Wednesday, October 14, 2009

'Ardi' May Rewrite the Story of Humans


"Ardi" is the nickname given to a shattered skeleton that an international team of scientists painstakingly excavated from the Ethiopian desert, analyzed over the course of 15 years, and declared Thursday to be a major breakthrough in the study of human origins. Ardi lived more than a million years before "Lucy," a much-celebrated, 3.2 million-year-old fossil of an early human progenitor found just 45 miles away.

If the scientists are correct, Ardi and her kind were the ancestors of our ancestors. She was a transitional figure, almost a hybrid -- a tree creature who could carry food in her arms as she explored the woodland floor on two legs.

The skeletal remnants of Ardi were recovered along with bones from at least 35 other members of a species that the scientists call Ardipithecus ramidus. Their arduous investigation had incited grumbling in a scientific community that had grown impatient to find out what exactly had been found in the silty clay of Ethiopia. The answers are dramatic, detailed in 11 papers published Thursday in the online edition of the journal Science and discussed in dual press conferences in Washington and Addis Ababa, Ethiopia.

The scientists who found Ardi do not contend that she necessarily evolved into Lucy. The human line of primates could have splintered, with some species turning into genetic dead ends. Lucy's line of primates could have diverged from Ardi's line long before Ardi lived. Even so, White said he believes that his team has documented an evolutionary sequence that shows, at the genus level, where people came from. Ardipithecus, then Australopithecus, then Homo.

Lucy, a member of the species Australopithecus afarensis, was a small-brained primate that had fully adapted to a bipedal life and had expanded her habitat beyond the forest into the savannah of Africa. Unlike Ardi, she lacked the grasping big toe. Ardi and Lucy had different teeth, with Lucy's enlarged molars more adapted to a wide-ranging diet on the savannah.

"Ardi tells us twice as much as Lucy did. We have hands and feet, a more complete environment, a more complete skeleton, it's older, it's more primitive, it shows us the process of transformation from common ancestor to hominid," said C. Owen Lovejoy, an anthropologist at Kent State University who was part of the Ardi team.

The origin of the human species via evolution from earlier primates is beyond scientific dispute. Field work over the past century has shown that the human line originated in Africa, and the fossil findings have been bolstered by laboratory analysis of the genetic codes of humans, chimpanzees and other primates. The fine details of human origin, however, become sketchier, and more subject to interpretation and debate, as the researchers dig deeper into the past and the fossils become scarcer.

Scientists continue to search for the "last common ancestor," sometimes abbreviated as the LCA. This is the creature to which both modern humans and modern chimpanzees can trace their ancestry. Many scientists think the common ancestor lived at least 7 million years ago. The new research on Ardi suggests that this ancestor didn't look nearly as much like a modern chimpanzee as had been previously suspected. Rather, the ancestor would have looked more like Ardi. This suggests that chimpanzees, far from being time machines for visiting the distant past, have themselves evolved significantly, including developing such skills as suspending from branches and knuckle-walking.

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