Copyright: Neanderthal Museum/H. Neumann
Copyright: Neanderthal Museum/H. Neumann

Neanderthals: Intimate Strangers

For my new “Matter” column in the New York Times, I look at the latest advance in our understanding of Neanderthal DNA. Neanderthals and humans interbred about 40,000 years ago, and their DNA is still in human genomes today. Scientists are mapping those Neanderthal genes we carry, and figuring out which ones have benefited us and which have made us sick.

One thing I didn’t have room to discuss is a question that I keep asking and to which scientists always respond with intriguingly noncommittal answers: Are Neanderthals members of our own species? Are they Homo sapiens? Are they a subspecies–Homo sapiens neanderthalensis? Or are they a separate species–Homo neanderthalensis?

For much of the 1900s, many scientists saw Neanderthals as the ancestors of living Europeans. But then in the late 1900s, some researchers argued that living humans descended from a small group of Africans that expanded out to the rest of the world. The discovery of Neanderthal DNA has wonderfully muddled that dichotomy. Humans and Neanderthals, the DNA suggests, share a common ancestor that lived 600,000 years ago. After hundreds of thousands of years, they came into contact and interbred. The fact that we carry some Neanderthal DNA shows that their hybrid offspring could have children of their own. One could argue that this ability to breed means that we belong to the same species. Perhaps we’re just subspecies that came to look different because we adapted to different conditions–Africa versus Eurasia.

But the latest evidence adds a new twist. Many genes from Neanderthals appear to have reduced the number of offspring that hybrids could have. That would explain why big segments of the human genome are free of Neanderthal DNA.

Why these genes were harmful isn’t yet clear, but the clues are fascinating. Take FOXP2, a gene involved in language in humans. Neanderthals have FOXP2 as well, but natural selection appears to have eradicated their version from the human gene pool. Did humans with the FOXP2 have trouble speaking? There are other clues that Neanderthal genes created infertile male hybrids. These effects didn’t have to be catastrophic to lead to the disappearance of Neanderthal genes. They might have just eroded away over many generations.

There are no known reproductive barriers between any living humans, no matter how distantly related they are to each other. These barriers are crucial to the origin of new species (although they can still allow some populations to interbreed even after millions of years). So perhaps we can say that Neanderthal, while not a separate species, were well on their way to separating.