Femur

Modern human Orrorin

Chimpanzee

Short femoral neck

Short femoral neck

Humanity may have arisen more than a million years earlier than a number of molecular studies had estimated. More important, it may have originated in a different locale.

to previously out-of-reach foods, or afforded them a better view above the tall grass. But paleoecological analyses indicate that Orrorin and Ardipithecus dwelled in forested habitats, alongside monkeys and other typically woodland creatures. In fact, Giday WoldeGabriel of Los Alamos National Laboratory and his colleagues, who studied the soil chemistry and animal remains at the A. r. kadabba site, have noted that early hominids may not have ventured beyond these relatively wet and wooded settings until after 4.4 million years ago.

If so, climate change may not have played as important a role in driving our ancestors from four legs to two as has been thought. For his part, Lovejoy observes that a number of the savanna-based hypotheses focusing on posture were not especially well conceived to begin with. "If your eyes were in your toes, you could stand on your hands all day and look over tall grass, but you'd never evolve into a hand-walker," he jokes. In other words, selection for upright posture alone would not, in his view, have led to bipedal locomotion. The most plausible explanation for the emergence of bipedalism, Lovejoy says, is that it freed the hands and allowed males to collect extra food with which to woo mates. In this model, which he developed in the 1980s, females who chose good providers could devote more energy to child rearing, thereby maximizing their reproductive success.

The Oldest Ancestor?

THE PALEOANTHROPOLOGICAL community was still digesting the implications of the Orrorin and A. r. kadabba dis coveries when Brunet's fossil find from Chad came to light. With Sahelanthropus have come new answers—and new questions. Unlike Orrorin and A. r. kadabba, the Sahelanthropus material does not include any postcranial bones, making it impossible at this point to know whether the animal was bipedal, the traditional hallmark of humanness. But Brunet argues that a suite of features in the teeth and skull, which he believes belongs to a male, judging from the massive brow ridge, clearly links this creature to all later hominids. Characteristics of Sa-helanthropus's canines are especially important in his assessment. In all modern and fossil apes, and therefore presumably in the last common ancestor of chimps and humans, the large upper canines are honed against the first lower premolars, producing a sharp edge along the back of the canines. This so-called honing canine-premolar complex is pronounced in males, who use their canines to compete with one another for females. Humans lost these fighting teeth, evolving smaller, more incisorlike canines that occlude tip to tip, an arrangement that creates a distinctive wear pattern over time. In their size, shape and wear, the Sahelanthropus canines are modified in the human direction, Brunet asserts.

At the same time, Sahelanthropus exhibits a number of apelike traits, such as its small braincase and widely spaced eye sockets. This mosaic of primitive and advanced features, Brunet says, suggests a close relationship to the last common ancestor. Thus, he proposes that Sahelanthropus is the earliest member of the human lineage and the ancestor of all later hom-inids, including Orrorin and Ardipithecus. If Brunet is correct,

HUNTING FOR HOMINIDS: Michel Brunet [left), whose team uncovered Sahelanthropus, has combed the sands of the Djurab Desert in Chad for nearly a decade. Martin Pickford and Brigitte Senut [center) discovered Orrorin in Kenya's Tugen Hills. Tim White [top right] and Yohannes Haile-Selassie [bottom right] found Ardipithecus in the Middle Awash region of Ethiopia.

humanity may have arisen more than a million years earlier than a number of molecular studies had estimated. More important, it may have originated in a different locale than has been posited. According to one model of human origins, put forth in the 1980s by Yves Coppens of the College of France, East Africa was the birthplace of humankind. Coppens, noting that the oldest human fossils came from East Africa, proposed that the continent's Rift Valley—a gash that runs from north to south—split a single ancestral ape species into two populations. The one in the east gave rise to humans; the one in the west spawned today's apes [see "East Side Story: The Origin of Humankind," by Yves Coppens; Scientific American, May 1994]. Scholars have recognized for some time that the apparent geographic separation might instead be an artifact of the scant fossil record. The discovery of a seven-million-year-old hominid in Chad, some 2,500 kilometers west of the Rift Valley, would deal the theory a fatal blow.

Most surprising of all may be what Sahelanthropus reveals about the last common ancestor of humans and chimpanzees. Paleoanthropologists have typically imagined that that creature resembled a chimp in having, among other things, a strongly projecting lower face, thinly enameled molars and large canines. Yet Sahelanthropus, for all its generally apelike traits, has only a moderately prognathic face, relatively thick enamel, small canines and a brow ridge larger than that of any living ape. "If Sahelanthropus shows us anything, it shows us that the last common ancestor was not a chimpanzee," Berkeley's White remarks. "But why should we have expected otherwise?" Chimpanzees have had just as much time to evolve as humans have had, he points out, and they have become highly specialized, fruit-eating apes.

Brunet's characterization of the Chadian remains as those of a human ancestor has not gone unchallenged, however. "Why Sahelanthropus is necessarily a hominid is not particularly clear," comments Carol V. Ward of the University of Missouri. She and others are skeptical that the canines are as hu

manlike as Brunet claims. Along similar lines, in a letter published last October in the journal Nature, in which Brunet's team initially reported its findings, University of Michigan pa-leoanthropologist Milford H. Wolpoff, along with Orrorin discoverers Pickford and Senut, countered that Sahelanthropus was an ape rather than a hominid. The massive brow and certain features on the base and rear of Sahelanthropus's skull, they observed, call to mind the anatomy of a quadrupedal ape with a difficult-to-chew diet, whereas the small canine suggests that it was a female of such a species, not a male human ancestor. Lacking proof that Sahelanthropus was bipedal, so their reasoning goes, Brunet doesn't have a leg to stand on. (Pickford and Senut further argue that the animal was specifically a gorilla ancestor.) In a barbed response, Brunet likened his detractors to those Dart encountered in 1925, retorting that

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