In 2010, scientists mapping the Neanderthal genome finally confirmed that our human ancestors got it on with our Neanderthal Earth-mates. Likewise, even though the last of the Neanderthals died out around 30,000 years ago, most of us still carry a little bit of the Neanderthal genes that we inherited as a result of that interspecies swinging.
How much Neanderthal DNA? Current estimates suggest that anyone of Eurasian descent—and that's pretty much everyone apart from those with strictly African ancestry—have at least 1 to 4 percent Neanderthal DNA. New research also suggests those small bits of Neanderthal genetic code can play a role in human health, and may even elevate your risk for certain diseases.
"The first thing we should understand is that Neanderthal and human DNA are very, very close—almost identical sequences," says Omer Gokcumen, an assistant professor of biological sciences at the University of Buffalo. "So even if you have Version A from modern humans and I have Version B from Neanderthals, most of the time this will not translate into any health consequences."
Gokcumen is stressing this point because of the misconceptions many people have about Neanderthals and the genetic shadow they cast. The personal gene-sequencing company 23andMe offers customers info about their Neanderthal genes—including the percentage of Neanderthal genetic code they carry. And a recent Reddit thread reads like a laundry list of the erroneous beliefs people have about Neanderthal genes. One commenter says his 23andMe report put him in the 99th percentile for Neanderthal gene variants. ("I can't even grow a beard!" he wrote.)
But having a high percentage of Neanderthal DNA doesn't mean you're going to be hairier, have a more pronounced brow, or somehow look more ape-like than your neighbor. In fact, Gokcumen says, the percentage of Neanderthal DNA you have really doesn't mean much at all. "Even if you have 4 percent Neanderthal DNA, which is on the high end, that may mean nothing functionally," he says. "Whereas you may have one individual piece of Neanderthal code that happens to hit a metabolism gene, and so you may have higher cholesterol content or lipid retention."
How could harmful Neanderthal gene mutations persist among modern-day humans? "It's just really difficult sometimes to get rid of bad mutations, especially if they're only a little bad, or if the negative effects appear late in life," says Rebekah Rogers, an assistant professor of bioinformatics at the University of North Carolina, Charlotte.
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Rogers has published research on the way chromosomes shuffle and rearrange DNA. She says Neanderthal genes may be able to stick around in part due to aspects of this complex genetic reorganization. Or it may just be that some of the unhealthy effects of Neanderthal DNA don't show up until a carrier is old, and so has already reproduced and passed that harmful mutation on to his or her offspring.
Gokcumen says Neanderthal genes related to immune function and metabolism seem to be especially clingy and, for some, may turn out to have significant health implications. Research suggests some Neanderthal gene variants may raise a carrier's risk for autoimmune diseases like lupus. Ditto for metabolic disorders like obesity and diabetes.
Why would Neanderthal immune system and metabolism genes stick around? "Modern humans are essentially a sub-Saharan African species, while Neanderthal is a northern species," Gokcumen says. Because of their cold-climate origins, a Neanderthal's DNA likely promoted fat storage and other metabolic and immune adaptations suited to surviving in chilly temperatures. The interbreeding that went on between humans and Neanderthals probably took place as modern humans moved north out of Africa to populate Europe and Asia. And as humans moved into harsher climates, it makes sense that their genome would latch onto Neanderthal variants that could help them survive.
Some research has also linked Neanderthal variants to schizophrenia and cholesterol shifts. But in some cases, those links are positive. One recent study in the journal Science found Neanderthal DNA was associated with elevated levels of LDL cholesterol—the "bad" kind. The same study found ties between some Neanderthal gene variants and autoimmune diseases like rheumatoid arthritis.
But even the evidence-backed ties between some Neanderthal genes and disease do not suggest carriers of those gene variants are sure to develop health issues. "The genetic basis of human disease is very complex," Gokcumen says. Scientists don't have a firm handle on the "overall architecture" of disease, so the impact Neanderthal DNA has on human health is still hazy. "I think that impact exists, but it's vague," he says.
So yes, you're likely carrying around some snippets of Neanderthal DNA. And yes, some of that DNA has been associated with an increased or decreased risk for certain diseases. But for now, science can't provide many firm answers about the Neanderthals in your family tree, or how their DNA is affecting your health.
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Correction: A previous version of this story referred to LDL cholesterol as "the 'good' kind," when in fact it's the "bad" kind.