NEW YORK — A fragment of jawbone found in Ethiopia is the oldest known fossil from an evolutionary 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.

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.

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.

Field work is continuing.