Scientists have collected evidence for years that modern humans interbred with our ridge-browed Neanderthal ancestors in Eurasia. But in Africa, where the Homo sapiens species is said to have emerged, a lack of genetic evidence has left researchers scratching their heads about exactly how we came to beat out not only the Neanderthals, or Homo neanderthalis, but also the other archaic species like Homo erectus and Homo habilis. A new paper published by Michael Hammer from the University of Arizona, however, provides new evidence that Homo sapiens not only interbred with Neanderthals in Eurasia, they also had sex with several species of our ancestors across the African continent. And they did it often. "We think there were probably thousands of interbreeding events," said Hammer. "It happened relatively extensively and regularly."

What we know about the history of our species has long been determined by what we can learn from our ancestors' remains. As recently as five years ago, researchers deduced that humans and Neanderthals had interbred at some point based on the shapes of skulls found in caves or buried under thousands of years worth of soil. Then, a ground-breaking paper published last year by Swedish evolutionary biologist Svante Pääbo in Science brought genetics into the equation. Pääbo provided genetic proof that Homo sapiens migrated out of Africa and into the Neanderthal-occupied Eurasian continent, where they met and mated with the more primitive men. Pääbo and his team made the discovery while comparing samples of Neanderthal DNA with that of modern human DNA. In a recent profile on Pääbo, The New Yorker's Elizabeth Kolbert describes what's become known as the "leaky replacement" hypothesis:

Before modern humans "replaced" the Neanderthals, they had sex with them. The liaisons produced chil- dren, who helped to people Europe, Asia, and the New World. 

The leaky-replacement hypothesis--assuming for the moment that it is correct--provides further evidence of the closeness of Neanderthals to modern humans. Not only did the two interbreed; the resulting hybrid offspring were functional enough to be integrated into human society. Some of these hybrids survived to have kids of their own, who, in turn, had kids, and so on to the present day. Even now, at least thirty thousand years after the fact, the signal is discernible: all non-Africans, from the New Guineans to the French to the Han Chinese, carry somewhere between one and four per cent Neanderthal DNA.

Hammer says that the interbreeding didn't stop with Neanderthals, however, but because of environmental conditions, we haven't been able to do the same genetic research with our African ancestors. "We don't have fossil DNA from Africa to compare with ours," Hammer explains. "Neanderthals lived in colder climates, but the climate in more tropical areas make it very tough for DNA to survive that long, so recovering usable samples from fossil specimens is extremely difficult if not impossible."

Lacking actual DNA, Hammer and his team did what any modern scientist would do: they wrote a computer program. Using modern human DNA, Hammer says, they were able to "simulate history" and sort of reverse-engineer human DNA. In doing so, they found evidence that Homo sapiens not only had sex with Neanderthals, they also interbred with Homo erectus, the "upright walking man," Homo habilis, the "tool-using man," and possibly others. Hammer says that despite earlier skepticism about interbreeding between human species and despite the belief that humans were an exception to certain laws of evolution, our DNA shows otherwise.

One big question remains: Why did modern man survive as the archaic species died off? Kolbert talked with Pääbo at length about this topic, and while we still don't know the answer, genetic research like Hammer's that tracks the migration of Homo sapiens around the globe provides some clues. In fact, it seems like what makes modern man different has a lot to do with traveling to new places and conquering them. Kolbert writes:

From the archeological record, it's inferred that Neanderthals evolved in Europe or western Asia and spread out from there, stopping when they reached water or some other significant obstacle. (During the ice ages, sea levels were a lot lower than they are now, so there was no English Channel to cross.) This is one of the most basic ways modern humans differ from Neanderthals and, in Pääbo's view, also one of the most intriguing. By about forty-five thousand years ago, modern humans had already reached Australia, a journey that, even mid-ice age, meant crossing open water. Archaic humans like Homo erectus "spread like many other mammals in the Old World," Pääbo told me. "They never came to Madagascar, never to Australia. Neither did Neanderthals. It's only fully modern humans who start this thing of venturing out on the ocean where you don't see land. Part of that is technology, of course; you have to have ships to do it. But there is also, I like to think or say, some madness there. You know? How many people must have sailed out and vanished on the Pacific before you found Easter Island? I mean, it’s ridiculous. And why do you do that? Is it for the glory? For immortality? For curiosity? And now we go to Mars. We never stop."