Geneticist David Reich used to study the living, but now he studies the dead.

The precipitating event came in the form of 40,000-year-old Neanderthal bones found in a Croatian cave. So well-preserved were the bones that they yielded enough DNA for sequencing, and it became Reich’s job in 2007 to analyze the DNA for signs that Neanderthals interbred with humans—a idea he was “deeply suspicious” of at the time.

To his surprise, the DNA revealed that humans and Neanderthals did interbreed in their time together in Europe. Possibly even more than once. Today, surprisingly, the people carrying the most Neanderthal DNA are not in Europe but in East Asia—likely due to the patterns of ancient human migration in Eurasia in the thousands of years after Neanderthals died out. All this painted a complicated but dynamic picture of human prehistory. Since the very beginning of our species, humans have been on the move; at times they replaced and at other times they mixed with the local population, first hominids like Neanderthals and later other humans.