When did modern humans first arrive in Asia? Skull pieces could hold clues. (+video)

No artifacts were found at the site, nor were signs of human occupation.

"We think this fossil was outside with other fauna and flora, and during the rainy season, rain washed it into the cave," Shackelford said. "In subsequent seasons, more sediment washed into the cave and covered it."

The shape of the bone and teeth is distinctly anatomically modern human, not like those of an extinct lineage such as the Neanderthals. A variety of dating techniques of the sediments surrounding the fossils suggests they are at least 46,000 to 51,000 years old, and direct dating of the bone suggests a maximum age of about 63,000 years. This makes these fossils the earliest skeletal evidence for anatomically modern humans east of the Middle East.

These findings "change the thinking regarding modern human migration routes into Asia, that there were more routes of dispersal than previously thought," Shackelford said.

"The typical thinking was that once modern humans hugged the coastline to go from India to Southeast Asia, they went southward into Indonesia and Australasia (the region comprising Australia, New Zealand and neighboring Pacific islands)," she explained. "We think they absolutely did that, but we're also suggesting other populations probably went north or northeast toward China, and some went through the mountains into mainland Southeast Asia, taking advantage of river systems. Beforehand, no one thought they would have gone into the mountains of Laos, Vietnam and Thailand."

The researchers are now attempting to extract DNA from these fossils to see how related they may or may not be to later humans that once lived or currently live in the area. In the future, "the work we have is pretty boundless — there are literally thousands of limestone caves we can work on in this area to look for early modern humans," Shackelford said. "We can work here for the rest of our careers or lives and not see all the caves."

The scientists detailed their findings online Aug. 20 in the journal the Proceedings of the National Academy of Sciences.

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