RENEE MONTAGNE, HOST:
Scientists working in Ethiopia say they've found the earliest known fossil on the ancestral line that led to humans. It's a jawbone with several teeth, and it's old - really old. NPR's Christopher Joyce reports the fossil could fill an important gap in the history of human evolution.
CHRISTOPHER JOYCE, BYLINE: It's risky to say you've got the first or oldest of anything. But Brian Villmoare is sure he's got the earliest specimen of Homo - the human genus.
BRIAN VILLMOARE: Oh, Yes. Absolutely - it definitely is.
JOYCE: Villmoare is a paleoanthropologist at the University of Nevada, Las Vegas.
VILLMOARE: I mean, we were looking for it. And by miraculous chance, we did happen to find it.
JOYCE: His team found a lower jaw with five teeth in a region of Ethiopia called Afar. They were working a hill that was full of fossils.
VILLMOARE: I was on the other side of the hill. And they say, Brian, Brian, come here.
JOYCE: And it was just on the surface?
JOYCE: Several dating methods confirmed its age - about 2.8 million years old. That's nearly half-a-million years older than the previous record for a human-related fossil. Here's why that's a big deal. There's this big gap in human history that has puzzled scientists. It starts 3 million years ago in East Africa. The place was home to a variety of 4-foot-tall ape-like creatures called Australopithecus. The famous Lucy is the best-known. Over the next half-million years, these creatures disappeared. Then, our genus, Homo, appears in Africa about 2 million years ago. Scientists believe that during that million-year gap, Homo evolved from Australopithecus. But there hasn't been physical evidence of that. This jaw from Ethiopia could be it. The first human branch in the big primate family tree. The jaw is primitive but definitely different from the ape-like jaws of Australopithecus.
VILLMOARE: The general story is it's a reduction. You know, it's becoming less. The teeth are not these big, blocky, rectangular things. They become slender.
JOYCE: Villmoare doesn't know anything about this creature's brain. But he says its jaw and teeth clearly have human traits. And Richard Potts agrees. Potts runs the Human Origins Program at the Smithsonian Institution's National Museum of Natural History.
RICHARD POTTS: This new jaw really seems to confirm that by 2 3/4 million years ago, we certainly had a lineage branching off.
JOYCE: And what defined the new branch was its mouth as much as its brain size.
POTTS: You have this fragment of a jaw that shows that the leading edge of the origin of the genus Homo was our teeth.
JOYCE: And apparently, changes in the mouth continued as humans kept evolving. Fred Spoor at the Max Planck Institute in Germany has been making computerized scans of the skulls of early humans. By two million years ago, early humans were established and had split into at least two species - Homo habilis and Homo rudolfensis. What made those two different from each other - mostly their jaws and teeth.
FRED SPOOR: They all seem to have a brain size that is pretty much the same. So it is not brain size that actually distinguishes these species. But it is the face. It's the way the jaws are built.
JOYCE: Villmoare and Spoor published their findings in the journals Science and Nature, respectively. Together, the research suggests that human evolution was partly driven by an ongoing remodeling of the jaws and teeth. What made that possible? Well, you don't need big jaws and teeth if you have stone tools to process food - pound it or grind it up before you eat it. Maybe it was these very first human ancestors who invented stone tools. But to prove that, scientists will have to find some tools as old as this earliest Homo. Christopher Joyce, NPR News.
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