Fish and SNPs: What fish are teaching us about human skin color

These fish can tell us a lot about ourselves.

Species often end up a different color when their environment changes. And humans are no exception.

When people moved out of Africa tens of thousands of years ago, they were dark-skinned. Now when we look around Northern Europe or parts of Asia, we see much lighter people. What happened?

A common explanation has to do with sunlight and vitamin D. When people moved north, they got less sun. Less sun means less vitamin D and awful diseases like rickets.

Anyone who moved north and had lighter skin ended up getting more vitamin D and did better than their darker neighbors. After awhile, most of the population had light skin.

This is all well and good, but what happened at the gene level to cause this transformation? One way scientists are learning about how humans ended up with lighter skin is by studying fish. For example, the zebrafish has taught us a lot about why Europeans are often so pale.

The zebrafish is an important model system that scientists use to study vertebrate development, human disease, and lots of other things. A common mutant fish that scientists use in these studies is called "golden." These fish have lighter, yellowish stripes instead of black ones.

Scientists discovered that these mutant fish had yellow stripes because of a single DNA difference (or SNP*) in their SLC24A5 gene. When fish have this DNA difference, they have yellow stripes.

These scientists next looked for this gene in people. What they found was that most of the people they looked at had two copies of the "black stripe" version of the gene. Except for Europeans. They tended to share a common SNP in their SLC24A5 gene that the scientists went on to show is a big part of why many Europeans have lighter skin.

Another group of researchers decided to dig a bit deeper and find out when this transformation happened. By looking at the DNA around SLC24A5, they found that lighter skin came to dominate Europe around 6,000-12,000 years ago. At first this result is a bit confusing because humans moved into Europe around 40,000 years ago. Why did it take so long for lighter skin to become the norm?

Scientists can't know for sure but one idea is diet. Around this time, Europeans started to grow their own food. And a farmer's diet has less vitamin D than does a hunter-gatherer's diet. Maybe the lack of sun only started to affect Europeans after they started growing their own food. Then, after a relatively brief time, most Europeans ended up fair-skinned to get enough vitamin D.

This gene doesn't explain all of skin color. For example, it doesn’t explain the difference in color between Northern and Southern Europeans. Or why some Asians have fair skin. But it does explain a good deal of European coloration. Thanks, zebrafish!

*SNP=single nucleotide polymorphism

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.

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