Did modern humans wipe out the competition, absorb them through interbreeding, outthink them or simply stand by while climate, dwindling resources, an epidemic or some other natural phenomenon did the job? Perhaps all of the above. Archaeologists have found little direct evidence of confrontation between the two peoples. Skeletal evidence of possible interbreeding is sparse, contentious and inconclusive. And while interbreeding may well have taken place, recent DNA studies have failed to show any consistent genetic relationship between modern humans and Neanderthals.
"You are always looking for a neat answer, but my feeling is that you should use your imagination," says Harvard University archaeologist Ofer Bar-Yosef. "There may have been positive interaction with the diffusion of technology from one group to the other. Or the modern humans could have killed off the Neanderthals. Or the Neanderthals could have just died out. Instead of subscribing to one hypothesis or two, I see a composite."
Modern humans' next conquest was the New World, which they reached by the Bering Land Bridge—or possibly by boat—at least 15,000 years ago. Some of the oldest unambiguous evidence of humans in the New World is human DNA extracted from coprolites—fossilized feces—found in Oregon and recently carbon dated to 14,300 years ago.
For many years paleontologists still had one gap in their story of how humans conquered the world. They had no human fossils from sub-Saharan Africa from between 15,000 and 70,000 years ago. Because the epoch of the great migration was a blank slate, they could not say for sure that the modern humans who invaded Europe were functionally identical to those who stayed behind in Africa. But one day in 1999, anthropologist Alan Morris of South Africa's University of Cape Town showed Frederick Grine, a visiting colleague from Stony Brook University, an unusual-looking skull on his bookcase. Morris told Grine that the skull had been discovered in the 1950s at Hofmeyr, in South Africa. No other bones had been found near it, and its original resting place had been befouled by river sediment. Any archaeological evidence from the site had been destroyed—the skull was a seemingly useless artifact.
But Grine noticed that the braincase was filled with a carbonate sand matrix. Using a technique unavailable in the 1950s, Grine, Morris and an Oxford University-led team of analysts measured radioactive particles in the matrix. The skull, they learned, was 36,000 years old. Comparing it with skulls from Neanderthals, early modern Europeans and contemporary humans, they discovered it had nothing in common with Neanderthal skulls and only peripheral similarities with any of today's populations. But it matched the early Europeans elegantly. The evidence was clear. Thirty-six thousand years ago, says Morris, before the world's human population differentiated into the mishmash of races and ethnicities that exist today, "We were all Africans."
Guy Gugliotta has written about cheetahs, Fidel Castro and London's Old Bailey courthouse for Smithsonian.