Evolution's Bad Girl: Ardi Shakes Up the Fossil Record

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She's the ultimate evolutionary party crasher. Dubbed Ardi, her partial skeleton was unearthed in Ethiopia near the scattered remains of at least 36 of her comrades. Physical anthropologists had known about the discovery of this long-gone gal for around 15 years, but few expected to see the 4.4-million-year-old hell-raiser that was unveiled in 11 scientific papers in October.

Like a biker chick strutting into a debutante ball, Ardi brazenly flaunts her nonconformity among more-demure members of the human evolutionary family, known as hominids. She boasts a weird pastiche of anatomical adornments, even without tattoos or nose studs. In her prime, she moved slowly, a cool customer whether upright or on all fours. Today, she's the standard bearer for her ancient species, Ardipithecus ramidus.

And in true biker-chick fashion, Ardi chews up and spits out conventional thinking about hominid origins, according to a team--led by anthropologist Tim White of the University of California, Berkeley--that unearthed and analyzed her fragile bones (SN: 10/24/09, p. 9). First, White and his colleagues assert, Ardi's unusual mix of apelike and monkeylike traits demolishes the long-standing assumption that today's chimpanzees provide a reasonable model of either early hominids or the last common ancestor of people and chimps--an ancestor which some scientists suspect could even have been Ardi, if genetics-based estimates of when the split occurred are borne out.

Second, the team concludes, Ardi trashes the idea that knuckle-walking or tree-hanging human ancestors evolved an upright gait to help them motor across wide ancient savannas. Her kind lived in wooded areas and split time between lumbering around on two legs hominid-style and cruising carefully along tree branches on grasping feet and the palms of the hands.

One member of White's team argues for a controversial possibility: that two-legged walking evolved because Ardipithecus males had small canine teeth. Many living and fossil male apes fight for mates by wielding formidable canines, but Ardi's male counterparts had to band together and forage over long distances to obtain mates, his thinking goes.

In a third slap at scientific convention, Ardi fits a scenario in which a few closely related hominid lineages preceded the larger-brained Homo genus that emerged around 2.4 million years ago, White says. In contrast, many anthropologists think of hominid evolution as a bush composed of numerous lineages that, for the most part, died out.

Each of Ardi's challenges draws plenty of fire. While lauding the new finds and the painstaking reconstruction of Ardi's bony frame, some critics dismiss White and company's reading of the fossils as incomplete and speculative. Presentations at the Royal Society of London in October by several members of the Ardi excavation team produced "much sparring," says anthropologist William McGrew of the University of Cambridge in England.

"There's legitimate disagreement," White says. "But Ardi provides a perspective on early hominid evolution that was previously missing. This is a really bizarre primate."

Chimp change

Ardi sports a peculiar skeletal medley that pushes chimps and gorillas out of the evolutionary spotlight, says anthropologist Owen Lovejoy, a member of White's team. Ardi's ancient remains indicate that the last common ancestor of humans and chimps must not have looked much like living chimps, as many researchers have assumed, asserts Love joy, of Kent State University in Ohio.

Since a split 8 million years ago or so, chimps and gorillas have evolved along evolutionary paths that eventually produced specialized traits such as knuckle-walking, he says.

In his opinion, Ardi indicates that a human-chimp ancestor had monkeylike limb proportions and feet, a flexible and unchimplike lower back, and an ability to move along tree branches on all fours, rather than swinging chimp-style from branch to branch and hanging by outstretched arms. …