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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All images courtesy of Science/AAAS

Ten kilometers of a lost millennium jut from the low hills of the Middle Awash, near a river in the Afar Region of northeastern Ethiopia. Afar is one of the most active volcanic regions of the world and is blanketed with the basalt remnants of old lava flows. But the lava spared a window where today the Awash River bends, and fossil hunters creep the ground, seeking 
ancient shards of bone. Fifteen years ago, bent beneath the equatorial sun, the anthropologist Yohannes Haile-Selassie laid his hands on, well, a hand.

Four and a half million years earlier, a volcanic eruption had scoured life from the Middle Awash with a thick corrosive drift of ash and glass. In time, the trees and grasses 
returned, and, as proven by the thousands of time-worn teeth found littering the ground, so did monkeys, bushbucks, and a species of ape
called Ardipithecus ramidus, which, together with other primate ancestors of humans and chimpanzees, are collectively known as “hominins.” A thousand years later, another eruption once again blasted hot rock, mud, and ash across the Middle Awash, sandwiching the landscape and all its creatures into a 3-meter thick layer, creating a sort of Pliocene epoch time machine. Fossil wood, hackberry seeds, even the balls rolled by dung
beetles were all trapped like flies in amber.

But nothing like a fossilized Ardipithecus hand was supposed to be there, or at least the team led by paleoanthropologist Tim White had almost given up hope of finding such things. White had been intermittently exploring the Middle Awash site since 1981; by 1994, it was clear that the ancient bones there crumbled almost as fast as nature could unearth them. And yet the hand bones were there, and even the fingers. The team brushed away the dirt, an inch a day, eventually rescuing a crushed female skeleton that they named “Ardi.”

Much of the process of Ardi’s discovery would’ve been recognizable to paleontologists 60 years ago. In 1947, Robert Broom and John Robinson unearthed the first partial skeleton of another hominin, Australopithecus, in the South African cave of
Sterkfontein. Two and a half million years old, as we now understand, it was at the time the earliest hominin skeleton. They set to work reconstructing and describing it, putting its anatomy into context with living apes and humans, as well as with other extinct ancestral species. Across those 60 years, one technological advance after another has deepened our knowledge of Australopithecus—better and more fossil specimens, new means of measuring their features and territorial ranges, chemical analyses of their diet, CT scans of their internal anatomy, and computer-aided reconstructions.

White’s team, drawing upon the work of more than 50 experts, has marshaled the accumulated techniques to produce an incredible series of 11 papers, published simultaneously in the journal Science. Like the Six Million Dollar Man, they had the technology, and they rebuilt
Ardipithecus. Their careful years of analysis clearly show that Ardi is the oldest known skeleton of an ancestral species to humans, dating from a hazy period of history much earlier than Australopithecus, only a scant few million years after humans diverged from chimpanzees.

Most paleoanthropological news stories are “firsts” —the first hominin, the first evidence of apes using tools or fire, the first indications that Neandertals ate dolphin meat (it’s true!). Such “firsts” are news of a sort, but a sterile kind. A place on a map, a date on a calendar, they’re the paleontological equivalent of 
“Kilroy was here!” Once, the teeth of Ardipithecus were all that was known of this species and were thought to possibly represent the “first” hominin. But they’ve since lost that claim to older fossils—originally to Orrorin tugenensis from Kenya in 2000, then to
Sahelanthropus tchadensis in 2002, which is nearly 7 million years old. The thousand-year ribbon of rock at Middle Awash from whence Ardi came sits not quite two-thirds of the way back to these earlier species that we know only by their teeth. What makes Ardipithecus singular is the skeleton: To anatomists, Ardi is not a mere point on a map. It is the map.

As paleoanthropologist C. Owen Lovejoy describes it, Ardi gives us a view of a previously unknown “adaptive plateau” among early hominins—a suite of anatomical and behavioral characteristics that lasted for a long, stable period in the early Pliocene environment. The Ardipithecus form might account for the bulk of the whole story of human evolution—a kind of hominin that was different from anything that came before or after.

Humans’ big brains, lanky bodies, and rock-hammering technology have formed our own adaptive plateau for almost 2 million years. Before
that, the familiar Australopithecus spanned another 2 million. Australopithecus walked upright in a very human-like way, with long
legs and short arms quite unlike those of living great apes. Looking beneath the neck, one might mistake an Australopithecus skeleton for that of a very little, very muscular human. What lay above the neck was unique—a small, almost ape-sized brain, combined with
massive grinding molars but without the big canine teeth (fangs) of 
today’s apes. A myopic time-traveling anthropologist just might mistake Australopithecus for an ape.

With Ardipithecus, seeing an ape would be no mistake. Ardi’s feet were ape feet, with a stout opposable big toe. Her hips weren’t at all like that of another famous girl, “Lucy,” the partial skeleton of an Australopithecus afarensis. With their odd morph between chimpanzee and human-like features, Ardi’s pelvic bones look like a Hollywood CGI creation. She was made for the trees—on the ground she walked with a kind of waddling gait. Ardi’s hand was longer than her shinbone. For comparison, sit down and bend your leg at the knee. With the heel of your hand just under your knee, chances are you can reach halfway down your shin. From that same position, Ardi could tickle her toes.

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