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You are a mutant. You were born with around 100 mutations that your parents don’t have. You’ll pass about half of these on to your children, if you have any, who will have 100 new mutations of their own. And their children too, and so on. So, are we humans accumulating harmful mutations generation after generation, resulting in a decline in 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 . Around this time, several studies reported declining IQ in a number of countries, including the UK, Australia, Denmark, Sweden and Norway. It seemed like this might be direct evidence of us becoming more stupid.

The idea of human degeneration was, of course, the basis for wildly unethical eugenic policies in the 20th century. There’s . But while the early eugenicists were largely making stuff up to justify their prejudices, it is now possible to sequence genomes and measure mutations directly to see what is really going on.

What this shows is that humans have compared with most other animals. Fathers are the main issue: while women are born with their eggs already formed, in men, sperm are continuously generated from stem cells that mutate over time. Because men can father children for many decades, there’s more time for mutations to accumulate than there is in shorter-lived species.

Now, most of the 100 or so new mutations we all have make no difference because most of our DNA is junk. But a few are likely to be harmful. They might occur in a protein-coding gene, resulting in a faulty protein, or in a regulatory sequence, altering gene activity.

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Severe mutations kill individuals unfortunate enough to get them. But mutations that have only a minor harmful effect can be passed on down the generations. So, what stops ever more detrimental mutations from building up in the population?

The conventional idea in genetics is that, by chance, some offspring end up with a lot more harmful mutations than other offspring do. These individuals will be more likely to die before they can reproduce, or will perhaps be unable to reproduce. This hideously unfair process stabilises the “genetic load” of harmful mutations at a certain level.

However, that level can change. Around before adulthood, but in higher-income countries almost all now survive thanks to vaccines, plentiful food and so on. 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.

That would be a serious problem. But some of the studies Lynch’s conclusions were based on were done in animals like flies and worms. So, at the University of Edinburgh in the UK decided to measure the accumulation of mutations in a mammal. His team bred 55 lines of mice over 21 generations in favourable conditions – that is, with relaxed selection.

The results, , 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 at the University of Arizona.

Being less fit isn’t always a bad thing

What’s more, fitness in the evolutionary sense isn’t always desirable. Infectious diseases were a big driver of the high child mortality in the past and still kill lots of children in some areas, but gene variants conferring resistance to these diseases can have big downsides – the classic example being the ones that protect against malaria but cause sickle cell disease. “If there’s no malaria, you really don’t want them,” says Masel.

Starvation and malnutrition were also big killers in the past, but gene variants that prevent against them are also probably often maladaptive when food is plentiful.

More broadly, Masel thinks that while evolution can eliminate almost all harmful mutations in organisms such as bacteria – which have tiny genomes and huge populations – it just isn’t possible in humans.

“Our genomes are monstrously bloated with all kinds of parasitic elements,” she says. “There’s more deleterious mutations coming in than we can get rid of. But we have ways of compensating for them.”

In essence, rather than trying to clean up each genetic “mess” individually, organisms evolve the equivalent of sewage systems for continually clearing up lots of messes, says Masel. In biological terms, what’s been overlooked is that rare beneficial mutations with a big effect can compensate for lots of slightly detrimental mutations. (Remember, rare mutations with a big harmful effect are quickly eliminated.)

This idea has profound implications. “Deleterious mutations may be the driving force of complexity, because they create the mess that needs to be cleaned up at higher levels of complexity,” says Masel. For instance, when mutations filled up genes with bits of junk DNA, cells evolved a system for cutting these bits of junk out of the RNA copies of genes.

Intriguingly, simulations her team has been running suggest that when mutation rates increase, than harmful ones do.

“You’re actually improving the garbage disposal system faster than you’re creating more mess,” says Masel. “The math counterintuitively, to our surprise, came out that way.”

If this is right, the higher mutation rate in humans may not be the big problem many biologists have assumed it is, and those studies reporting declining IQ could just be down to chance. The science isn’t settled, but it’s looking like there’s no reason to panic about humans degenerating – which is just as well as there would be no easy way to reverse it.

In the meantime, there are other things we should be worrying about far more, 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.