JANUARY 17, 2000 VOL. 155 NO. 2

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SPLIT FROM THE APES

As recently as five years ago, all that scientists could really tell about our earliest ancestors was when they first appeared. Molecular biologists had measured the differences between human and chimpanzee DNA, then averaged the rate of genetic change over time. By calculating backward, they determined that great apes and hominids branched from a common ancestor between 6 million and 4 million years ago. But no fossils were on hand to support this scenario. The oldest hominid species known, Australopithecus afarensis (southern ape of the Afar), could be dated back only 3.6 million years. Its most famous member, Lucy, unearthed in Ethiopia's bleak Afar Triangle in 1974, is a mere 3.2 million years old.

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Then, in 1994 and 1995, teams working in Ethiopia and Kenya announced that they had each found a new species of hominid. Both discoveries smashed the 4 million-year barrier. The first--and at 4.4 million years, the oldest--was dug up by an international team in the Middle Awash region of Ethiopia, about 80 km south of where Lucy was discovered.

All told, the scientists excavated the bones and teeth of 17 individuals. Given their age, no one was surprised that they showed a mix of chimpanzee-like and human traits that as a whole are more primitive than those of A. afarensis: smaller molars, larger canines and thinner tooth enamel, suggesting a diet rich in easy-to-chew fruits and vegetables. The new species, says paleontologist Tim White of the University of California at Berkeley, a co-leader of the expedition, "is way closer to an ape than to an australopithecine and is significantly different from any other hominid."

Because the fossils were too distinctive to be included in Lucy's extended Australopithecus family, the researchers called the new species Ardipithecus ramidus (ardi means ground or floor in the local Afar language, and ramid means root). White and his colleagues have since found other ramidus fossils at their site but are giving out precious few details until they complete their methodical analysis of the bones. Says ramidus co-discoverer Berhane Asfaw of the Rift Valley Research Service in Addis Ababa: "It will be worth the wait."

One prize specimen, they acknowledge, is a partial skeleton found by Berkeley graduate student Yohannes Haile-Selassie (no relation to the Emperor). Alas, the back of the skull is badly crushed. A hippo or elephant probably trampled it soon after the creature died. "It looks like roadkill," quips White. Given the small skulls of A. afarensis and other later australopithecines, however, this specimen undoubtedly had a pint-size brain. At this point in evolution, says White, "we're in the minor leagues of brain development."

But the skeleton does include many bones that will help White's team answer the much more important question of how Ardipithecus got around. Paleoanthropologists believe that bipedalism was the first significant modification separating our ancestors from the great apes. By studying the bones and fossil footprints of A. afarensis (Lucy and her line) as well as those of half a dozen other australopithecine species, scientists already knew that our ancestors walked upright long before they acquired other human traits--and that bipedalism gave them a huge edge.

According to conventional wisdom, this evolutionary breakthrough came at a time when climate change was transforming eastern and southern Africa from dense forest into open grassland. Standing upright in such an environment could have offered our ancestors many advantages. It could have let them scan the horizon for predators, exposed less body surface to the scorching equatorial sun (and, conversely, more to the cooling wind) or freed their hands for carrying food.

But these ideas may be in trouble. Fossilized seeds, petrified wood and animal bones found by White and his colleagues at the digging site, near the village of Aramis, indicate that it was quite densely wooded when A. ramidus lived there. If the hominid turns out to have been bipedal, as preliminary studies indicate, this could wash away existing theories--though the scientists can't say for sure until other hominid fossil sites of comparable age are found.

Even if ramidus didn't walk upright, however, another of the recently discovered human ancestors certainly did. Less than a year after A. ramidus made headlines, a team led by Meave Leakey of the National Museums of Kenya (wife of well-known fossil hunter Richard Leakey) and Alan Walker of Pennsylvania State University revealed that it too had found fossils of an ancient human ancestor at two sites near Lake Turkana, in Kenya. Not only is the new hominid very old, dating to 4.2 million years B.P., but it is similar in some ways to A. afarensis--though clearly more primitive. Given the family resemblance, Leakey and Walker assigned the fossils to the same genus, Australopithecus, and gave the new species the name anamensis (anam is the Turkana word for lake).

Several of the bones underscore that A. anamensis did indeed walk upright, some 500,000 years before the next oldest two-legged hominid known. But these creatures didn't walk in the modern sense. As Leakey explains, "They weren't nearly as efficiently upright as we are, and they had relatively short legs. They had a form of locomotion that we don't know today because there isn't anything equivalent."

Precisely where do A. ramidus and A. anamensis fit into the scheme of human evolution? Leakey believes the latter is a direct ancestor of A. afarensis and thus a direct ancestor of modern humans. White and his colleagues have tentatively labeled the older ramidus a "sister species" of all later hominids; it's either our direct ancestor or a close relative of that ancestor. Whichever ramidus turns out to be, it's clear that paleontologists are closing in on the split between apes and humans. "We're in the ballpark. Five or 10 years ago, we couldn't even have conceived of this," asserts White. "Ardipithecus is the closest thing we currently have to the common ancestor of African apes and humans, but its derived characteristics, particularly its teeth, suggest that it postdates that ancestor."

As for the ancestor, White hints that his team has already discovered hominid fossils that are more than 5 million years old, though he refuses to elaborate before detailed studies are completed. But Leakey and Walker readily acknowledge that they are studying two 5.5 million-year-old hominid teeth and a similarly ancient jaw fragment with an embedded tooth from a site in northern Kenya. "They look like australopithecines with lots of primitive features," Walker says, but there isn't enough evidence from these fossils alone to claim a new species.

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