Evidence from a new large-scale study suggests that humans are still evolving, and this process is weeding out genes that make developing Alzheimer's or becoming a heavy smoker more likely. The landmark research examines how the human genome is evolving over just one or two generations.

A team from Columbia University analysed the genomes of 210,000 people in the UK and United States showing that these genes are found less often in people who live longer, suggesting that natural selection is working to remove these unfavourable traits from both populations.

It is generally thought that people who live longer are more likely to pass on their genes and, therefore, their traits. Because most people in more affluent countries live well past reproductive age, many argue that humans have stopped evolving. However, recent evidence is hinting that we are still evolving in subtle ways, even over just one or two generations.

The scientists from Columbia University also found that certain genetic mutations which predispose people to heart disease, asthma, high cholesterol or obesity, are also found less often in people who live longer. Their findings have been published in the journal PLOS Biology.

"It's a subtle signal, but we find genetic evidence that natural selection is happening in modern human populations," said Joseph Pickrell, co-author of the study.

Favourable traits evolve when they offer an advantage to survival and become more widespread through a population as each generation passes them on. Complex traits, for example the ability to walk on two legs, have taken millions of years to evolve. The process of evolution itself however, occurs due to tiny changes in the genetic makeup of each passing generation as certain mutations become more prevalent.

Because of the revolution in gene sequencing, researchers can now pinpoint this process in action by comparing the genes of thousands of people and tracing which specific mutations rise and fall across generations.

The team looked at the genes of 150,000 people in the UK and 60,000 people of European ancestry in the US. They observed two significant trends in the data. Firstly, there was a drop in the frequency of the ApoE4 gene, which is linked to Alzheimer's. Secondly, they found that a specific mutation in the CHRNA3 gene, which is associated with heavy smoking in men, occurred less often after middle age.

"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," said Molly Przeworski, a co-author of the study.

As more people have their genes sequenced, scientists will be able to shed further light on the ways in which humans are evolving.

Human Genome Illustration
New research finds 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 weeding out these unfavorable variants in some populations.National Institutes of Health