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Were our ancestors pea-brained? Maybe not, say scientists. (+video)

New reconstructions of an iconic fossil could shift our understanding of how humans evolved.

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    This is a movie taken of a cast of the type specimen of Homo habilis found at Olduvai Gorge in Tanzania.
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What if the feature that made you uniquely “human” wasn't your superior intellect, but your jawbone?

For one early human ancestor at least, that seems to be the case.

With X-ray and digital modeling equipment, researchers at the Max Planck Institute for Evolutionary Anthropology were able to reconstruct a damaged fossil of a human jawbone. In doing so, they determined that mandibular, or "gnathic" features – not brain size – are crucial to understanding humankind's most primitive forebears. Their findings appear today in Nature.

In 1964, famed paleoanthropologist Louis Leakey announced a new species of early human: Homo habilis, or Handy Man. The announcement was based heavily on OH 7 – a fossilized mandible unearthed by Mary and Jonathan Leakey in the Olduvai Gorge, Tanzania. Decades of controversy followed – some academics objected to Leakey’s placement of the specimen within the Homo genus, citing its primitive and apelike appearance.

But in Leakey’s opinion, H. habilis’s less-protruding face and apparent use of tools suggested that it was a transitional species, bridging the gap between the older Australopithecus genus and more modern species within Homo. If so, H. habilis was the earliest known member of the genus, possibly giving rise to the more recent progenitors. That proved difficult to substantiate, however, because the OH 7 mandible was too distorted and damaged to accurately compare it to other fossil evidence.

But now, aided by an imaging process called computer tomography, lead author Fred Spoor and his team have a way to work out the kinks, as it were, in the OH 7 jaw – thereby opening it up to comparative studies.

And if the reconstruction is any indication, it may be time for an anthropological paradigm shift. For decades, the prevailing assumption had been that human ancestors developed larger brains over evolutionary time. As such, hominid species are often distinguished by their brain size. By that logic, the chronologically older H. habilis should have a very small brain. But in Dr. Spoor's reconstruction, OH 7 has a bigger cranium than expected – comparable to that of H. erectus, a species 500,000 years younger – indicating that it had a larger brain.

But the lower jaw from OH 7 was primitive, resembling that of the much older Australopithecus afarensis (the famous ‘Lucy’ was a member of A. afarensis). In other words, H. habilis was simultaneously more and less “human-like” than expected. It implies a notable diversity among members of the genus – and according to Spoor, it could also change how we classify early hominids.

"Differences between species of early Homo appear to be characterized more by gnathic diversity than by differences in brain size, which was highly variable," Spoor wrote in the study.

The fossil record of H. habilis goes back 2.3 million years, but this primitive mandible could also be a sign that the species originated even earlier, Spoor says. New papers published today in Science describe a similar mandible that dates back to 2.8 million years, and potentially belonged to a transitional species between Australopithecus and Homo.

"By digitally exploring what Homo habilis really looked like we could infer the nature of its ancestor," Spoor said in a statement, "but no such fossils were known. Now the Ledi-Geraru jaw has turned up as if 'on request,' suggesting a plausible evolutionary link between Australopithecus afarensis and Homo habilis."

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