October 2, 2009 in Nation/World

Skeleton provides clues on humans

‘Ardi’ stood upright 4 million years ago
Joel Achenbach Washington Post
 
Associated Press photo

This image provided by the journal Science shows the reconstructed frontal view of the skeleton “Ardi.”
(Full-size photo)

WASHINGTON – 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 from 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.

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.

Key find in 1994

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.

The scientists found scores of other specimens, from both males and females, though the bones were for the most part scattered and isolated. Although Ardipithecus quickly entered the paleontology lexicon in the mid-1990s, and scientists knew that this was potentially a major discovery, it was not until Thursday – and after some complaints by fellow scientists over how long the process was taking – that White and his colleagues produced a detailed description of the species.

“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 Owen Lovejoy, an anthropologist at Kent State University who was part of the Ardi team.

Search for common ancestor

The origin of the human species via evolution from earlier primates is beyond scientific dispute. Even when the fossil record of Africa was virtually nonexistent, Charles Darwin argued that human beings probably evolved from African primates. Field work over the past century confirmed Darwin’s hypothesis, which was bolstered further by laboratory analysis of the genetic codes of humans, chimpanzees and other primates.

The fine details of human origin, however, have become sketchier, and more subject to interpretation and debate, as the researchers dig deeper into the past and the fossils become scarcer, more fragmentary and in many cases more enigmatic.

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 believe the common ancestor lived about 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 Ardipithecus. 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 the Ardipithecus genus, 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, and supplied them with food. The upright walking would have made food transport easier.

Lovejoy sees male parental investment in the survival of offspring as a hallmark of the human lineage.

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