Evidence that humans are still evolving
Wednesday 20 September 2017
A study of over 200,000 people in the US and the UK has found that the genetic variants linked to Alzheimer’s disease and heavy smoking are less frequent in people with longer lifespans – suggesting that natural selection is happening in modern human populations.
The study by researchers from Columbia University in the US involved analysing the DNA and genes of 60,000 people in the US and 150,000 people in the UK. The aim was to look at how humans are evolving over one or two generations.
The researchers were looking for gene mutations that changed across different age groups. They found that two common mutations – from more than 8 million tested – appeared to become less common as people get older. A variant of the APOE gene, which is linked to Alzheimer’s disease, was rarely found in women over 70 years old. And a mutation in the CHRNA3 gene, which is associated with heavy smoking in men, got less common as people reached middle-age.
People who don’t have these gene mutations are likely to live longer. This research could suggest that natural selection is weeding out these unfavourable variants in the particular populations studied.
The fact that only two common mutations were found during this huge research study was surprising. This suggests that other variants have been purged from the population.
Molly Przeworski, co-author of the study said: “It may be that men who don't carry these harmful mutations can have more children, or that men and women who live longer can help with their grandchildren, improving their chance of survival.”
Delayed puberty and childbearing
The researchers also found that there are certain groups of genetic mutations that are less common in people who were expected to have long lifespans than those who weren’t. These include the mutations that predispose someone to asthma, a high body mass index, high cholesterol and heart disease.
They also made the surprising discovery that people who are genetically predisposed to delayed puberty and delayed childbearing were those who live longer. A one-year delay in puberty lowered the death rate by 3–4%, and a one-year delay in childbearing lowered the death rate by 6% in women.
So can these results be taken as evidence that genetic variants that influence fertility are evolving? Or is the link between a long life and late fertility due to wealth and education, leading to people having children later in life?
The study’s lead author, Hakhamenesh Mostafavi, advised some caution, saying that: “The environment is constantly changing. A trait associated with a longer lifespan in one population today may no longer be helpful several generations from now or even in other modern day populations.”
This research follows a similar study that analysed whole DNA and gene sequences, and was able to reveal how over the past 2,000 years, British people have become taller and, blonder, are more likely to have blue eyes and are better able to digest milk.
That study took advantage of the fact that genetic variants that are favoured under natural selection are increasing in frequency – meaning that the population they came from is smaller in size than the population they are currently found in.
These new techniques of analysing DNA and genes in large populations are shedding light on where changes are happening. And it is also allowing scientists to find out why certain people live for longer.
For example, a study of healthy people over the age of 80 found that they had a lower genetic risk of Alzheimer’s disease and heart disease than those who either died earlier or lived a long life due to medical interventions.
These studies will continue to pinpoint how our genetics impact outcomes in life, and how natural selection will impact future generations.