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When modern humans and Neanderthals met up we shared some genes. But we may have shared something else: germs. Work by David Enard and Dmitri Petrov shows Neanderthal diseases may have also been introduced into modern human populations at this time1.
Fortunately, their genes held the cure for these diseases (or at least, some resistance to them). Enard and Petrov think this may be why some of their genes still survive. They’re the antidote to a poison they gave us1!
What’s left of Neanderthals
Humans and Neanderthals hooked up at some point(s) within the last 60,000 years. Many of the genes we inherited from this encounter(s) were rapidly purged from our genome. Natural selection was working overtime, indicating that Neanderthal DNA was harmful to our health. Particularly male fertility, as every associated Neanderthal gene has disappeared2.
As a result of this, many human populations only have a few percent of Neanderthal DNA left. These dregs of DNA seems to have stuck around because this DNA doesn’t do anything, so natural selection doesn’t care3.
Or the stuff it does do is too minor for natural selection to care about. Like impact how tall you sit up. Some people still have a Neanderthal gene involved in that, weirdly3.
However, there are a few Neanderthal alleles that don’t fit that pattern, being more common than you’d expect for something non-functional. These genes aren’t just hitching along for the ride, they’ve been positively selected for as they confer some advantage3.
The most prevalent of these, present in 66% of Europeans. . . increases our risk of sunburn3?
Neanderthal antidotes to Neanderthal diseases
Clearly, something weird is going on here. Maybe these genes offer some other, undetected benefit. Or maybe getting a sunburn is better than it cracked up to be.
Enter Enard and Petrov, who looked at these more common Neanderthal sequences which are likely under positive selection. Specifically, they were looking at how this Neanderthal DNA influences proteins. Maybe there would be some clue there as to what impact this DNA had1.
Sure enough, they did find something. It turns out that viruses exploit 1/4 – 1/3 of the proteins influenced by Neanderthal DNA. They’ll attach to these proteins to trick the host into making more virus copies, or to inhibit an immune response, or to sneak into our cells1.
Or something else that is probably also evil.
Based on this, Enard and Petrov infer that the reason this Neanderthal DNA was beneficial was that it provided some protection against these viruses. What’s more, they could have served a similar function within Neanderthals; also helping their immune system.
Which would mean they were also infected with these viruses. Thus, these Neanderthal diseases could also have been passed to us at the same time as these protective genes. That’s the crux of Enard and Petrov’s “poison-antidote” idea. It’s early days for it yet, but it could help explain a good chunk of beneficial Neanderthal DNA.
However, the sunburn gene was not one of them. So whatever is going on there remains a mystery.
- Enard, D. and Petrov, D.A., 2018. Evidence that RNA viruses drove adaptive introgression between Neanderthals and modern humans. Cell, 175(2), pp.360-371.
- Sankararaman, S., Mallick, S., Dannemann, M., Prüfer, K., Kelso, J., Pääbo, S., Patterson, N. and Reich, D., 2014. The genomic landscape of Neanderthal ancestry in present-day humans. Nature, 507(7492), p.354.
- Dannemann, M. and Kelso, J., 2017. The contribution of Neanderthals to phenotypic variation in modern humans. The American Journal of Human Genetics, 101(4), pp.578-589.