Ardi Is a New Piece for the Evolution Puzzle

Ardi, the oldest hominid skeleton ever discovered, predating Lucy, offers unexpected clues to what our even more ancient ancestors might have looked like

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T. White

Science magazine with the skeleton of Ardipithecus ramidus, aka "Ardi."

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Scientists know this because they've studied not only Ardi's fossils but also 110 other remnants they uncovered, which belonged to at least 35 Ar. ramidus individuals. Combine those bones with the thousands of plant and animal fossils from the site and they get a remarkably clear picture of the habitat Ardi roamed some 200,000 generations ago. It was a grassy woodland with patches of denser forest and freshwater springs. Colobus monkeys chattered in the trees, while baboons, elephants, spiral-horned antelopes and hyenas roamed the terrain. Shrews, hares, porcupines and small carnivores scuttled in the underbrush. There were an assortment of bats and at least 29 species of birds, including peacocks, doves, lovebirds, swifts and owls. Buried in the Ethiopian sediments were hackberry seeds, fossilized palm wood and traces of pollen from fig trees, whose fruit the omnivorous Ar. ramidus undoubtedly ate.

This tableau demolishes one aspect of what had been conventional evolutionary wisdom. Paleoanthropologists once thought that what got our ancestors walking on two legs in the first place was a change in climate that transformed African forest into savanna. In such an environment, goes the reasoning, upright-standing primates would have had the advantage over knuckle walkers because they could see over tall grasses to find food and avoid predators. The fact that Lucy's species sometimes lived in a more wooded environment began to undermine that theory. The fact that Ardi walked upright in a similar environment many hundreds of thousands of years earlier makes it clear that there must have been another reason.

No one knows what that reason was, but a theory about Ardi's social behavior may hold a clue. Lovejoy thinks Ar. ramidus had a social system found in no other primates except humans. Among gorillas and chimps, males viciously fight other males for the attention of females. But among Ardipithecus, says Lovejoy, males may have abandoned such competition, opting instead to pair-bond with females and stay together in order to rear their offspring (though not necessarily monogamously or for life). The evidence of this harmonious existence comes from, of all things, Ardipithecus' teeth: its canine teeth are relatively stubby compared with the sharp, dagger-like upper fangs that male chimps and gorillas use to do battle. "The male canine tooth," says Lovejoy, "is no longer projecting or sharp. It's no longer weaponry."

That suggests that females mated preferentially with smaller-fanged males. In order for females to have had so much power, Lovejoy argues, Ar. ramidus must have developed a social system in which males were cooperative. Males probably helped females, and their own offspring, by foraging for and sharing food, for example — a change in behavior that could help explain why bipedality arose. Carrying food is difficult in the woods, after all, if you can't free up your forelimbs by walking erect.

Deducing such details of social behavior is, admittedly, speculative — and several researchers are quick to note that some of the authors' other major conclusions need further discussion as well. One problem is that some portions of Ardi's skeleton were found crushed nearly to smithereens and needed extensive digital reconstruction. "Tim [White] showed me pictures of the pelvis in the ground, and it looked like an Irish stew," says Walker. Indeed, looking at the evidence, different paleoanthropologists may have different interpretations of how Ardi moved or what she reveals about the last common ancestor of humans and chimps.

But Science doesn't put out special issues very often, and the extraordinary number and variety of fossils described in these new papers mean that scientists are arguing over real evidence, not the usual single tooth here or bit of foot bone there. "When we started our work [in the Middle Awash]," says White, "the human fossil record went back to about 3.7 million years." Now scientists have a trove of information from an era some 700,000 years closer to the dawn of the human lineage. "This isn't just a skeleton," he says. "We've been able to put together a fantastic, high-resolution snapshot of a period that was a blank." The search for more pieces continues, but the outlines of the puzzle, at least, are coming into focus.

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