More than skin deep

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Geneticists view “race” in a far more complex way than the average census taker.

For centuries, authors, researchers, and politicians have tried to use science to classify the “races.” Their attempts have resulted in divisive books, theories such as eugenics, and even genocide. One such book, Nicholas Wade’s 2014 tome A Troublesome Inheritance, inspired over 140 experts in population genetics and human evolution to speak out against it.

Wade, a journalist, used the work of leading geneticists to hypothesize that behavior and IQ, like some physical differences, vary by race as a result of natural selection. The 143 scientists signed a letter condemning the book and said their research did not support his claims that genetics, rather than culture, explains differences in global economies, political structures, and social norms.

Geneticists’ rejection of Wade’s book begs the question: How then did the races come to be? One challenge in addressing that question has to do with the term “race.” Genetically, “races” don’t fall into distinct categories or groups like those on a census survey. Geneticists recognize far more genetic variation among humans than we consciously recognize in the physical differences we see on the street. This diversity tells the story of migration, natural selection, and random chance.

More than just three

Wade’s book focuses on Caucasians — made up of Europeans and people of the Near East and the Indian subcontinent — Africans, and East Asians as the three major races and acknowledges natives of the Americas and of Australia and Papua New Guinea as fourth and fifth races. But these classifications don’t mean much to scientists. “The meaning of [race] in genetics doesn’t overlap neatly with the meaning commonly used in the broader community,” says Michael Eisen, a geneticist at the University of California, Berkeley.

We may make distinctions among people whose skin pigment is different from our own, but our genes don’t divide us along those same lines. In fact, genetically speaking, two people from different parts of Africa could be more different than a person from East Asia and someone from Europe.

“It’s not surprising that we see the most genetic variation in Africa,” says Sarah Tishkoff, a geneticist at the University of Pennsylvania’s Perelman School of Medicine and the School of Arts and Sciences. “They’ve had a larger population size for a longer time. They’ve adapted to very different environments within Africa.”

In her research, Tishkoff discusses how modern humans first appeared in Africa about 200,000 years ago. Before leaving Africa 100,000 years ago to fill the rest of the earth gradually, humans had already migrated and reproduced within Africa for 100,000 years — diversifying their gene pool with every migration and every generation.

Tishkoff’s research compared the DNA of more than 2,400 Africans, nearly 100 African Americans, and about 1,400 non-Africans worldwide. The research revealed that 14 clusters — not three or five races — best reflect human diversity. Most of those clusters are comprised of Africans.

All the other people of the world are less diverse. Every time a small group migrated out of Africa, they lost some genetic diversity.

“It’s a bottleneck that reduces genetic variation,” Tishkoff says. “Every migration to a new region takes a subset of African diversity along with it.”

This is not to say that scientists deny there are differences among us, including differences that influence how we identify ourselves racially. But these outward differences don’t account for a whole lot of our genetic makeup.

“The majority of the variation we see in the human genome is shared among populations,” Tishkoff says. “Only a small proportion actually differs.”

Migration and distinction

Anthropological and genetic research shows that humans migrated out of sub-Saharan Africa via East Africa to North Africa, probably along the western coast of the Red Sea. Researchers continue to uncover clues as to the precise routes of the multiple migrations, but most agree that a first major migration led to the population of most of Asia, and then another into Indonesia, southern India, Australia, and Oceania. In a later migration out of East Asia, people traveled across the Bering Sea Land Bridge into the Americas.

Simply separating into smaller populations made people genetically distinct from others over time. In a smaller population, a rare variation of a particular gene can rapidly increase or disappear, making that group genetically distinct from the ancestors they left behind. The variation could represent a physical trait or something invisible to the naked eye. But, most of the time, the change in frequency of the variation brings no benefit. It’s chance.

The blond, dark-skinned natives of the Solomon Islands (about 1,100 miles northeast of Australia) are a recently discovered example of genetic drift. Twenty-six percent of Solomon Islanders, whose usually dark hair has a texture typically associated with African people, carry one copy of a specific recessive gene for blond hair. This means they inherited the gene from one parent, but it will not result in blond hair. But 5 to 10 percent of the islanders got the gene from both parents; they have blond hair. Researchers see no survival benefit to carrying the gene, which means it’s not a result of natural selection. The gene simply rose in frequency with ease in the small population.

Some genetic changes are adaptations to new environments. Migrating out of Africa, our ancestors faced different altitudes and temperatures and fewer hours of sunlight in their new world.

“Skin pigmentation is an adaptation, but it’s one of the worst indicators of race because it’s completely correlated with latitude and UV exposure,” Tishkoff says. “In the tropical belt, you can be equally dark skinned whether you’re from Africa, India, Australia, or Melanesia.”

Dark skin brings decreased susceptibility to sunburn, sun damage, and skin cancer. In the tropical belt, where people get more sun exposure, natural selection protects against those consequences.

What makes skin dark, light, or somewhere in between isn’t simple though. Researchers have identified many genes that explain the diversity in skin, hair, and eye pigment within a population of people and from one population to the next. More than one gene may be at work in each individual.

One example is the gene MC1R. It varies little among African people and more among Europeans and Asians. Researchers conclude that the gene’s relative inflexibility among Africans ensures that they have dark skin and hair. MC1R tends to relax, however, in Europeans and East Asians, resulting in variations that allow for lighter skin and hair color.

Another explanation for skin color lies in a specific mutation in the SLC245 gene. Called A111T, this mutation is thought to have arisen between the Middle East and the Indian subcontinent and is found in nearly all people who have European ancestry.

Skin and hair color aren’t the only traits we share with our geographic counterparts. For example, natural selection gives Tibetans and Andeans circulatory and respiratory systems better suited than those of most others to life at high altitudes.

Selection also explains why sickle cell anemia is most common among people with origins in malaria-prone regions, such as parts of Africa and the Mediterranean. Like the blond hair of the Solomon Islanders, people with sickle cell disease inherited a copy of the recessive gene from both parents. They carry this seemingly harmful gene because one copy of it brings a survival benefit — greatly reduced susceptibility to malaria.

Natural selection also explains lactose tolerance. Sixty-five percent of the world’s population is lactose intolerant. The gene that allows babies to digest milk usually shuts off after they are weaned. Some populations, however, evolved to be lactose tolerant. Geneticists have connected the rise of lifelong lactose tolerance in particular populations to the time in history when that group began to domesticate cows.

Complex traits

Sickle cell anemia, lactose tolerance, the blond hair of Solomon Islanders — these are each determined by a different single gene. But dozens or even hundreds of genes determine most of the traits that make us who we are.

“Skin color, heart disease, IQ, the majority of traits that we perceive as differences among individuals and among populations are complex traits,” Tishkoff says. “We’re just at the beginning of even understanding the genetic basis of complex traits.”

We don’t know enough about the genes that impact complex traits, such as IQ, to hypothesize that these traits vary by population and that natural selection might have caused it. Individual personalities and the collective behavior of our population are particularly difficult to study. It is extremely hard to determine the role genes play in behavior, let alone which genes, considering the roles that culture and upbringing play.

“You can’t simplify down to one gene that regulates an entire set of complex behaviors,” Tishkoff says. “That’s not how genetics works.”

This article originally appeared in Genome.


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