Ancient jaw helps fill void in evolution

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A Cleveland Museum of Natural History-led research team scouring the sun-baked badlands of central Ethiopia has unearthed jaws and teeth of early human ancestors that lived 3.5 million to 3.8 million years ago.

The specimens, whose discovery was announced Tuesday in Addis Ababa, Ethiopia, are from a crucial period in human evolution that had been sorely lacking in fossils for study.

The Cleveland team found a partial skeleton of comparable age in 2005. Time-wise, the skeleton, jaws and teeth collectively fill a void between an older hominid group called Australopithecus anamensis that lived in Africa 3.9 million to 4.2 million years ago, and the younger Australopithecus afarensis species of 3 million to 3.6 million years old represented by the famous “Lucy” fossil.

The newly uncovered fossils should help scientists sort out the relationship between those two species – whether there was a direct line of descent, or a branching of the family tree.

“The current hypothesis, which so many people seem to accept, is that they were ancestral descendents – that anamensis gave rise to afarensis,” Yohannes Haile-Selassie, expedition co-leader and anthropology curator at the Cleveland museum, said in a phone interview from Addis Ababa, Ethiopia. “To test that, we need fossils. That’s why we think these specimens are really, really important.”

The partial skeleton’s discovery two years ago was a major find, since its pelvis and leg bones, once cleaned and reassembled, should help researchers judge how well the creature walked upright. Anthropologists consider a two-legged gait a defining evolutionary feature that distinguishes early humans from their more-primitive ancestors.

But a complete lower jaw and teeth the team found in February about a mile from the skeleton site and in the same geological layer of time has the potential to be even more significant. It is more than 3.6 million years old, Haile-Selassie said.

The previously discovered skeleton lacked a skull and jaw.

Scientists rely on cranial and dental features to pin down the evolutionary group to which a hominid belongs.

Both A. anamensis and A. afarensis walked upright. But their jaws and teeth were markedly different.

A. anamensis’ jaw was narrow, its sharply pointed canine teeth were quite large, and the crowns atop its molars were short. A. afarensis, which lived as much as 1 million years later and considerably farther north, under presumably different living conditions, had a wider-spread lower jaw, less prominent canines, and molars with taller crowns suited for tougher chewing and grinding. Fossil jaws slightly younger than those found by the Cleveland-led team and classified as afarensis show a mixture of those features.

“We don’t have enough evidence to show this, but one hypothesis would be that this is one continuously evolving population and anamensis evolved gradually into afarensis,” said Pennsylvania State University anthropologist Alan Walker, the co-discoverer of the anamensis species in 1995.

Rather than a single line of hominid descent from 4 million to 3 million years ago, some researchers contend evolution produced a “bushy” family tree, with more than one species splitting away and existing at the same time. By some accounts, A. afarensis – whose most celebrated specimen is the stubby partial skeleton nicknamed “Lucy” – is on a side branch, not the one leading to anatomically modern humans in the Homo family.

The newly found jaw and teeth could help clarify Lucy’s lineage and address the “bushy versus straight” debate.

If A. anamensis directly evolved into afarensis, fossils from the time period between the two species should “bridge the anatomical distance,” said University of California, Berkeley, anthropologist Tim White, who co-discovered the afarensis species along with Donald Johanson, Haile-Selassie’s predecessor at the Cleveland museum in the 1970s, White said. “Those are the characters that Yohannes will focus on – the teeth and jaws. It’s really important to get a jaw.”

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