Shovelled teeth, the ice age, and breastfeeding

Today, something a little different – a short mystery story about teeth shaped like shovels, the last Ice Age, and breastfeeding in ancient Native Americans.

Welcome to the tundra

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A set of images showing Beringia, and how it has become Alaska and Siberia over time as sea levels rose.
Credit: U.S. National Oceanic and Atmospheric Administration, public domain.

Our tale begins roughly 20,000 years ago in Beringia, a landmass that used to link Alaska with north-eastern Russia. A band of humans, numbering a few thousand, were trapped during the Ice Age, between vast sheets of ice to the east, and barren uninhabitable Siberia to the west. They settled here for 5,000 years, isolated, in what is known as the “Beringia Standstill”. Afterwards, they emerged to populate the Americas and North-Eastern Asia. How do we know this? Well, one part of the evidence is in the unusual “shovel” shape of their teeth, still common in many Native Americans and some East Asians today.

Shovelled teeth

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Photograph of human upper incisors with significant “shoveling”.
Credit: Christy G. Turner, II, courtesy G. Richard Scott

Skulls unearthed during archaeological digs have shown that shovelled teeth were present in all indigenous people throughout the Americas before European colonialists arrived. Scientists have also found a genetic cause for shovelled teeth, in the form of a mutation in a gene called EDAR.

But here’s the mystery – why are shovelled teeth so common? The assumption so far has been that they must give some sort of advantage, such that those with shovelled teeth survive and pass them onto their children – in other words, evolution by natural selection. What possible advantage could shovelled teeth provide?

All sorts of explanations have been put forward. One such theory was that the shovelling of teeth helps with the eating of animals, part of the staple diet amongst the Beringians.

But Leslea Hlusko, an expert in the evolution of teeth in early humans based University of California Berkeley, was not convinced. She looked elsewhere – maybe the explanation could be found in the diet, but not in the way everyone had thought.

Sunny D

sunset-3291263_1920

Though Beringia was relatively speaking a safe-haven for our group of humans 20,000 years, one of the hazards was low levels of sunlight. The UV light from the Sun helps our bodies produce vitamin D, a nutrient essential for bone development and for fat tissue. But being so close to the Arctic Circle, our band of Beringians were barely exposed to any UV light throughout the year, and so were at risk of a dangerous vitamin D deficiency.

The adults could make up for this by eating a diet rich in animal fat, which contains a lot of vitamin D. But the babies and infants of this group had to get all the nutrients they needed – including the vitamin D – from breastfeeding. So how did their mothers’ bodies ensure that enough vitamin D got into the milk?

Branching out

illu_breast_anatomy
Cross-section of human breast, with breast duct branching shown.

Looking at all the evidence, Hlusko has put forward a clever solution, published with colleagues in the scientific journal Proceedings of the National Academy of Sciences. It comes back to the EDAR gene – one of many effects that the mutation in EDAR causes, in addition to shovelled teeth, is an increase in the number of branches in the mammary gland of the breast.

The mammary gland is the organ that produces breastmilk, and it gets what it needs to do this from the surrounding fatty tissue of the breast. Previous research in mice has shown that when this fatty tissue lacks vitamin D, the mammary gland develops more branches – sort of like a tree developing more extensive roots when water is scarce.

Leslea Hlusko therefore offered the theory that the mutation in EDAR is an adaptation which ensured that babies got enough vitamin D in their breast milk. The Beringian mothers who carried the mutation had mammary glands with more branches, which provided the Vitamin D needed to help their Beringian babies survive, to eventually pass on the same mutation to their children, along with the breastmilk rich in vitamin D. Evolution in action.

But what about those shovelled teeth? Far from being a carnivorous tool for survival, the teeth are in fact just a by-product of something a little gentler, but equally essential – the passing of nutrients from mother to baby through breastfeeding.

The mutation in EDAR is still common amongst many Native Americans and East Asians today, and if Hlusko’s theory proves to be true, it would be first example of natural selection based upon the bond between mother and infant. She says that it “highlights the importance of the mother-infant relationship and how essential it has been for human survival.”

 

Read more about the research on Leslea Hlusko’s article for The Conversation.

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