A fragment of jaw bone dating back 2.8 million years is evidence that the first humans evolved more than 400,000 years earlier than previously thought, scientists reported Wednesday. 

The fossil, which was uncovered in the Afar region in northern Ethiopia, is dated very close to the time that the human, or "Homo" genus, or group, split away from more ape-like ancestors like Australopithecus afarensis, best known for the fossil skeleton Lucy discovered in 1974.

Africa is a hotbed for human ancestor fossils, and scientists from Arizona State University have worked for years at the Ethiopia site, trying to find fossils from the dimly understood period when the Homo genus 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 in January 2013 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, which has been dated at around 3.2 million years old.

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.

The find also bolsters the argument that Homo arose from Lucy's species rather than a related one, said Susan Anton of New York University.

The new paper's analysis is first-rate, but the fossil could reveal only a limited amount of information about the creature, said Eric Delson of Lehman College in New York.

"There's no head, there's no tools, and no limb bones. So we don't know if it was walking any differently from Australopithecus afarensis," which was Lucy's species, he said.

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, he said.

"This fills a gap, but it hasn't yet given us a complete skeleton. It's not Lucy," Delson said. "This is always the problem. We always want more."

Also on Wednesday, another research team reported in a paper released by the journal Nature that the lower part of the face of Homo habilis, the earliest known member of the Homo branch, was surprisingly primitive. That came from reconstruction of a broken jaw that was found 50 years ago.

The finding means the evolutionary step from the Ethiopian jaw to the jaw of Homo habilis is "not so large," said an author of the Nature study, Fred Spoor of University College London and the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

The Associated Press contributed to this report.