Why Humans Are Black or White?

Humans and stickleback fish

First humans might have been black, but once they started the migration out of Africa about 100,000 years ago, their skin color gradually paled, in the new colder climes. 10,000 years ago, at the end of the Ice Age, marine stickleback fish started to colonize lakes and streams in Europe, Asia and North America, and they seem to have experienced a similar color change.

A new research led by Howard Hughes, Medical Institute investigator, and David Kingsley, published in the journal "Cell", reveals how humans and the three-spined stickleback fish (even if so different) have shared the same genetic mechanisms, enabling them to get their skin pigmentation in new environments.

"Sticklebacks have undergone one of the most recent and dramatic evolutionary radiations on earth. They created a multitude of little evolutionary experiments, in which these isolated populations of fish adapted to the new food sources, predators, water color, and water temperature that they found in these new environments (freshwater)", said Kingsley.

Coloration changes enabled the fish to camouflage and lure mates in new conditions.

"Human populations have also undergone pigmentation changes as they have adapted to life in new environments. The ecological reasons for those changes may be quite different from the forces driving the evolution of pigmentation in sticklebacks", said Kingsley.

In northern cooler climates, darker pigmentation is no further required; by contrary, a more transparent skin allows the diminished sunlight, in those climates, to produce sufficient vitamin D.

The team crossed stickleback types with different pigmentation patterns, and genetic markers allowed them detect chromosome stretches in the hatchlings, connected with inheritance of dark/light gills and skin. The team detected a gene named Kitlg (short for "Kit ligand") that was linked to pigmentation inheritance.

"Kitlg was an excellent candidate for regulating pigmentation because mutant forms of the corresponding gene in mice produce changes in fur color", said Kingsley.

But, Kitlg also controls germ cell development, pigment cell development, and hematopoiesis. Light-colored fish present a decreased activity of the Kitlg in gills and skin, but not in other tissues.

"By altering expression of this gene in one particular place in the body, the fish can fine tune the level of expression of that factor in some tissues but not others. That lets evolution produce a big local effect on a trait like color while preserving the other functions of the gene", said Kingsley.

Kitlg gene is also found in humans, and it could control the color of human skin. Other researches had found that human Kitlg gene differs among various human populations, thus it experienced selection, and all variants were connected with various skin color. Two copies of the African allele (gene type) of Kitlg meant darker skin, in comparison to the presence of one or two copies of the Kitlg alleles in European or Asian population.

"Although multiple chromosomal regions contribute to the complex trait of pigmentation in both fish and humans, we have identified one gene that plays a central role in color changes in both species", said Kingsley.

"Since fish and humans look so different, people are often surprised that common mechanisms may extend across both organisms. But there are real parallels between the evolutionary history of sticklebacks and humans. Sticklebacks migrated out of the ocean into new environments about ten thousand years ago. And they breed about once every one or two years, giving them five thousand to ten thousand generations to adapt to new environments", said Kingsley.

"Humans breed about once every 20 years, giving them about 5,000 generations or so to emerge from an ancestral environment and colonize and adapt to new environments around the world. So despite the difference in total years, the underlying process is actually quite similar. And the genetic mechanisms that can produce these changes may be so constrained that evolution will tend to use the same sorts of genes in different organisms", he added.

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