This Jawbone Is the Oldest Specimen of the Human Genus

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Chalachew Seyoum didn't even have to dig to change the timeline of human origins. He just looked around. On January 29, 2013 at a research site in northeastern Ethiopia, the Arizona State graduate student said he found the jawbone in the shifting sediment on top of the hill they were surveying that day without so much as lifting his trowel. The region of Ethiopia is on the East Africa Rift System, where the movement of tectonic extension shifts the nearly 3-million-year-old rocks and whatever lies within, allowing them to become exposed through erosion and found by keen-eyed graduate students. Seyoum's discovery is now helping shed light in one of the murkiest eras of human evolution.

The jawbone looks to be the oldest example of the genus to which humans belong. At between 2.8 and 2.75 million years old, this 8 centimeters of a lower left jawbone pushes back the earliest evidence of the human genus back by 400,000 years.

As laid out in a paper just published today in the journal Science, it is not yet clear if the specimen is from a heretofore unidentified species, or if it pushes the origins of a known member of the genus, for instance, back earlier than had yet been observed.

The fossil, discovered in the Ledi-Geraru research area of Ethiopia, dates back to just 200,000 years after the latest Australopithecus afarensis specimen, the famously complete "Lucy" who was also discovered in Ethiopia, not far away. At a teleconference this morning, researchers explained that this fossil dates very close to when the two genuses split.

The new, which is to say very old, specimen has resemblance to ancestor and progenitor. It has the "slim molars, symmetrical premolars and an evenly proportioned jaw, that distinguish early species on the Homo lineage such as Homo habilis at 2 million years ago, from the more apelike early Australopithecus," the study states. But "the primitive, sloping chin links the Ledi-Geraru jaw to a Lucy-like ancestor."

"I think we need to know a lot more about the population it represents before we can be sure as to whether it represents an already known species or perhaps even a new one," study co-author, William Kimbel, from the Institute of Human Origins and School of Human Evolution and Social Change at ASU said at a teleconference this morning.

"We have a jaw with teeth that preserves enough anatomy to be quite confident that it does represent an early part of the Homo lineage, but I would suggest that we need to know more about it before we can make any firm decisions about what species it might belong to."

While volcanic ash built up around the jawbone gives a timeframe where it can be placed, questions remain about what kind of the environment the subject was living in and how he or she related to it. It's not clear what other adaptations that the Homo genus would become known for—tool making, bigger brains, etc—developed at the same time as the teeth and jaw were changing.

"There's a lot that remains unknown," Kimbel said. "The precise nature of the transition, for example, whether it was a gradual transformation along a single lineage or a rapid convergence of lineage and whether the early changes in the teeth and jaws were accompanied by changes in other systems, such as brain or technology."

The discovery "significantly narrows the time range in which fieldworkers should now focus their search," he said, which is why fieldwork will continue.

It took finding a lot more than a mandible to figure out what Homo habilis may have looked like, but with the fossils from this 2-3 million-years-ago range so rare, a jawbone that changes the time frame is exciting.