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Fossil Find Is Missing Link in Human Evolution, Scientists Say

John Roach
for National Geographic News
April 13, 2006
 
When the famous skeleton of an early human ancestor known as Lucy was discovered in Africa in the 1970s, scientists asked: Where did she come from?

Now, fossils found in the same region are providing solid answers, researchers have announced.

Lucy is a 3.5-foot-tall (1.1-meter-tall) adult skeleton that belongs to an early human ancestor, or hominid, known as Australopithecus afarensis.

The species lived between 3 million and 3.6 million years ago and is widely considered an ancestor of modern humans.

The new fossils are from the most primitive species of Australopithecus, known as Australopithecus anamensis. The remains date to about 4.1 million years ago, according to Tim White, a biologist at the University of California, Berkeley.

White co-directed the team that discovered the new fossils in Ethiopia (map) in a region of the Afar desert known as the Middle Awash.

The team says the newly discovered fossils are a no-longer-missing link between early and later forms of Australopithecus and to a more primitive hominid known as Ardipithecus.

"What the new discovery does is very nicely fill this gap between the earliest of the Lucy species at 3.6 million years and the older [human ancestor] Ardipithecus ramidus, which is dated at 4.4 million years," White said.

The new fossil find consists mainly of jawbone fragments, upper and lower teeth, and a thigh bone.

The fossils are described in today's issue of the journal Nature.

Found Links

According to White, the discovery supports the hypothesis that Lucy was a direct descendent of Australopithecus anamensis.

Previously, Au. anamensis was known only from fossils discovered about 470 miles (750 kilometers) away in Kenya.

The new fossils also link the older Ardipithecus with the more recent Australopithecus.

In 1992 White's team discovered 4.4-million-year-old Ardipithecus ramidus fossils in the Middle Awash.

But the relationship between Australopithecus and Ardipithecus remained unclear because of the million-year gap in the fossil record.

The newfound fossils narrow this gap to 300,000 years, which is still long enough to keep the scientists from saying that Ardipithecus ramidus is the direct ancestor of Australopithecus anamensis.

However, White believes the two species are closely related.

"This discovery strengthens the ties between Ardipithecus and Australopithecus at the genus level, but what happened at the species level is going to require more fossils," he said.

Awash in Fossils

The discovery of the new fossils in the Middle Awash highlights the region as a treasure trove for the study of human origins.

Its mile-thick (1.6-kilometer-thick) column of sediment spans more than 6 million years, encompassing three major phases of human evolution.

"At 4.4 [million years] you have ramidus, at 4.1 you have anamensis, and at 3.4 in the same rock column—less than a day's walk away—you have the Lucy species," White said.

"That is unique," he added. "There is no other site like this."

In addition to the ancient hominids, the site includes fossils of more modern early-human species.

There are also thousands of fossils of animals, such as colobus monkeys, pigs, birds, rodents, and even carnivores like hyenas and big cats. Petrified wood has also been found in the area.

These fossils together "show that a closed wooded habitat type persisted over a long period in this part of the Afar," team member Giday WoldeGabriel, a geologist with the Los Alamos National Laboratory in New Mexico, said in a statement.

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