A massive, multi-generational study of the human genome suggests that natural selection is still ridding our genome of harmful mutations that shorten lifespan. This work is one of the first attempts to directly show how humans are evolving over one or two generations.
Hakhamanesh Mostafavi, lead author of the study, and his colleagues analyzed over 215,000 DNA samples from large U.S. and U.K. genetic databases. They scoured the databases for mutations that changed prevalence across different age groups. Of the more than 8 million common mutations, they found two that seemed to become less prevalent with age: the APOE gene and the CHRNA3 gene.
The APOE gene is strongly linked to Alzheimer’s disease and was rarely found in women over 70 years old. The CHRNA3 gene is associated with men who are heavy smokers, but tapers off around the middle age. According to the data, people who carry a harmful genetic mutation have shorter life spans, so the mutation is less prevalent among older populations.
But there’s more to life than living a long time. From an evolutionary standpoint, it’s more important to have lots of offspring who survive into adulthood—who then birth their own offspring. So the question remains: why is longevity important?
Scientists have two hypotheses. First, parents who live to an old age in good health can raise their children and can help care for grandchildren—what’s known as the “grandmother hypothesis.” This would increase the later generations’ chances of surviving and reproducing and would explain why women tend to live decades after menopause.
Second, it’s possible that malignant genetic variations in old age also harmful earlier in life. Though, researchers would need extremely large samples to see these small effects. They also looked at genetic mutations that influenced overall health. Here’s Bruno Martin, reporting for Nature:
The researchers also found that certain groups of genetic mutations, which individually would not have a measurable effect but together accounted for health threats, appeared less often in people who were expected to have long lifespans than in those who weren’t. These included predispositions to asthma, high body mass index and high cholesterol. Most surprising, however, was the finding that sets of mutations that delay puberty and childbearing are more prevalent in long-lived people.
It’s notoriously difficult to study ongoing evolution—mainly because scientists need to observe the frequency of a mutation across multiple generations, from parents to children, to grandchildren. “That would be very hard to do well,” says Gil McVean, a statistical geneticist at the University of Oxford, UK for Nature. “You would need vast samples.”