Scientists: 'Ardi' Fossil Sheds Light on Origin of Human Species
Thursday, October 1, 2009; 10:33 AM
Ardi lived 4.4 million years ago in the woodlands of East Africa. She spent most of her time in the trees. She stood about four feet tall, weighed 110 pounds, and had long arms, short legs, and a grasping big toe that was perfect for clambering branch to branch. She ate in the trees, raised her offspring in the trees, slept in the trees.
But sometimes she came down to the ground, and stood upright. She could walk on two legs. She was, in a sense, taking baby steps on a journey that would change the world.
"Ardi" is the nickname given to a remarkable, shattered skeleton that an international team of scientists believes is a major breakthrough in the study of human origins. The skeletal remains were painstakingly recovered from the Ethiopian desert along with bones from at least 35 other members of a species scientists call Ardipithecus ramidus. The 15-year investigation of Ardipithecus culminated Thursday in the publication of a raft of papers in the online edition of the journal Science, as well as dual press conferences in Washington and Addis Ababa, Ethiopia.
"This is huge. This is the biggest discovery really since the 'Lucy' skeleton of the 1970s," said Carol Ward, a University of Missouri paleoanthropologist who was not involved with the research but had been given a preview so that she could offer an independent assessment.
Human origins is a field with high stakes and small bones, and the elaborate roll-out of the Ardipithecus research probably will trigger debate about the message contained in fossils so fragile they had to be excavated with dental picks and porcupine quills. If the scientists who found Ardi are correct, she represents 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.
Ardi lived more than a million years before Lucy, the name given to a 3.2 million-year-old skeleton found in 1974 that is the best example of Australopithecus afarensis, a small-brained primate that had fully adapted to a bipedal life and had expanded its habitat beyond the forest into the savannah of Africa.
Unlike Ardi, she lacked the grasping big toe that extends laterally from the foot. Lucy's big toe pointed forward, aligned with the other toes, and was used for propulsion. Ardi and Lucy had different teeth, with Lucy's enlarged molars more adapted to a wide-ranging diet on the savannah. But Ardi and Lucy had rather similar faces, skulls, hands, and pelvises.
The scientists who found Ardi do not contend that Ardi necessarily evolved into Lucy, or that Ardipithecus ramidus was necessarily a direct human ancestor. The human family of primates could have splintered into multiple species along the way, with some winding up as genetic dead ends. If that were the case, Ardi would be more of a distant cousin to human beings rather than a direct forebear.
"The individual, Ardi, that female individual, is she our ancestor?" said Tim White, a University of California at Berkeley paleoanthropologist who led the research team. "And the answer is, probably not. If she didn't have any kids, tough luck, she's nobody's ancestor."
The Ardi team, however, does make the case that the genus Ardipithecus, which could have encompassed a number of species, is ancestral to the genus Australopithecus. Thus the general body plan of Ardi would evolve into the general body plan of Lucy, and on down the line until the genus Homo appears.
"The Ardipithecus genus gave rise to Australiopithecus even though we can't say exactly what species did. Maybe ramidus did. But certainly something like ramidus did," White said.
White and colleagues found the first signs of Ardipithecus in 1994 in what is known as the Middle Awash, a treeless desert that 4 million years ago would have been much wetter, teeming with birds, reptiles, primates and thickly covered with fig and palm trees. A key moment came Nov. 5, 1994, when a Berkeley graduate student, Yohannes Haile-Selassie of Ethiopia, found fragments of two finger bones. Further digging turned up scraps of a pelvis, feet, hands, chips from a skull. By January 1995 the scientists realized they'd found a paleontological treasure, a partial skeleton, broken up and ravaged by time. This was Ardi.