It's just a single row of teeth, but a primitive jawbone has given archaeologists much to chew on. The fossil appears to belong to the oldest known member of the human family.
The lower jawbone, discovered in the Ethiopian Rift Valley, is approximately 400,000 years older than the earliest-known species, Homo habilis, in the lineage that led to modern humans.
For decades scientists have searched Africa for fossils that reveal insights about the origin of the Homo lineage, but specimens from this time period between 3 million and 2.5 million years ago have been noticeably absent or poorly preserved.
An international team of geologists and anthropologists have dated the new fossil, known as Ledi-Geraru after the research area where it was uncovered, to this critical interval in human evolution, between 2.8 and 2.75 million years old.
One of the team leaders Brian Villmoare said in spite of a lot of searching, fossils on the Homo lineage older than 2 million years ago are very rare.
"To have a glimpse of the very earliest phase of our lineage's evolution is particularly exciting," said Dr Villmoare, from the University of Nevada.
Between 3 and 2 million years ago humans made the important transition from the more ape-like Australopithecus to the more modern adaptive pattern seen in Homo.
Dr Villmoare said the Ledi-Geraru fossil was also valuable because it possessed a unique combination of traits, from the height of the mandible to the shape of the teeth, that made it clearly a species transitioning between Australopithecus and Homo.
"The fact that it has features that so clearly ally it with Homo by 2.8 million years ago helps us narrow the time of transition, and suggests that the transition itself was relatively rapid," he said.
The team used a combination of techniques to date the fossil, including radiometric analysis of volcanic ash layers.
"We are confident in the age of LD 350-1," said Erin DiMaggio, another research leader, from Penn State University.
A collection of slightly younger animal fossils were also unearthed in the area - a prehistoric antelope, water dependent grazers, prehistoric elephants, a type of hippopotamus and crocodiles and fish. These animals lived between 2.84 to 2.54 million years ago, say the team.
The fossil discovery has been published in the prestigious journal Science.
