A fossil human jawbone discovered in southern China is upsetting conventional notions of when our ancestors migrated out of Africa.

The mandible, unearthed by paleontologists in China's Zhiren Cave in 2007, sports a distinctly modern feature: a prominent chin. But the bone is undeniably 60,000 years older than the next oldest Homo sapiens remains in China, scientists say.

In fact, at about a hundred thousand years old, the Chinese fossil is "the oldest modern human outside of Africa," said study co-author Erik Trinkaus, an anthropologist at Washington University in St. Louis.

(Also see "Oldest Skeleton of Human Ancestor Found.")

Popular theory states that Homo sapiens migrated out of Africa about 60,000 years ago, at which point modern humans quickly replaced early human species such as Homo erectus and Homo neanderthalensis across the world.

Finding such an ancient example of a modern human in China would drastically alter the time line of human migration. The find may also mean that modern humans in China were mingling—and possibly even interbreeding—with other human species for 50,000 or 60,000 years.

(Related: "Neanderthals, Humans Interbred—First Solid DNA Evidence.")

What's more, the find seems to suggest that anatomically modern humans had arrived in China long before the species began acting human.

For example, symbolic thought is a distinctly human trait that involves using things such as beads and drawings to represent objects, people, and events. The first strong evidence for this trait doesn't appear in the archaeological record in China until 30,000 years ago, Trinkaus said.

(Related: "Oldest Jewelry Found in Morocco Cave.")

Hoping for DNA Evidence

So far, genetic evidence largely supports the traditional timing of the "out of Africa" theory. But the newly described China jawbone presents a strong challenge, said anthropologist Christopher Bae of the University of Hawaii, who was not associated with the find.

"They actually have solid dates and evidence of, basically, a modern human," he said.

Still, the jaw and three molars were the only human remains retrieved from the Chinese cave, and the jaw is "within the range" of Neanderthal chins as well as those of modern humans, added paleoanthropologist John Hawks of the University of Wisconsin, Madison.

(Related: "Oldest Human Footprints With Modern Anatomy Found.")

"If this holds up, we have to reevaluate" the human migration time line, he said.

"Basically, I think they're right, [but] I want to see more evidence," Hawks added. "I really, really hope that there can be some sort of genetic extraction from this [fossil]."

The oldest human jawbone from China is described in this week's issue of the Proceedings of the National Academy of Sciences.