October 1, 2009

Oldest Hominid Remains Offer Clues Of Human Evolution

A 17-year investigation into the discovery of the fragile remains of a small "ground ape" discovered in Ethiopia is described today in a special issue of the journal Science.

The report includes 11 papers about the discovery of the Ardipithecus fossils, which include a partial skeleton of a female nicknamed "Ardi", the earliest known skeleton from the human branch of the primate family tree.  The branch includes Homo sapiens as well as species closer to humans than to chimpanzees and bonobos.

Nearly 15 scientists from 10 different countries were responsible for the 1994 discovery, which provides new insights about how hominids"”the family of "great apes" comprising humans, chimpanzees, gorillas and orangutans"”may have emerged from an ancestral ape.

The last common ancestor shared by humans and chimpanzees is thought to have lived six or more million years ago. Though Ardipithecus is not itself this last common ancestor, it likely shared many of this ancestor's characteristics.

Until Ardi's discovery, the earliest well-known stage of human evolution was Australopithecus, a small-brained, bipedal "ape man" that lived between 4 million and 1 million years ago. 

The most famous Australopithecus fossil is the 3.2-million-year-old "Lucy," discovered in 1974 about 45 miles north of where Ardi would later be found. However, Ardi's skeleton and associated Ardipithecus ramidus remains are older and more primitive than Australopithecus.

After Lucy's discovery, there was some expectation that when earlier hominid remains were found, they would converge to a chimpanzee-like anatomy, based on the genetic similarity of humans and chimps. However, the Ardipithecus ramidus did not bear that out.

Ardi's skeleton contains enough of the skull, teeth, pelvis, legs, feet, arms, and hands to estimate her body weight and height.  They also show that she walked on two legs on the ground, but also climbed trees and spent time in them.  She was likely omnivorous.

Remarkably, Ardi and her companions did not have limb proportions like chimps or gorillas, but rather like those of extinct apes or even monkeys.  Her hands also are not chimpanzee- or gorilla-like, but are rather more closely related to earlier extinct apes.

Los Alamos geologist Giday WoldeGabriel, one of the scientists involved in the discovery of Ardi, led the field geology investigations and sampling of ancient lavas and ashes that were used to determine the age of the fossilized remains. He was also able to precisely characterize the environment in which Ardi lived.

Ardi's woodland home included fresh-water springs and small patches of fairly dense forest with palm trees at the edges and grasslands extended perhaps many kilometers away.

"It is a privilege to have the opportunity to look back in time into the lives of mankind's oldest relatives," said WoldeGabriel.

"This is a fascinating and important discovery."

Because of its antiquity, Ardipithecus takes us closer to the still-elusive last common ancestor. However, many of its traits do not appear in modern-day African apes.

One surprising conclusion is that it is likely that the African apes have evolved extensively since we shared that last common ancestor, making living chimpanzees and gorillas poor models for the last common ancestor and for understanding our own evolution since that time.

"In Ardipithecus we have an unspecialized form that hasn't evolved very far in the direction of Australopithecus. So when you go from head to toe, you're seeing a mosaic creature, which is neither chimpanzee, nor is it human. It is Ardipithecus," said Tim White of the University of California Berkeley, one of the lead authors of the research.

"With such a complete skeleton, and with so many other individuals of the same species at the same time horizon, we can really understand the biology of this hominid," said Gen Suwa of the University of Tokyo, Project paleoanthropologist and also a lead author of the Science report.

"These articles contain an enormous amount of data collected and analyzed through a major international research effort. They throw open a window into a period of human evolution we have known little about, when early hominids were establishing themselves in Africa, soon after diverging from the last ancestor they shared with the African apes," said Brooks Hanson, deputy editor, physical sciences, at Science.

Other fossils associated with Ardi included fig and hackenberry trees, land snails, birds such as owls, parrots, and peafowl, and small mammals such as shrews, mice, and bats.  Other animals, such as porcupines, hyenas, bears, pigs, rhinos, elephants, giraffes, two kinds of monkey, and several different types of antelope, were also associated with Ardi.

This research, in the form of 11 detailed papers and more general summaries, will appear in the October 2, 2009 issue of the journal Science, published by the nonprofit science society AAAS.


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