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The number of mildly harmful mutations in the European population has gradually increased over the last 45,000 years, ever since modern humans arrived on the continent. The mutations may be a lingering effect of the original migration into Europe.

鈥淭hese mutations that today are associated with genetic disease do not decrease over time,鈥� says St茅phane Aris-Brosou of the University of Ottawa in Canada.

However, while many of the mutations are linked to diseases, their effects are minor and it is unlikely that they are causing the people who have them significant harm.

Into Europe

Our species evolved in Africa and only moved into Europe in a big way 45,000 years ago. Geneticists have known for decades that African populations contain much more genetic diversity than non-African groups. This is because the first groups that moved out of Africa were fairly small.

To find out how European genomes have been affected by this, Aris-Brosou examined the genomes of 2062 Europeans, including 1179 ancient genomes dating back to up to 45,000 years.

For each genome, he looked at 1.2 million sites where a single 鈥渓etter鈥� on the DNA varies from person to person. Many of these genetic variants have previously been found to be statistically associated with diseases like asthma and diabetes, although their effects are often small: having a single harmful variant would only slightly increase a person鈥檚 chances of developing diabetes.

Aris-Brosou found that the number of mildly harmful variants in the European population has steadily increased over time.

The key factor was probably the small initial populations in places like Europe, which allowed the harmful mutations to become common. 鈥淭hen it鈥檚 very hard to get rid of them,鈥� says Laura Botigu茅 of the Centre for Research in Agricultural Genomics in Barcelona, Spain.

A lingering effect

In many ways this is in line with existing studies, says Botigu茅. 鈥淲hat is significant and new is the fact they include ancient specimens,鈥� she says. studies only used , for example .

It may seem odd that harmful mutations have become more common, because natural selection is supposed to winnow out disease-causing genes. However, in small populations the power of natural selection is reduced.

鈥淚 think everyone would agree now that highly deleterious mutations do not increase with human expansions,鈥� says Botigu茅. This is because people carrying really harmful mutations are unlikely to live long enough to have children, even in small groups.

However, existing studies have found that mildly harmful mutations do seem to be more common in populations that have gone through bottlenecks. In 2016, Botigu茅 helped show that .

Nothing to worry about

Aris-Brosou points out that his study has a big limitation: he could only look at genetic variants that are known to be harmful in modern populations. 鈥淭here could have been some mutations that were terribly deleterious, but that have been driven to extinction, so the mutations disappeared,鈥� he says. There is no way to identify such harmful mutations in the ancient DNA, if they are not present today.

Both Aris-Brosou and Botigu茅 agree that the lingering mutations in European populations are nothing to worry about. 鈥淚 think it鈥檚 obvious that it鈥檚 fine, because we are fine,鈥� says Botigu茅. 鈥淗umans are still around,鈥� says Aris-Brosou.

There is also very little we could do about them. Although the European population as a whole carries more mildly harmful mutations than African populations, if you compared individuals from the two continents you would not see a difference, Botigu茅 says. This is because we all carry so few of the mutations anyway. As a result, it is not possible to identify individuals or groups in Europe who have greater or smaller shares of the mutations.