown species of hominid strode upright at least 4 million years ago. Their find, reported in last week's Nature, consists of complete upper and lower jaws, teeth from several individuals, a piece of skull, arm bones and a leg bone. Taken together, the fossils push the emergence of two-legged walking, or bipedalism, 500,000 years earlier than any other data had indicated. Says Walker: "This gets close to the hypothesized time of splitting of the ape and human lineages."

Scientists have been scouring the African continent for fossils from the earliest humans ever since 1871, when Charles Dar win first proposed that people and apes had a common ancestor. Kanapoi and Allia Bay, the sites where Leakey and Walker made their discoveries, lie about a day's drive north of Nairobi along the shores of Lake Turkana in the East African Rift Valley. An 1,800-mile-long gash in the surface of the earth, the Rift has yielded many important clues to early human history, because of its unique geology. Layers of sediment preserved animal specimens, while the volcanic eruptions that periodically shook the valley produced ash and lava whose radioactive elements make the fossils easy to date. Probably the most famous inhabitant of the valley was the diminutive creature known as Lucy (Australopithecus afarensis), whose fossil skeleton was discovered in 1974 and who lived more than 3 million years ago.

The new discovery, which Leakey and Walker have named Australopithecus anamensis (anam is the Turkana word for lake), is yet another reason why paleontologists are reconsidering some of their ideas about why the earliest humans stood up. According to one theory, as a change in climate transformed Africa's moist forests into drier grasslands, evolution favored hom inids that could stand upright in order to spot predators lurking in the tall grasses. Other researchers argue that an upright posture lessened the heat the animals absorbed from the fierce tropical sun. Still others believe bipedalism freed the hands for carrying food or children over long distances. Such explanations appear to share a common flaw. A growing body of evidence strongly suggests that the earliest hominids did not, in fact, move out onto the savanna. The fossils they left behind were found in areas that were once densely wooded. Apparently they learned to walk in the relative safety of the forests while living next to their cousins, the apes.

Leakey and Walker have good reason to state so confidently how A. anamensis moved about. Their team unearthed pieces of the right tibia, or shinbone. "There probably isn't another bone more linked with balance and walking, and this one is absolutely remarkable," says Lee Berger, a paleontologist at the University of the Witwatersrand in South Africa who has seen the new fossils. "It's morphologically very advanced--surprisingly human-like." The top portion of the tibia is thick, in order to support the extra weight of walking on two limbs instead of four. Furthermore, a knob on top of the tibia is concave--rather than convex as it is in apes--creating the more stable knee joint needed for balance.

Unfortunately, not enough of a skeleton could be recovered to give paleontologists an idea of how large the creature was. "Its forearm is longer than mine, and I stand taller than six feet," Walker notes. "But that doesn't tell me whether anamensis was taller than I am or whether it just had long arms." Detailed measurements of the tibia, however, indicate that the hominid weighed between 104 lbs. and 121 lbs.

Anamensis differs most dramatically from all later hominids in its skull. The tooth rows in the jaw are parallel, like those of a chimp or gorilla, whereas those of humans widen in the back. Even the earholes in the skull seem more fitting for an ape than a person. "All hominids after this one have very large earholes--large enough to stick your fingers in, even though your mother tells you not to," Walker explains. "Chimps, on the other hand, have tiny earholes. Anamensis is clearly a mosaic. One part evolved and the other part didn't."

However, not everyone agrees with all of the authors' claims. Because the jaws and the tibia were recovered from separate layers of rock at Kanapoi, some paleontologists question whether they represent the same species or different ones. The lower jaw was found in the older layer of sediment, which has been dated at between 4.12 million and 4.17 million years old. The arm and leg fragments come from the upper layer, which is at least 3.5 million years old but cannot be older than 4.1 million years. By looking at rocks from Allia Bay and other sites around Lake Turkana, Leakey and Walker infer that their find must be at least 3.9 million years old. The numbers are close enough that the bones could come from the same species, but Leakey and Walker will have to find more fossils to prove the point to everyone's satisfaction.

Then there is the matter of how to fit such an unusual creature into the hominid family tree. Like all australopithecines, anamensis had relatively large teeth covered in thick enamel, indicating that its diet included nuts and hard fruits. But its jaws and skull were much more primitive than those of afarensis, even considering their difference in age.

Just as intriguing is anamensis' relationship to its forebears. When paleontologist Tim White of the University of California, Berkeley, announced last year that his team had discovered a 4.4 million-year-old hominid in Ethiopia, they named it Australopithecus ramidus and suggested that it had given rise to all australopithecines after it. Eight months later, after collecting more samples-including leg and pelvic bones-they decided to classify the fossils as a wholly separate genus , Ardipithecus ramidus. They have yet to explain why they changed their minds, but some researchers speculate that their new findings, which have not yet been published, indicate that ramidus either did not walk on two legs or had developed a gait that was neither four-legged nor two-legged but something in between.

It may simply be that walking upright was not so difficult, from an evolutionary point of view. If that is the case, then several different species could have developed variations of the trait. And they may have accomplished that feat on vastly different leg bones and knee joints. "There's nothing theoretically to preclude different types of bipedalism early in hominid evolution," says William Kimbel of the Institute of Human Origins in Berkeley, California. Argues paleontologist Berger: "We probably all have to sit back and take a hard look at how we believe early hominids evolved. It's certainly clear that even during the early stages of human evolution, there wasn't just a single species but multiple ones." The issue then becomes which species survived and eventually gave rise to modern humans.

Although neither the Ethiopian nor the Kenyan discoveries resolve the issue, they have provided paleontologists with an extraordinary peek at the very beginnings of humanity. "I think we're going to look back at the 1990s as one of the most important periods in the study of human evolution," Kimbel says. "We'll be able to answer a lot of questions as new discoveries are made." Some of the answers and, no doubt, confounding new questions lie somewhere in the ancient cradle of East Africa. "There is no time to relax," Meave Leakey declares. "We'll be returning to the same site next year."

--Reported by Andrea Dorfman/New York and Joseph Ngala/Nairobi

With reporting by ANDREA DORFMAN/NEW YORK AND JOSEPH NGALA/NAIROBI