When the ancestors of Native Americans ventured across the Bering land bridge from today’s Siberia to Alaska about 20,000 years ago, they struggled to get enough sunlight during the long, dark winters. Living so far north with scant sunshine should have led to rickets and other health problems, yet somehow the population survived and even thrived enough to live there for thousands of years. Their lucky break, according to a new study, was that they carried a genetic mutation—revealed in ancient teeth—that boosted the development of milk ducts in women’s breasts, which may have helped nursing mothers pass more nutrients to their infants.
“Teeth telling us something about fertility? That’s really amazing,” says biological anthropologist Julienne Rutherford of the University of Illinois in Chicago, who was not involved with the work.
The gene in question is known as EDAR. Native Americans and Asians carry a version of the gene that is linked to thicker hair shafts, more sweat glands, and shovel-shaped incisors. A variant of this gene—V370A—arose about 30,000 years ago or so in China when the climate was hotter and more humid, which prompted researchers to speculate initially that it was advantageous to have more sweat glands in that environment. But the gene variant swept through the ancestors of Asians and Native Americans about 20,000 years ago, when the climate where they lived in Asia and Beringia (the now-submerged land between Asia and Alaska) was colder and dryer. So the actual cause of the gene’s spread has been unknown.
The new study reveals that the variant was so beneficial it spread to everyone in the Americas. When researchers led by biological anthropologist Leslea Hlusko of the University of California, Berkeley, examined data on the teeth of more than 5000 people from 54 archaeological sites in Europe, Asia, and North and South America, they found shoveled teeth—and hence, the gene variant that causes them—in about 40% of the individuals in Asia and all of the 3183 fossils of Native Americans they examined (who all lived before European colonization).
This suggests that some members of the first group to arrive in Beringia probably carried the gene, which arose in Asia. Then it quickly swept through the rest of the small isolated population of people who settled there between 28,000 and 18,000 years ago.
Living at such a high latitude puts nursing infants at risk of not getting enough sunlight in winter to synthesize vitamin D in their skin. Unlike adults, nursing infants can’t eat marine foods and organ meat rich in vitamin D to compensate. Vitamin D deficiency can trigger serious problems with bone development, such as rickets. It also interferes with the many ways fat insulates and fuels our bodies, as well as how the immune system wards off disease.
The EDAR variant led to the development of more elaborate branching of milk ducts in studies of mice. Hlusko and her colleagues hypothesize that those extra branches cause mothers to produce more milk or deliver more nutrients in their milk. If so, children of mothers with the EDAR variant would have been more likely to survive, thus spreading the variant throughout the population, the team proposes today in the Proceedings of the National Academy of Sciences.
The study shows how natural selection can work rapidly when humans move into extreme environments, such as the Arctic, exerting strong selection on genes critical for development and metabolism, says biological anthropologist William Leonard of Northwestern University in Evanston, Illinois, who was not involved with this work.
Follow-up studies need to be done to explore just how an increase in breast ducts might deliver more nutrients. One possibility, Hlusko says, is that the EDAR gene variant works with fatty acid genes to deliver more fats in breast milk. Researchers need to see whether there is a connection between the genes, says geneticist Tábita Hünemeier of the University of São Paulo in Brazil, who found selection for fatty acid genes in Native Americans and is not involved with the work.
Hlukso is now collaborating with others to explore how the EDAR gene variant affects breast development and density. “This is not just a Native American story,” Hlusko says. “Everyone with shovel-shaped incisors has this gene that may compensate for vitamin D deficiency.”