Fossil jaw sheds light on a point in human evolution.

Byline: Malcolm Ritter

NEW YORK -- A fragment of jawbone found in Ethiopia is the oldest known fossil from an evolutionary tree branch that eventually led to modern humans, scientists reported Wednesday.

The fossil comes from very close to the time that our branch split away from more ape-like ancestors best known for the fossil skeleton Lucy. So it gives a rare glimpse of what very early members of our branch looked like.

At about 2.8 million years old, the partial jawbone pushes back the fossil record by at least 400,000 years for our branch, which scientists call Homo.

It was found two years ago at a site not far from where Lucy was unearthed. Africa is a hotbed for human ancestor fossils, and scientists from Arizona State University have worked for years at the site in northeast Ethiopia, trying to find fossils from the dimly understood period when the Homo genus, or group, arose.

Our species, called Homo sapiens, is the only surviving member of this group.

The jaw fragment, which includes five teeth, was discovered in pieces one morning by Chalachew Seyoum, an Ethiopian graduate student at Arizona State.

He said he spotted a tooth poking out of the ground while looking for fossils.

The discovery is described in a paper released Wednesday by the journal Science.

Arizona State's William Kimbel, an author of the paper, said it's not clear whether the fossil came from a known early species of Homo or whether it reveals a new one. Field work is continuing to look for more fossils at the site, said another author, Brian Villmoare of the University of Nevada, Las Vegas.

Analysis indicates the jaw fossil came from one of the earliest populations of Homo, and its age helps narrow the range of possibilities for when the first Homo species appeared, Kimbel said.

The fossil dates to as little as 200,000 years after the last known fossil from Lucy's species.

The fossil is from the left lower jaw of an adult. It combines ancestral features, like a primitive chin shape, with some traits found in later Homo fossils, like teeth that are slimmer than the bulbous molars of Lucy's ilk.

Despite that mix, experts not involved in the paper said the researchers...