New Scientist
Humans are born with new mutations every generation, but that does not necessarily mean we are sliding into genetic decline.
Are we evolving to be more stupid?
Author: Michael Le Page
YOU are a mutant. You were born with around 100 mutations that your parents don’t have. You will pass about half of these on to your children, if you have any, who will have 100 mutations of their own. So, are we accumulating harmful mutations over time, driving down our physical and mental fitness?
Some think so. “A substantial reduction in human fitness can be expected over the next few centuries in industrialized societies,” geneticist Michael Lynch wrote in 2010. Around that time, several studies reported declining IQ in a number of countries, including the UK and Australia.
The idea of human degeneration was the basis for wildly unethical eugenic policies in the 20th century. But while eugenicists were largely making stuff up to justify their prejudices, it is now possible to measure mutations directly to see what is really going on.
What this shows is that humans have a relatively high mutation rate compared with other animals. Severe mutations kill individuals unfortunate enough to get them. But mutations that have only a minor harmful effect can be passed down. So, what stops ever more detrimental mutations building up in the population?
The conventional idea is that, by chance, some offspring end up with more harmful mutations than other offspring. These individuals will be more likely to die before they can reproduce, stabilising the “genetic load” of harmful mutations at a certain level.
However, that level can change. Around half of all children used to die before adulthood, but in higher-income countries, almost all now survive. This relaxed natural selection is causing harmful mutations to build up, Lynch suggested, leading to a reduction in fitness in people of at least 1 per cent per generation and possibly as high as 5 per cent.
But some of the studies Lynch’s conclusions were based on were done in animals like flies and worms. So, Peter Keightley at the University of Edinburgh in the UK decided to measure mutations in a mammal. His team bred 55 lines of mice over 21 generations in favourable conditions – that is, with relaxed selection.
The results, published in 2024, would equate to a fitness reduction of less than 0.4 per cent per generation in humans, and Keightley thinks there are many reasons why it would be much smaller in reality.
For starters, natural selection is still acting on people. At least a third of conceptions result in miscarriage, for instance. “There’s always selection,” says Joanna Masel at the University of Arizona.
What’s more, evolutionary fitness isn’t always desirable. Infectious diseases were a big driver of child mortality in the past and still kill lots of children in some areas, but gene variants conferring resistance to these diseases can have downsides – like ones that protect against malaria but cause sickle cell disease.
More broadly, Masel thinks that while evolution can eliminate almost all harmful mutations in organisms such as bacteria – which have huge populations and tiny genomes – it can’t do so in humans.
“There’s more deleterious mutations coming in than we can get rid of. But we have ways of compensating for them,” she says.
In biological terms, what has been overlooked is that rare useful mutations can compensate for lots of slightly detrimental ones. Intriguingly, simulations run by Masel and her team suggest that when mutation rates increase, beneficial mutations accumulate faster than harmful ones do.
So, while the science isn’t settled, the higher mutation rate in humans may not be the problem many have assumed it is.
In the meantime, there are other things we should be worrying about, says Masel. “I think there are things out there, like climate change, where the science is settled and we should be panicking,” she says. I completely agree.
Credits: TCA, LLC.