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Mutations Helped Humans Survive Siberian Winters
sciencehabit writes "Researchers have identified three genetic mutations that appear to have helped humans survive in the frigid climate of Siberia over the last 25,000 years. One helps the body's fat stores directly produce heat rather than producing chemical energy for muscle movements or brain functions, a process called 'nonshivering thermogenesis.' Another is involved in the contraction of smooth muscle, key to shivering and the constriction of blood vessels to avoid heat loss. And the third is implicated in the metabolism of fats, especially those in meat and dairy products—a staple of the fat-laden diets of Arctic peoples."
I can do the same thing with Rockstar and CHEETOS.
I read a theory once that I found interesting: that arterial plaque is a legacy of survival in an ice age or in an extremely cold environment like Siberia.
Here's how it goes, from memory: Humans get antioxidants from plants, but in extreme conditions plants were less available and humans may not have gotten enough antioxidants. Absent the antioxidants, free radicals posed a greater health risk.
Arterial plaque provided some defense against the deleterious effects of the free radicals, and helped the humans survive the freezing times... long enough to reproduce. Maybe in middle age the hardening of the arteries had deleterious effects of its own, but evolution is all about what helps reproduction, not so much what helps the individual live to a ripe old age.
This sounds sort of plausible but I don't have the background to evaluate it. It could also be one of those "wet streets cause rain" theories that invert cause and effect... is arterial plaque not the body's defense against free radicals but simply damage caused by them?
1. Just like the article says and unlike the Slashdot summary suggests, shiver-free thermogenesis is old and all mammals share it.
2. The researchers found traces of positive selection in a gene involved in shiver-free thermogenesis.
3. How do you look for traces of selection? A mutation in a DNA fragment coding for a protein can have two effects: either it changes the corresponding amino acid in the protein sequence (non-synonymous mutation), or it does not (synonymous mutation). This is because genetic code is redundant and different codons code for same amino acids, so a change from one codon to another does not have to change the protein. Synonymous mutations are assumed to be neutral for evolution (although they are not, not always).
Now, if you look at many possible variants of a gene and collect many different mutations, you can calculate whether the ratio of non-synonymous to synonymous mutations (called the dN/dS ratio) is (i) higher (ii) lower or (iii) quite like expected. Depending on the outcome of the test, you can say:
- if it is higher than expected, then there is a positive selection force at work (the gene is pushed towards change)
- if it is lower than expected, then we have a case of purifying selection; the gene is being actively maintained as it is, and any non-synonymous mutations are being removed from the population
- if it is neither lower nor higher, the gene is just not important
4. So, nice, you found that a gene related to non-shiver thermogenesis shows traces of positive selection. So what?
The answer is, not much. You do not always know which mutation was the one being selected. And even if you can pinpoint it, very often you will not be able to say what it actually does. So fine, you have a leucine replaced by arginine at position 186 in a protein chain; you might be able even to model the new sequence and see a delicate shift in the structure of the protein. How does it relate to the protein function? What has been modified or improved? No idea.
5. OK, why is that important? It is important because much of the genetic variability of the humans that we know is thought to have been fixated by genetic drift and other neutral evolutionary effects (like surfing the wave of colonization) - rather than selection. There are few examples of selection known. Light skin is one of them, and is thought to be an adaptation to the vitamin D deficiency caused by lack of sun at high latitudes. Mutation that keeps lactase being produced throughout life is another one. There were independent (convergent) events in both cases, by the way.
Look, humans are special. Special in the sense that humans are genetically extremely uniform, and the genetic differences between, say, native Australian, a blond-haired, blue-eyed Swede and a member of the Mbuti people from Africa are all together much smaller than between two chimpanzee individuals from groups living a few hundred kilometers apart. And moreover, these few mutations specific for some people but not for other seem to be more or less neutral in their character.
Finding differences that are *not* neutral, that are actually doing something is therefore an interesting thing. Notably, the few existing differences like that are linked to mundane things like metabolism or immune response (yes, some people are special because they don't fart after drinking milk, how is that for a superior race), and not, for example, to cognitive and brain development. The latter differences are found between humans and other primates.
"...and the weight loss industry. ;-)"
I don't see why--the adaptation allows Eskimo and Inuit (among others) to STORE fat--in large quantities--to allow for the generation of heat directly rather then through the much slower chemical process that the rest of us utilize. While you and I would sit there shivering our asses off, the Eskimo sitting next to you would simply be burning fat reserves, comfortably. I've experienced this exact situation while ice fishing--shivering makes it really hard to bait a hook without including your finger in the deal.
Most Eskimo/Inuit that I've met (I lived in Central Alaska for 10 years) were what most people would call "chubby"--they had a consistent, yet normal layer of fat that could provide them with emergency heating in the event they REALLY needed it. Falling through the ice in the middle of winter would be an example of such an emergency. Otherwise, they carry that fat around all year. Even in summer, fatty foods are a large part of the Eskimo/Inuit diet. But, I'd like to point out that I've never met a truly obese Native Alaskan--I'm guessing because they don't eat all the crap that most other Americans do--it costs too much to ship it there. While most of us would trim fat off of our meat, the fat is the important part of the catch up North. Muktuk (a common Eskimo/Inuit food) is pure whale fat--there is no meat whatsoever.
Interestingly, the Athabaskan peoples traditional range overlaps that of the Eskimo and Inuit where there are no large mountains that block travel between the interior and the coast (like the Brooks Range does). This is interesting because Athabaskan folks are built much different--they tend to be much slimmer in both bone and tissue--yet share many of the same foods and climate. They one thing missing from the Athabaskan diet is marine mammals of the large and fatty variety--whales, seals and walrus. Eskimo and Inuit eat them but the Athabaskans do not really, unless social circles overlap (more of a modern development). Both groups eat salmon, as the rivers from the coast reach far inland where the Athabaskan group historically occupied, and the interior (Athabaskan group) can actually get much colder then the North Slope of Alaska. The three characteristics discussed in this article also exist in most marine mammals.
Perhaps there is some difference to the fat in marine mammals that isn't present in the other main source of fat, namely salmon, and that consuming it led to the adaptation, rather then this being an environmental adaptation? After all, the Athabaskan peoples live in much the same environment, yet are built totally different (ie, do not have large fat reserves).