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Natural selection was at work on聽Bronze Age Britons, ancient DNA reveals. Within the past 4500聽years, evolution has acted on genes involved in the production of vitamin D 鈥� which people living in Britain are sometimes short of due to a lack of sunlight for much of the year.

The genetic changes have had knock-on effects on other traits, from the ability of people to
digest milk to their skin colour.

One of the ways evolutionary change can happen is through natural selection: genetic variants that are beneficial become more common in the population because individuals that carry them are more likely to reproduce.

In recent years, geneticists have collected DNA from the remains of thousands of people who lived in Britain over the millennia, so it is possible to see natural selection by聽looking for genetic variants becoming more or less common.

鈥淚n some cases, the change is so聽dramatic that you can rule out this happening by chance, and that鈥檚 when we would posit that selection is driving this,鈥� says at the University of Michigan.

Terhorst has developed a new聽method of analysing ancient DNA for signs of natural selection. Unlike previous techniques, it doesn鈥檛 assume that selection is equally intense throughout the study period, as that is unrealistic. 鈥淭he novelty here is that we can really localise selection to within a聽few thousand years, and say 鈥榯his is what鈥檚 being selected鈥�,鈥� says at the University of Pennsylvania, who has worked with Terhorst to apply the technique to ancient Britons.

The new approach impresses , who is at the University of York in the UK. 鈥淚t鈥檚 really amazing,鈥� she says. 鈥淲e鈥檙e all going to use the method.鈥�

Terhorst and Mathieson have now used the new technique to examine DNA from 529 ancient Britons from the past 4500 years, enhanced with genetic data from 98 present-day individuals. They found seven regions of the genome with strong evidence of selection.

To their surprise, there was a pattern. 鈥淎ll these genes that are under selection can plausibly be linked to natural selection for increased vitamin D and calcium,鈥� says Mathieson.

That finding fits into an existing body of knowledge about the role of vitamin D in recent human evolution, which has driven genetic and cultural adaptations in some populations. At the heart of this is the health function of vitamin D. It helps us to absorb dietary calcium, strengthening bones. Children who are deficient in vitamin D can develop soft bones, a condition called rickets.

Our bodies make vitamin D when our skin is exposed to ultraviolet radiation in sunlight. When humans first evolved in Africa, there was no shortage of sunlight. However, when people migrated away from the tropics, they found themselves in places where the sunlight reaching them is less intense and the days can聽be shorter. Britain鈥檚 cloudy skies didn鈥檛 help either.

at Pennsylvania State University, who has spent聽decades unpicking the significance of vitamin D on human populations, says Britain has 鈥渁 punishingly low and highly seasonal UV regime鈥�.

Unable to produce enough vitamin聽D, Bronze Age Britons adapted. One shift, which Mathieson and Terhorst saw in the DNA they examined, was towards lighter skin. Skin pigmentation protects against UV, which is good聽in the tropics as it guards against skin cancer, but can聽limit vitamin D production聽in Britain.

It may seem odd that this shift聽only happened in the past few thousand years, when people have lived in Britain for much longer than that. However, the earlier inhabitants were hunter-gatherers who could get vitamin D by eating oily fish.

Cheddar Man, who lived in Britain about 10,000 years ago, had very dark skin, and some Irish people from about 5000 years ago had moderately dark skin.

Natural selection for paler skin聽only really kicked in when people started getting most of their food from crop farming, leaving them prone to vitamin D deficiency, says Mathieson.

Jablonski agrees this would create evolutionary pressure. 鈥淭hey鈥檙e going to be at the sharp end of natural selection,鈥� she says.

People can also compensate for a lack of vitamin D by consuming lots of calcium in their diet. Milk is聽an excellent source, but for most聽of our species鈥� existence we could only digest it as babies. After that, our bodies stopped making the lactase enzyme that digests the lactose sugar, so drinking milk caused indigestion.

However, in Bronze Age Britain there was strong selection for producing lactase even into adulthood, allowing people to drink milk throughout their lives. Today, most adults of European origin can drink milk, whereas many other populations remain lactose-intolerant beyond infancy.

The study illustrates the importance of vitamin D, says Jablonski, because the lack of it forced evolution to find 鈥渕ultiple strategies鈥� to help Bronze Age Britons survive. 鈥淰irtually everything in the body requires vitamin D,鈥� she says, so it isn鈥檛 surprising that a shortage provoked rapid evolution.

Reference: bioRxiv,

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