Oldest Skeleton in Human Family Tree Surprises Scientists

THURSDAY Oct. 1, 2009 -- New details about the oldest skeleton from the human family tree suggest that human evolution was much more complicated than ever imagined.

Fossil remains of "Ardi," a female member of the hominid species Ardipithecus ramidus who lived 4.4 million years ago, do not resemble a chimp, as was long supposed.

Instead, Ardi was a combination of the features of chimps, humans and other creatures.

"Our ancestors didn't come from something that looked like a chimp, and we now have a very good picture of what it was like and what kind of environment it lived in," said Leslea Hlusko, associate professor of integrative biology at the University of California Berkeley. "Our last common ancestor [shared with chimpanzees] was not a monkey but a different primate. It wasn't a human and it wasn't a chimp. It was something else."

Hlusko is one of the researchers publishing a series of 11 articles about Ardi and her environment in the Oct. 2 issue of Science.

"This is earth-shaking. It challenges the simple paradigm that we've had for a long time of our earliest ancestors being very chimp- or gorilla-like," added Jeffrey Laitman, director of the Center for Anatomy and Functional Morphology at Mount Sinai School of Medicine in New York City. "Now we have this odd beast that is tantalizingly closer to the elusive last common ancestor, and already it's showing features that are different than chimps and gorillas, and in some cases closer to humans."

The findings also suggest that chimps and gorillas have followed their own evolutionary path, quite separate from humans.

"This is sort of a declaration of independence for gorillas and chimps. They're not just transitional types of humans," Laitman said.

Fossil remains of Ardi, who weighed about 110 pounds and stood close to four-feet tall, were first discovered in a remote part of northeastern Ethiopia in 1994. She is 1.2 million years older than "Lucy," an Australopithecus, who, until now, was the earliest existing evidence of the path of human evolution.

It has taken 17 years for scientists to piece together a portrait of Ardi's life and times.

Ardi has smaller canines than chimps, was able to walk on two feet on the ground and, thanks to a "grasping big toe," could also travel easily in trees. Her limbs and hands more closely resembled those of now-extinct apes.

And the fragments of Ardi's skeleton had remained in the same place for 4.4 million years. "Where the animal fell to the ground was where it got buried," said Hlusko. "The animal and plant material stayed exactly where it fell, which is just amazing."

This enabled Giday WoldeGabriel, another author on the papers and a geologist with Los Alamos National Laboratory in New Mexico, to accurately (give or take 60,000 or 80,000 years) pinpoint a date using three different methodologies.

"We made sure they hadn't been moved or mixed with something else," said WoldeGabriel, who is originally from Ethiopia.

Researchers were also able to reconstruct Ardi's environment to a degree not possible for Lucy, because bits and pieces of her history had long been washed away or moved.

Ardi lived in a forested area with grasslands possibly several miles away. This detail challenges one school of thought which holds that bipedalism came about as a response to living in grasslands.

Scientists also found thousands of specimens from small mammals in the area, along with indications of fig trees, birds (including owls and parrots) and larger mammals such as elephants and giraffes.

Ardi is not the last common ancestor but she is "tantalizingly close," Laitman said.

"It's showing us that if you went even earlier and found the Great Elusive One, that that individual would give rise to distinctive features of chimps and gorillas and distinctive features of Ardi and eventually us," he added. "That's why this is such a tantalizing paradigm shift. It's telling us that the template we were all looking at was too simplistic. This is really going to shake a lot of tree branches."

More information

The University of California Museum of Paleontology has more on evolution.

Posted: October 2009


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