The emergence of the human genus, Homo, which ultimately gave rise to modern humans, can be traced back to 2.8 million years ago, about 400,000 years earlier than previous evidence had indicated, according to research out Wednesday that analyzed a jaw fossil from East Africa.

The fossil, unearthed from the Ledi-Geraru research area at the Afar Regional State in Ethiopia in 2013, preserves the left side of a lower jaw, or mandible, along with five teeth, and researchers said that it is 2.8 to 2.75 million years old, representing the earliest known record of the Homo genus that includes Homo Sapiens -- humans -- and species closely related to them.

Previously, the first Homo species were believed to diverge from earlier, more ape-like Australopithecus around 2.3 or 2.4 million years ago.

"What is special about this jaw is not only the date, which is much older than any specimen of Homo known until now, but that has a unique combination of traits, from the height of the mandible to the shape of the teeth, that makes it clearly transitional between Australopithecus and Homo," coauthor Brian Villmoare of University of Nevada, Las Vegas told reporters at a teleconference.

According to the researchers, the fossil, known as LD 350-1, has advanced features such as slim molars, symmetrical premolars and an evenly proportioned jaw that distinguish early Homo species from Australopithecus. But the primitive, sloping chin links the jaw to an ancestor similar to 3.2-million-year-old Lucy, the first Australopithecus afarensis skeleton ever uncovered in the same area and probably the world's most famous fossil.

"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," Villmoare said.

For decades, scientists have been searching for African fossils documenting the earliest phases of the Homo lineage, but specimens recovered from the critical time interval between 3 and 2.5 million years ago have been frustratingly few and often poorly preserved.

As a result, there has been little agreement on the time of origin of the lineage that ultimately gave rise to modern humans.

Directly dating a fossil this old is impossible, so geologists used a variety of methods to date the layers of rock in which the fossil was found and then determined its age.

The area of Ethiopia where LD 350-1 was found is part of the East African Rift System, an area that undergoes tectonic extension, which enabled the 2.8 million-year-old fossil to be deposited, they reported in a pair of papers published by the US journal Science.

When the fossil was deposited, the site was mostly mixed grasslands and shrubs with some gallery forests. There were also a lake, rivers in the area with hippos, crocodiles, and fish, they said.

"The take home message from this paper is that we have determined that by 2.8 million years ago members of our genus, Homo, were living in the Afar region of Ethiopia in open grassland environments near a lake, rivers, active volcanoes and likely active faults," coauthor Erin DiMaggio of the Pennsylvania State University added.