Selection for Skin Color

That anatomically modern humans originated in Africa suggests that the first humans likely had dark skin. But then how might varying skin tones have later evolved? Nina Jablonski’s research has shown that environmental factors likely played a role in the evolution of different skin tones. Environment alone doesn’t produce evolution, however. Rather, the environment acts on traits, or phenotypes, increasing or decreasing the frequency of alleles in a population by natural selection. Where did these alleles come from?

Recall that each time a cell replicates, mutations caused by errors in replication can occur. If these mutations occur in germ cells during meiosis (see Chapter 10), they will permanently change the genome of the next generation. This process continually introduces new alleles into the population. Some of these alleles can be negative or harmful, as in the case of hereditary cancer or cystic fibrosis. But new alleles can also be benign or even beneficial. Indeed, sometimes alleles can be so positive and confer such a survival advantage that they become more common in succeeding generations and can eventually become fixed in a population, reaching 100% frequency (INFOGRAPHIC 20.9).

Sometimes alleles that are harmful in one environmental context may be beneficial in another. For example, the recessive allele responsible for cystic fibrosis (CF) can cause this serious disease when it occurs in homozygotes, who have two copies of the allele. However, research has suggested that being heterozygous for CF—that is, having only one CF allele—may have reduced the severity of diarrhea caused by cholera or some other infection in times past. Consequently, carrying a CF allele provided an advantage during epidemics. This would help explain why the CF allele became relatively common.

Skin color is another example of a trait that likely conferred an advantage to humans and underwent natural selection at some point in human history. Otherwise, dark or light skin color wouldn’t be so common among specific populations. In fact, the dark skin of those Homo sapiens who evolved in Africa was probably an early adaptation; it is likely that before dark skin evolved, our earliest ancestors had light skin, just as chimpanzees do today.

Fossil and genetic evidence suggests that about 2 million years ago hominids became “bipedal striders, long-distance walkers, and possibly even runners,” according to Jablonski. But to sustain such activities, hominids needed an effective cooling system, a feature they could have developed only by losing excessive body hair and gaining more sweat glands. In contrast, hairy chimpanzees, our closest living animal relatives, can sustain only short bouts of activity without getting overheated. “It’s like sweating in a wool blanket,” Jablonski explains. “After that blanket gets saturated, you can’t lose very much heat.”

Eventually, some factor—food scarcity, perhaps—forced ancient hominids out of the forests and into the open savannahs to hunt for food. Hominids with less hair and more sweat glands were likely better hunters because they could sustain long bouts of activity without getting overheated. Like modern-day chimpanzees, these hominids likely had fair skin under their hair. Without hair to protect their light skin, they were exposed to the intense African sun. And, scientists hypothesize, exposure to the sun would have reduced their folate levels and thus their fitness in the sun-drenched environment.

Any of these ancient hominids that carried an allele or developed or inherited a mutation that increased their ability to produce more melanin would have been able to spend more time in the sun without the detrimental effects. Darker coloration would have protected their skin, and consequently their folate levels, from the sun, enabling them to hunt and travel in the open fields.

Evidence to support this hypothesis comes from genetics. In 2004, Alan Rogers of University of Utah and his colleagues studied a gene that influences skin color. They discovered that more than a million years ago, an allele that contributes to dark skin became fixed—that is, its frequency approached 100%—in the African population of hominids. “This is critical,” Jablonski says. “It shows that darkly pigmented skin became extremely important to us” around the time that hominids became more humanlike.

The allele for darker skin was such an advantage in terms of survival and reproduction that hominids with darker skin left more offspring than their lighter-skinned relatives. Hominids born with light skin weren’t able to survive and reproduce in great enough numbers for the trait to persist in the population. The allele for darker skin eventually increased in the population until it essentially reached 100%.

Populations that migrated north, away from the African sun, however, faced a different environment. Folate was not as easily destroyed in this lower-UV-light environment. But the high levels of melanin present in dark skin were a disadvantage; they prevented bodies from producing enough vitamin D. In this low-UV-light environment, fair skin allowed the body to soak up more ultraviolet light and produce essential vitamin D. In these environments fair-skinned people thus were more fit and left more descendants than did dark-skinned people. Consequently, the frequency of light skin in northern climates increased with each generation.

If the earliest humans migrating from Africa to northern regions all had dark skin, where then did alleles for light skin come from? There are two possibilities. One is that the alleles were introduced as new mutations into the population either shortly before or shortly after humans migrated out of Africa and were subsequently selected by natural selection. The other is that the alleles were still present in the African population, albeit at very low frequencies, and then subsequently became more common under selection in a new environment.

Both explanations may be true. Genetic studies show that the frequency of alleles for light skin increased and swept through populations as they migrated north—most likely more than once. Alleles of at least three different genes have been found to confer light skin in people from northwestern Europe and light skin in people from eastern Asia—suggesting that mutations for light skin arose independently and spread through those two populations separately. These mutations were genetically minor, yet phenotypically significant: single nucleotide changes in two of these genes have been shown to account for a large proportion of the difference in skin color between light- and dark-skinned people—two nucleotides among the 3 billion total in our DNA. Skin color really is skin deep.