For the first time, scientists say they have "evidence of selection on the human maternal-infant bond"—and it has something to do with an incredible connection between breastfeeding and the dental patterns of some population groups during the last ice age.
According to researchers out of the University of California-Berkley, Native Americans and northern Asians who lived some 20,000 years ago were the first to develop uniquely shaped incisor teeth, which are still common only among these populations. The origin of this adaptation stumped biologists until they determined the gene that controls the development of those teeth is the same gene responsible for mammary duct growth.
As for the reason this trait is just found among Native Americans and northern Asians who lived during the ice age: The researchers believe the women's bodies were adapting to provide more fat and more vitamin D to support their hardy northern babies.
"This highlights the importance of the mother-infant relationship and how essential it has been for human survival," says Leslea Hlusko, an associate professor of integrative biology at the University of California-Berkeley, in a press release.
According to the research published this week in the journal Proceedings of the National Academy of Sciences, the researchers began by looking into the development of the shovel-shaped incisors that were found among nearly 100% of Native Americans before European colonization and remain common in East Asia.
With no obvious reason for the development of the tooth shape, Hlusko says they figured it was packaged with another, more rational evolutionary development. As she says, "When you have shared genetic effects across the body, selection for one trait will result in everything else going along for the ride."
Unsatisfied with the previous hypothesis on the development of those teeth, Hlusko and her co-author launched an investigation by studying the genetics of the population that lived in the high-latitude Beringian refugium during the last ice age, where it was hard to come by natural vitamin D. They found that the gene EDAR was tied to both mammary duct growth and the incisor teeth.
"We know they had a diet that was attempting to compensate for it from the archaeological record, and because there is evidence of selection in this population for specific alleles of the genes that influence fatty acid synthesis," Hlusko says. "It looks like this mutation of the EDAR gene was also selected for in that ancestral population, and EDAR's effects on mammary glands is the most likely target of the selection."
This doesn't mean much for humans living today. But it is amazing proof that women's bodies have been responding in significant ways to the needs of their babies for thousands of years—because we're incredible like that.