Scientists have just done something that would have been unimaginable even a few years ago—they have sequenced the entire set of DNA from a 7000-year-old Spaniard. And this isn’t all. They have also managed to learn that he was most likely a dark-skinned, blue or green-eyed man who had trouble digesting milk as an adult.
Most of this is consistent with the way scientists think humans changed over time in Europe. But it is very cool to actually see it there, spelled out in the A’s, G’s, C’s and T’s of his DNA.
The current theory is that the humans who came to Europe weren’t too different from those in Africa or parts of the Middle East. They were dark-skinned, hunter-gatherers that hadn’t yet had to become lactose tolerant to succeed in their new environment.
As these Europeans began to settle down and grow their own food, these traits became a hindrance to their survival. For example, many scientists think that when they became farmers, their diet no longer provided enough vitamin D. These new farmers now needed to get the lion’s share of this important vitamin from the sun and their dark skin prevented this from happening because of Europe’s short winter days and lousy weather.
So what happened next is classic natural selection. Those Europeans who were lighter-skinned did better than their darker-skinned brethren and over time, lighter skin became the norm. For Europeans, a good chunk of this skin lightening has been traced to a small DNA difference in the SLC24A5 or golden gene. Our 7000-year-old man still had the African version of this gene (or ancestral allele as it is also called) which is now extremely rare in Europe.
Something similar is thought to have happened for lactase persistence (the scientific term for being able to drink milk as an adult). Conditions in parts of Europe favored being able to drink raw milk as opposed to eating cheese or yogurt and so, like lighter skin, lactase persistence spread across parts of Europe. Again, like 15% of modern Spaniards, our Iberian friend had the ancestral allele—he most likely could not tolerate milk as an adult.
There are also traits specific to agricultural societies in general that our Iberian man lacked. For example, people in agricultural societies tend to have more copies of the amylase gene which makes them better able to digest the starchy foods found in an agricultural diet. Our Spaniard had five copies which is more typical of a hunter gatherer (although five copies is at the lower end of what is typical for an agricultural society).
So he definitely fits what scientists thought early Europeans looked like. But before everyone starts popping champagne bottles it is important to point out that all of this information is coming from a single man from a part of Europe where these pressures were not very intense. One wouldn’t necessarily predict that his darker skin and lactose intolerance would even be a problem this far south in Europe. In fact, a recent study showed that eight Spaniards from 5000 years ago lacked the change in the lactase gene needed to be lactose tolerant. This means even 2000 years later, lactose intolerance was still relatively common in Spain (or at least it was in the eight skeletons they tested).
Still, this man is predicted to look the very picture of what scientists think an ancient European would look like. Except maybe for those blue eyes…
Blue eyes are sort of an enigma in our recent evolutionary history. There is no obvious nutritional advantage to having them like there is for lighter skin or being able to drink milk or digest starch. The most likely theories so far have been either that it hitched a ride somehow with lighter skin or maybe that it made those people who had blue eyes so irresistible that they had more children than their brown eyed compatriots. This last one is called sexual selection and is akin to a peacock’s feathers.
The man studied here does not fit the first model. He has the ancestral alleles for darker skin color combined with the European allele for blue eyes, a very rare combination these days. Keeping in mind again our sample size of one, it looks like blue eyes might have appeared and spread before lighter skin.
So our hunter-gatherer wasn’t simply an African who took up residence in Europe. On his way there, his ancestors picked up the blue eye version of the OCA2/HERC2 gene. Again, this is consistent with previous theories that blue eyes became established around the Black Sea around 6000-10,000 years ago. His ancestors must have stopped there or interacted with some recent emigrants from that region.
We are able to get this snapshot of history because sequencing ancient DNA keeps getting cheaper and easier and because scientists keep coming up with better and better ways to recover ancient DNA. And hold on to your hats, we’re just getting started here. We’ve already found a relative we didn’t even know existed, the Denisovan, and found out that there was quite a bit of hanky panky between non-African ancestors and both Neandertals and Denisovans. We may find even more relatives that we might have missed because of incomplete fossil records.
And if we get lucky, we might even have a shot at watching history unfold over the last 10,000 years or so. We may actually see blue eyes spread across Europe or follow lighter skin and lactase persistence as they suddenly become more common. We may also be able to see how lighter skin spread across parts of Asia or when and how Neandertal and Denisovan DNA spread across nonAfrican peoples.
This is an absolutely fascinating and until a few years ago, unexpected way to be able to trace human history. Stay tuned to learn more about who we are and how we got this way.
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