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Ancient Skeleton Could Rewrite the Book on Human Origins

By Joel Achenbach
Washington Post Staff Writer
Friday, October 2, 2009; A01

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 4 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 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 discovery of Ardi "further confirms that Ethiopia is the cradle of humankind," said Yohannes Haile-Selassie, the paleontologist who found the first two bones of Ardi in 1994.

Human origins is a field with high stakes and small bones, and the elaborate roll-out of the new research probably will trigger debate about the message contained in fossils so fragile they had to be excavated with dental picks and porcupine quills.

"It was a sort of a time capsule from 4.4 million years ago with contents that nobody had ever seen before," said Tim White, a University of California at Berkeley paleoanthropologist who led the Ardi research team. "We worked for years at opening that time capsule by collecting every shred of evidence that we could find."

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.

White and colleagues found the first signs of the new species in what is known as the Middle Awash, a site in a treeless desert that 4 million years ago would have been much wetter, teeming with birds, reptiles and primates and thickly covered with fig and palm trees. On Nov. 5, 1994, Haile-Selassie, then a Berkeley graduate student and now a curator at the Cleveland Museum of Natural History, found two finger bones. Further digging turned up scraps of a pelvis, feet, hands, chips from a skull. By the end of three years of digging, the scientists realized they'd found a paleontological treasure, a partial skeleton, broken up and ravaged by time. This was Ardi.

It was not until Thursday, after a decade and a half of work in the field and the lab, that White and his colleagues produced the first comprehensive analysis of the new species. The original fossils are being housed in a museum in Ethiopia.

"It's just a treasure-trove of surprises," Lovejoy said.

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.

"The common ancestor looked like Ardi. It's the chimp and gorilla that have evolved enormously, not hominids. Hominids have concentrated their evolution in two things -- upright walking and brain. Everything else is pretty primitive," Lovejoy said.

In Ardi's species, the males are not significantly different in size from the females. The males also lack the dagger-like teeth that male chimps use to fight one another for access to ovulating females. Lovejoy argues that this is a sign of a different social organization. The males, he argues, pair-bonded with females. Lovejoy sees male parental investment in the survival of offspring as a hallmark of the human lineage.

David Pilbeam, a Harvard paleontologist, noted that there has been some impatience in the scientific community as White and his team conducted the analysis of the new species, but he suggested that the wait was worth it: "This is an extraordinary achievement, of discovery, recovery, reconstitution, description and analysis, which will keep many others busy for at least another 15 years."

Ardi did not look like a human by any stretch. And there is no way to read her mind and measure her sense of self, her awareness of her place in the universe. But if the scientists are correct, her path in life proved to be fruitful over time, and the planet witnessed the rise of a new animal that could run on two legs, invent tools, tame fire, and perhaps eventually -- with much digging and scraping -- decipher its own origin.

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