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Developmental biology - Human Evolution|
Need for vitamin D may have increased breast density
"This highlights the importance of the mother-infant relationship and how essential it has been for human survival."
Hlusko specializes in the evolution of teeth among animals, in particular primates and early humans. She discovered these connections after being asked to participate in a scientific session on the dispersal of modern humans throughout the Americas at the February 2017 American Association for the Advancement of Science meeting. In preparing her talk on what teeth can tell us about the peopling of the New World, she pulled together the genetics of dental variation with the archaeological evidence to re-frame our understanding of selection on incisor shape.
For the study, Hlusko and her colleagues assessed the occurrence of shovel-shaped incisors in archeological populations in order to estimate the time and place of evolutionary selection for the trait. They found that nearly 100 percent of Native Americans prior to European colonization had shoveled incisors, as do approximately 40 percent of East Asians today.
The team then used genetic effects that are shared with dental variation as a way to discern the evolutionary history of mammary glands as they share a common developmental path.
Incisors are called "shovel-shaped" when the tongue-side of the incisors - the cutting teeth in the front of the mouth, four on top, four on the bottom - have ridges along the sides and biting edge. It is distinctive of Native Americans and populations in East Asia - Korea, Japan and northern China - with an increasing incidence as you travel farther north. Unconvinced by a previous idea that shoveled incisors were selected for their use in softening animal hides, Hiusko looked at explanations unrelated to teeth shape.
"People have long thought that this shoveling pattern is so strong that there must have been evolutionary selection favoring the trait, but why would there be such strong selection on the shape of your incisors?" Hlusko thought. "When you have shared genetic effects across the body, selection for one trait will result in everything else going along for the ride."
Understanding the origins of dense breast tissue could also have implications in breast cancer.
The frequency of the human-specific EDAR V370A isoform is highly elevated in North and East Asian populations. The gene is known to have several pleiotropic effects, among which are sweat gland density and ductal branching in the mammary gland. The former has led some geneticists to argue that the near-fixation of this allele was caused by selection for modulation of thermoregulatory sweating. We provide an alternative hypothesis, that selection instead acted on the allele’s effect of increasing ductal branching in the mammary gland, thereby amplifying the transfer of critical nutrients to infants via mother’s milk. This is likely to have occurred during the Last Glacial Maximum when a human population was genetically isolated in the high-latitude environment of the Beringia.
Because of the ubiquitous adaptability of our material culture, some human populations have occupied extreme environments that intensified selection on existing genomic variation. By 32,000 years ago, people were living in Arctic Beringia, and during the Last Glacial Maximum (LGM; 28,000–18,000 y ago), they likely persisted in the Beringian refugium. Such high latitudes provide only very low levels of UV radiation, and can thereby lead to dangerously low levels of biosynthesized vitamin D. The physiological effects of vitamin D deficiency range from reduced dietary absorption of calcium to a compromised immune system and modified adipose tissue function. The ectodysplasin A receptor (EDAR) gene has a range of pleiotropic effects, including sweat gland density, incisor shoveling, and mammary gland ductal branching. The frequency of the human-specific EDAR V370A allele appears to be uniquely elevated in North and East Asian and New World populations due to a bout of positive selection likely to have occurred circa 20,000 y ago. The dental pleiotropic effects of this allele suggest an even higher occurrence among indigenous people in the Western Hemisphere before European colonization. We hypothesize that selection on EDAR V370A occurred in the Beringian refugium because it increases mammary ductal branching, and thereby may amplify the transfer of critical nutrients in vitamin D-deficient conditions to infants via mothers’ milk. This hypothesized selective context for EDAR V370A was likely intertwined with selection on the fatty acid desaturase (FADS) gene cluster because it is known to modulate lipid profiles transmitted to milk from a vitamin D-rich diet high in omega-3 fatty acids.
Authors: Leslea J. Hlusko, Joshua P. Carlson, George Chaplin, Scott A. Elias, John F. Hoffecker, Michaela Huffman, Nina G. Jablonski, Tesla A. Monson, Dennis H. O’Rourke, Marin A. Pilloud and G. Richard Scott.
The synthesis presented here is a product of the American Association for the Advancement of Science’s 2017 annual meeting, as the ideas came together in the session entitled “Beringia and the Dispersal of Modern Humans to the Americas.” We thank Leslie Aiello, Marianne Brasil, Sarah Greenlee, Ophir Klein, Peter Kloess, Owen Lovejoy, Kunxin Luo, Whitney Reiner, Cat Taylor, Tim White, and three anonymous reviewers for critical feedback on earlier versions of this manuscript. Development of the ideas presented herein also benefited from conversations with Christopher Barrett, Michael Bell, Angelique Corthals, Liliana Davalos-Alvarez, Robert Dudley, Jennifer Raff, Ellen Simms, and Justin Tackney. L.J.H. also thanks the Department of Ecology and Evolution at the State University of New York at Stony Brook for feedback when she shared this research in their departmental colloquium series. Many thanks also to Christy G. Turner II, as his extensive study of incisor shoveling variation provided the foundation for the dental anthropology presented herein.
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Inuit mom nursing twins, Alaska. Date: [ca. 1903-1908]
Image Credit: Lomen Brothers, Nome, Alaska