Image: Jawbone
The jawbone of a Stone Age hominid -- a rare find with all teeth in good condition -- offers new insight into the evolution and behavior of humankind's earliest known ancestors to use stone tools.
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Science

A 1.8-million-year-old jawbone and other fossils uncovered in Tanzania’s Olduvai Gorge have reignited a longstanding controversy about the family tree of humankind’s earliest ancestors. At the same time, the finds offer a new look at how and where early humans lived, according to a study in the journal Science, published by the American Association for the Advancement of Science.

Anthropologist can extract a remarkable amount of information from a few skeletal fragments, and the new find was no exception. Team members quickly realized that the upper jaw and bones of the lower face belonged to one of the oldest species in the genus Homo — which includes modern humans, Homo sapiens. According to scientific custom, the fossils are now known as Olduvai Hominid (OH) 65.

OH 65 was a spectacular discovery. The fossils are beautifully preserved, according to team co-leader Robert Blumenschine of Rutgers University, and for the first time an upper jaw of earliest Homo had been found with all teeth in good condition. Today, he says OH 65 may be “one of the top five” of the 50 or so known fossils of these early human ancestors. Ironically, Blumenschine has also described the discovery as “a mixed blessing.”

Although it was “tremendously exciting to hold the bones, knowing there are so few in the world,” the discovery carried with it the daunting responsibility of meticulously analyzing the fossils and publishing the findings, he said. Moreover, Blumenschine’s group had started with quite different goals. “Finding body fossils is not the end-all and be-all” in studies of human evolution, he said.

With Fidelis Masao of Tanzania and Charles Peters of the University of Georgia, Blumenschine co-directs the Olduvai Landscape Paleoarchaeology Project. These researchers focus on stone tools and animal bones bearing butchery marks to reveal the activities of long-ago human ancestors. Fortunately, such specimens occurred in abundance along with OH 65, so the find has shed light on both the evolution and behavior of early humans.

Question mark in the family tree
Olduvai Gorge is one of the world’s richest troves of early human fossils, and archaeologists have been at work in the area for nearly 100 years. In 1960, researchers there discovered a lower jaw of the first stone-tool-using human ancestors.

Anthropologists Louis Leakey, Phillip Tobias and others dated the find to about 2 million years ago and named the species Homo habilis. At that time, experts believed that H. habilis evolved directly into later hominids and eventually into modern humans.

However in 1972, Leakey’s son Richard unearthed part of a skull in Kenya that dated to the same time period. It was similar to H. habilis, but displayed a larger cranial capacity and differences in the brow ridges and cheekbones.

Researchers debated where in humankind’s family tree the new fossil belonged, eventually describing it as a new species called Homo rudolfensis. They drew a question mark in diagrams of human evolution, wondering how these two groups of humans interacted and which one gave rise to the peoples of today.

Enter OH 65: Its upper jaw and teeth provide what Blumenschine called “a key anatomical link” between the lower jaw of H. habilis and the toothless H. rudolfensis cranium.

In their new paper in Friday’s issue of Science, Blumenschine and 16 co-authors conclude that all of the specimens are similar enough to be called H. habilis, and H. rudolfensis is not a separate species at all. At the same time, they suggest that several smaller-brained specimens do not belong in H. habilis.

So for the moment, it seems our understanding of the hominid family tree has come full circle. Such reshuffling of evolutionary trees is common — and in the case of human evolution, likely to continue — as ideas are put forth and later revised to incorporate new findings.

A broader view of behavior
As originally hoped, the Olduvai Landscape Paleoarchaeology Project’s research also provided valuable new insights into the behavior of our stone-tool-using ancestors. For one thing, the Science team found OH 65 and the other fossils in the little-known western region of Olduvai Gorge, on the side of ancient Lake Olduvai opposite from the famous sites where the Leakeys and others had explored.

According to Blumenschine, his team expanded across a broader geographic area to gain “a more ecologically realistic” view of early human behavior. The team recovered more than 200 stone knives and hammers, and the animal bones at the site included relatives of antelopes, giraffes and baboons. This suggests an environment of mixed grasses and trees that was probably drier and less productive than the more hominid-friendly area across the lake.

Two key pieces of evidence lead the Science authors to believe that early humans used the arid western area only intermittently. First, three of the stone tools were made not of local quartzite but of lava from the volcanic highlands in the southeastern basin. Second, only a few of the animal bones showed cuts or hammer marks, in contrast to much more pervasive evidence of butchery at sites used more intensively.

Much of Blumenschine’s research over the past two decades has focused on whether our earliest ancestors were hunters or scavengers. Not withstanding the ubiquitous cartoons of cavemen taking down mastodons with stone spears, he believes early hominids cleverly became “part-time carnivores” by encroaching on the kills of skilled hunters like lions and jackals. An animal bone found with OH 65 that bears both crocodile tooth marks and stone-cut marks suggests intriguing new possibilities as well.

A lowly start for the human race? Not so, says Blumenschine. “You’re talking about early hominids — which, compared to modern humans, were relatively small, dim-witted and poorly equipped — interacting with major carnivores. It required great skill and adaptation.”

Amazing Olduvai
The work of the Science team in western Olduvai yielded at least one other definitive finding. “The Olduvai Gorge in Tanzania has shown its potential is far from exhausted,” wrote erstwhile Leakey colleague Phillip Tobias of South Africa’s University of the Witwatersrand, in a commentary that accompanies the Science paper.

Blumenschine agreed: “There’s a perception that the Leakeys found everything, but that’s the furthest thing from the truth. As long as the Tanzanians continue to treasure and conserve Olduvai, the whole world will continue to be amazed by it.”

© 2013 American Association for the Advancement of Science

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