Current genetic evidence says the ancestors of modern humans left Africa 60,000 years ago. But recently discovered stone tools in the Arabian peninsula and India suggest humans actually started exploring the world as much as 120,000 years ago.
Although earlier relatives - like Homo erectus and Neanderthals of modern humans - left Africa earlier, genetic evidence suggests anatomically modern humans stayed on our home continent until about 60,000 years ago. That's why new stone tools found far inland on the Asian continent are so puzzling. Researchers believe they are clearly the work of Homo sapiens and not one of our relatives, and yet their date of origin requires humans to have left Africa at least 70,000 to 80,000 years ago, and likely much earlier than that.
The tools, which were mostly stone spears or scrapers, were found sandwiched in the ash of the Toba eruption, which geologists have dated very confidently to 74,000 years ago. The fact that the tools were found hundreds of miles inland means it's likely the humans migrated on foot from Africa, which means it probably would have taken many thousands of years of gradual movement to reach these spots.
Assuming the tools are indeed the work of our human ancestors, then there are really only two explanations: either the genetic evidence is wrong, or humans left Africa in multiple waves, and only the last one 60,000 years ago proved successful. Chris Stringer of London's Natural History Museum favors the latter interpretation:
"These tools show that people were in these regions, but the genetic data show an exit from Africa of later than 60,000 years ago. The people in India could have died out."
However, Dr. Michael Petraglia of Oxford suggests there may really be some basic problems with the genetic testing method. He points out that we are only using data from today's humans, which doesn't give us a very good sense of how accurate our projects into the past actually are. He argues we would benefit from applying similar methods ancient DNA to know whether the 60,000 year figure is really reliable.
In the end, these stone tools suggest we are looking at either a much longer period of humans traveling the globe or multiple waves of modern human migration. Either way, human prehistory just got a lot more complicated.