The genus Homo, the lineage that spawned modern man, may be 400,000 years older than previously thought. That's the conclusion scientists have made after pulling a 2.8-million-year-old jawbone from the dirt of a hilltop in Ethiopia.

The team of U.S. researchers who discovered the jawbone, which boasts five teeth, says it is evidence of the first human from the Homo genus -- showcasing traits from the earlier, ape-like lineage Australopithecus afarensis, which existed 3.9 million to 2.9 million years ago, as well characteristics unique to the lineage that eventually produced Homo sapiens.

"By finding this jaw bone we've figured out where that trajectory started," Brian Villmoare, a researcher at the University of Nevada in Las Vegas, told The Guardian. "This is the first Homo. It marks in all likelihood a major adaptive transition."

The jawbone's discovery is detailed in two separate articles published in the scientific journal Science this week -- one focusing on the location of the fossil's discovery and the other looking at the anatomical characteristics.

That adaptive transition can now be placed more firmly at about three million years ago, when a related paper published in the journal Nature suggests Australopithecus afarensis died out and two new human-like lineages emerged -- Paranthropus, trading brain size for heftier teeth and stronger jaw muscles, and the all-important Homo lineage, which opted for the more cerebral approach. It turned out, the bigger brain was the better bet.

"We now have a very plausible scenario where early Homo emerges from Australopithecus afarensis," Fred Spoor, researcher at the University College London and author of the Nature paper, told The Verge. "This is represented by the jaw that is 2.8 million years old, and somewhere between 2.8 million and 2.3 million years ago it breaks up into different lineages."

In other words, the jawbone marks an evolutionary transition from more ape-like lineages to those that boasted a bigger brain.

"It's an excellent case of a transitional fossil in a critical time period in human evolution," William H. Kimbel, director of ASU's Institute of Human Origins, said in a press release.

Kimbel and his colleagues expected to find the fossils of the relatives of Lucy, the famous ancient female Australopithecus afarensis specimen -- one of the oldest hominids ever unearthed. Instead, they found something much more exciting.