Wow

Chalachew Seyoum, an Ethiopian graduate student at Arizona State, has found a fragment of jawbone which "is the oldest known fossil from an evolutionary tree branch that eventually led to modern humans."

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.

...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 make a convincing case that the fossil belongs in the Homo category.

And they present good evidence that it came from a creature that was either at the origin of Homo or "within shouting distance," said Bernard Wood of George Washington University.

...It's the first time that anything other than isolated teeth have turned up as a possible trace of Homo from before 2.3 million years ago.

That is truly remarkable. What a fascinating find.