A fragment of a jawbone dating back 2.8 million years is evidence that humans evolved more than 400,000 years earlier than previously thought.
The fossil was uncovered in the Afar region in northern Ethiopia and dates very close to the tie that the "Homo" genus, or human group, split away from the more ape-like ancestors such as Australopithecus afarensis, best known by the fossil known as Lucy that was discovered in 1974.
Scientists from the Arizona State University have spent years working in Ethiopia in search of fossils of the earliest humans in an attempt to shed some light on the little understood period when the Homo genus arose. Our species, known as Homo sapiens, is the only surviving member of this group.
The jaw fragment was discovered in pieces one morning in January 2013 by Chalachew Seyoum, an Ethiopian graduate student at Arizona State. The fragment includes five teeth and Seyoum said he discovered it by spotting one of the teeth poking out of the ground while searching for fossils.
Arizona State's William Kimbel, one of the author's of the paper, stated that at this time it isn't clear if the fragment came from a known early species of Homo or it is the first signs of a new species that has yet to be discovered. Brian Villmoare of the University of Nevada, Las Vegas, another author of the study, said field work is continuing to search for more fossils in the area.
Early analysis indicates the fossil came from one of the earliest populations of Homo, and its age narrows down the range of possibilities. The fossil dates to as little as 200,000 years after the last known fossil from Lucy's species, which has been dated to around 3.2 million years ago.
The fossil is from the left lower jaw of an adult and it combines ancestral features such as a primitive chin shape with some traits found in later Homo fossils, like teeth that are slimmer than the molars from Lucy's species.
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, Delson says.
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.
"This fills a gap, but it hasn't yet given us a complete skeleton. It's not Lucy," Delson says. "This is always the problem. We always want more."
