What Neanderthal DNA Is Doing To Your Genome

Neanderthals and modern humans, Homo sapiens,
have shared a long and intertwined history. We split from a common ancestor around 700,000
years ago, spent some time apart, but then hooked back up again before the Neanderthals
died out 40,000 years ago. Even though they’re gone, a part of them
lives on – their DNA. That’s because for some of our history together,
early humans had occasional… dalliances with Neanderthals. Our species were similar enough that the resulting
offspring were still fertile. And so that history is still within us today. Scientists estimate that for many of us, about
2% of our DNA is from Neanderthals. And it’s not just sitting there, doing nothing. A paper out this week in the journal Cell
shows that those bits of Neanderthal DNA can control how active your genes are. In some cases, it turns out, your Neanderthal
genes get turned on more, while in others, the Neanderthal versions get turned on less
— especially in your brain, and if you have them, your testicles. These findings reveal a new way that all that
ancient interbreeding is still showing up today, in you body. Because we’ve sequenced the Neanderthal
genome from fossils, we already know about a bunch of gene variants that we’ve inherited
from our archaic cousins. Some of these relate to height or how well
our immune systems work. Others change our risk for diseases like schizophrenia
and lupus. There are probably a lot that we don’t even
know about yet, and different people have different ones. But one way these genes might affect us is
by altering how often these genes are expressed, or turned on. Turning on a gene means using that DNA sequence
to make a kind of copy, in the form of RNA, which is then used ultimately to make proteins. If a gene is highly expressed, that means
it’s getting used to make lots of RNA, which usually means more of that gene’s protein
is getting made. So if the remaining bits of Neanderthal DNA
in our genome are changing how genes are expressed, that could be really important for human health. To find out what’s going on, geneticists
at the University of Washington measured levels of RNA in different tissues from more than
200 people who had Neanderthal ancestry. They looked at over 5,000 spots in their genomes,
where the people had inherited both a Neanderthal version of a gene — from either their mom
or dad — and a modern human copy, from the other parent. This way, they could see whether one was expressed
more than the other. We usually think of these two versions of
a gene — what geneticists call alleles — as being used pretty much equally. But it turned out, a quarter of the time,
there was a difference in how much the Neanderthal copy was expressed compared to the modern
human version. That’s a lot of places where having Neanderthal
DNA might change something! The geneticists also noticed something else. Sometimes Neanderthal sequences were used
less than the modern human counterparts, and sometimes they were used more, but overall,
they balanced out. But when they compared different kinds of
tissues, from different parts of the body — and they tested more than 50 of them — some
interesting patterns emerged. In brains and testicles, specifically, Neanderthal
variants weren’t expressed as much. This included a gene whose protein helps develop
neurons and create synapses, and another one that works in the whip-like tail of sperm. For some reason, we’re just not using those
genes that much. This, the scientists say, is a sign that the
brain and testicles have evolved pretty rapidly since Neanderthals and modern humans diverged
some 700,000 years ago. After all, our brains are structurally quite
different from Neanderthals. And as for the testicles, well, reproduction
tends to be very species specific – that’s why cross-species hybrids are often less fertile. What scientists think happened is that in
many cases, when Neanderthal genes got mixed in with ours, the proteins that help turn
on a gene were no longer as compatible with some of the Neanderthal DNA, which led to
a lower expression of those genes. But there are still plenty of instances where
Neanderthal genes are going strong. And since early humans also had…you know,
special relations… with other early hominins, like the Denisovans as well, the research
team plan next to check whether a similar thing might have happened with them, as well. Thanks for watching this episode of SciShow,
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