Uncovering Ardi

What We Know / by John Hawks /

Anthropologist John Hawks explains why Ardi, the oldest known skeleton of a human-like primate, matters so much to the science of human origins.

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So what makes Ardi a hominin, a clear ancestor of you and me? Like Australopithecus, she had small, human-like canine teeth. Her molars were smaller, but stout—not at all like those of chimpanzees or gorillas. Her skull, like the earlier skull of Sahelanthropus, was carried above her spine most of the time, an indication that she saw the world from a vertical, upright posture.

Lovejoy writes that this form is such a contrast from apes that we should rethink our view of the human-chimpanzee common ancestors: We didn’t evolve from living chimpanzees, which have their own specializations not found in any ancient ape. Longer fingers and toes, knuckle-walking, very thinly enameled teeth—all these things arose in chimpanzees sometime after we diverged from them. The real “missing link” —the common ancestor of humans and chimpanzees—may
have been a lot like Ardi.

So how close is Ardipithecus to the last common ancestor? In the current issue of Genetics, yet another study of the human and chimpanzee genomes places the divergence between them at only 4.3 million years—a shade younger than Ardipithecus. So far it is not clear how accurate such estimates are, but ever since a draft chimpanzee genome became available earlier this decade, estimates have not placed the common ancestor much older. Ardipithecus was clearly at the very initial stage of human
evolution, taking the first step on the road to bipedality.

The incredible paleoecological work of the Middle Awash team also shows the environment in which Ardipithecus arose. The team followed the 3-meter-thick layer of volcanic ash and rock along its 10-kilometer extent. At one end, they collected no fossil wood, just vertebrate species whose living relatives inhabit open country. At the other, the environment had been forested, with bird and mammal species that depend on closed habitats. The team also recovered wood and pollen from abundant fossil trees, and found isotopic evidence that the ancient mammals there did not graze on grass. Ardipithecus lived only where there was clear evidence of a wooded environment—not a single tooth or bone was found in the open country. In this thousand-year ecological “transect,” the team has found the habitat in which the earliest hominins arose.

Chemical analysis of five of the Ardipithecus teeth shows that this species depended on the fruits and other woodland foods that surrounded it. Similar analyses revealed that Ardi derived some of her energy ultimately from grass, but probably by dining on grass-munching mammals and insects. Australopithecus teeth from more than a million years later give us isotopic evidence that these creatures ate a lot more of this “grass-derived” diet. This probably means that upright-walking, open-country living and a broad protein-rich diet went together. Lucy’s world, the Australopithecus world of 3 million years ago, had changed from Ardi’s. The dry season grew longer, and the East African woodlands evaporated, bringing savannas and grasslands in their stead.

This environmental shift created opportunities for a new wave of Australopithecus colonization. The earliest species, called Australopithecus anamensis, has been found living more than 4 million years ago in a territory that ranges from Middle Awash and
stretches southward into Kenya. The earliest examples of Australopithecus in far-off South Africa may be nearly as old. Did the last Ardipithecus evolve into this cosmopolitan, fully bipedal ape? Or were the last populations of Ardipithecus limited to where we’ve found them, in a tiny window of the Afar?

We won’t know until we have a better understanding of this critical period. Fossil hunters will still be laboring in the dirt and crawling over ancient rocks for a long time yet. The search for humanity’s origins goes on.

John Hawks is an associate professor of anthropology at the University of Wisconsin–Madison.

Originally published October 5, 2009

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