Origin: A Genetic History of the Americas

often have a morphology known as shoveling: an indentation
that you can feel as you rub your tongue along the inside
surface. This shoveling trait is also found at high frequencies
among East Asian populations, but it is uncommon among other
groups around the world. It was one of the clues that physical
anthropologists used to infer a connection between Native
Americans and Asiatic peoples, before it was possible to
sequence their DNA.
This shoveling trait is linked to a particular variant (called
V370A) in a gene known as EDAR. EDAR has been under
strong selection, and V370A is found at very high frequencies in
the Indigenous peoples of the Americas—not just in the Arctic
but in all populations so far examined.
It doesn’t make a lot of sense for natural selection to be
working so hard to maintain the shoveling trait—it’s not
particularly useful for anything, just one of those morphological
variants that humans have, like the ability to roll one’s tongue.
Evolutionary biologist Leslea Hlusko has a hunch that there is
much more to the story. She suspects that the shoveling feature
is just incidental; V370A influences quite a lot of different
aspects of human phenotypes including sweat glands, hair
thickness… and the branching of mammary glands.
In the Arctic, as Hlusko explained to me, you can’t get
enough UV light to make enough vitamin D to stay healthy, even
if you stood outside naked (which really isn’t an option for very
long). Humans have developed effective cultural adaptations to
deal with this, including a diet rich in the parts of animals that
can give them more than enough vitamin D. However, infants
are utterly dependent on their mothers for nutrition, so they are
particularly vulnerable when it comes to vitamin D. Perhaps,
Hlusko thought, instead of the shoveling trait that’s being
selected for, it’s the role V370A plays in mammary gland
branching that’s the critical factor in its selection. If V370A leads
to increased ability to absorb vitamin D (and perhaps other
nutrients as well) via increased mammary ductal branching,
then it would explain the intense selection pressure that