Humans and Neanderthals Interbred

It's one of the great questions of human evolution: Did Homo sapiens interbreed with Homo neanderthalensis? The two species had many similarities: they lived in caves, used similar types of tools and hunted the same prey. And they lived in the same place for long periods of time, most notably in Europe from 45,000 to 30,000 years ago. But analysis of the Neanderthal's mitochondrial DNA provided no evidence that they had interbred with modern humans.

However, scientists now have a draft sequence of the Neanderthal genome (published today in Science), and comparing it with the genomes of modern-day humans shows that interbreeding must have happened and that there's evidence of Neanderthal genes in some humans.

The Neanderthal and H. sapiens genomes are 99.84 percent identical, but there is variation in similarity depending on where a modern-day human is from. The researchers compared the Neanderthal genome to those of people from China, France, Papua New Guinea, southern Africa and West Africa and found that the Neanderthals were slightly more similar to non-Africans. Further analysis revealed that the non-Africans had a small number of Neanderthal genes.

How did those genes get there? A likely scenario may be "the movement of a few Neanderthals into a group of humans," University of California at Berkeley population geneticist Montgomery Slatkin told Science. One potential time and place for such an event would have been Israel about 80,000 years ago, where the two species overlapped for about 10,000 years. Interbreeding would have been rare, the scientists say, but they do not yet know what would have been preventing it from occurring more frequently.

The researchers are using the new genome sequence to tease out the ways in which Neanderthals and H. sapiens are the same and the ways they are different (including differences in metabolism, skin, skeleton and the development of cognition) in the hopes of discovering what truly makes us human.

Inspired by this new research, I finally visited the new Hall of Human Origins at Smithsonian's National Museum of Natural History this week (where I snapped the picture above, of the bones of a Neanderthal child). If you're in the Washington, D.C. area, I recommend stopping by. I've read, and written, so much about human evolution, but seeing the evidence in person still brings surprises—I had no idea the hobbit people of Flores were so tiny or that hand axes were that hefty—that's the sort of thing you just can't realize from words and pictures alone.

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Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for Smithsonian.com and blogs at Wild Things, which appears on Science News.