Do humans degenerate genetically and become dumber as a result?

You are a mutant. You are born with about 100 mutations that your parents don’t have. You pass about half of that on to your children, if you have any, who will have 100 new mutations of their own. And also their children and so on. So do we humans accumulate generation after generation of harmful mutations, leading to a decline in our physical and mental fitness?

Some think so. “A substantial decline in human fitness can be expected in industrialized societies over the next few centuries,” said geneticist Michael Lynch. wrote in 2010. At this time, several studies reported a decline in IQ in a number of countries, including the United Kingdom, Australia, Denmark, Sweden, and Norway. It seemed like it might be direct evidence that we’re getting dumber.

The idea of ​​human degeneration was, of course, the basis for wildly unethical eugenic policies in the 20th century. Here it is here is a very ugly history. But while early eugenicists largely made things up to justify their prejudices, it’s now possible to sequence genomes and measure mutations directly to see what’s really going on.

This shows that people have relatively high mutation rate compared to most other animals. The main problem is fathers: while women are born with eggs already formed, in men sperm are constantly generated from stem cells that mutate over time. Since males can father children for many decades, there is more time for mutations to accumulate than in shorter-lived species.

Now, most of the 100 or so new mutations we all have don’t change anything because most of our DNA is junk. But some are probably harmful. They can occur in a protein-coding gene, resulting in a defective protein, or in a regulatory sequence that alters gene activity.

Severe mutations kill individuals unlucky enough to get them. But mutations that have little harmful effect can be passed down from generation to generation. So what prevents more and more harmful mutations from accumulating in the population?

The conventional wisdom in genetics is that by chance some offspring end up with far more harmful mutations than other offspring. These individuals are more likely to die before they can reproduce, or they may not be able to reproduce. This hideously unfair process creates a “genetic load” of harmful mutations at some level.

However, this level may change. Around half of all children were dying before adulthood, but in higher income countries almost everyone now survives thanks to vaccines, enough food and so on. This relaxed natural selection causes deleterious mutations to accumulate, Lynch suggested, leading to a decline in fitness in humans of at least 1 percent per generation and possibly as much as 5 percent.

That would be a serious problem. But some of the studies on which Lynch’s conclusions were based were conducted on animals such as flies and worms. i know Peter Keightley at the University of Edinburgh in the United Kingdom decided to measure the accumulation of mutations in a mammal. His team bred 55 lines of mice over 21 generations under favorable conditions – that is, with relaxed selection.

results, published in 2024that would equate to less than a 0.4 percent decline in fitness per generation in humans, and Keightley thinks there are many reasons why it would actually be much less.

For starters, natural selection still acts on humans. At least a third of conceptions lead to abortion, for example. “There’s always a choice,” he says Joanna Masel at the University of Arizona.

Being less fit isn’t always a bad thing

What’s more, fitness in the evolutionary sense is not always desirable. Infectious diseases were a big driver of high child mortality in the past and still kill many children in some areas, but gene variants conferring resistance to these diseases can have major drawbacks – the classic example being those 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 have also been big killers in the past, but the gene variants that prevent them are also likely to be often maladaptive when food is plentiful.

More generally, Masel thinks that while evolution can eliminate almost all harmful mutations in organisms like bacteria — which have small genomes and huge populations — it’s simply not possible in humans.

“Our genomes are monstrously bloated with all kinds of parasitic elements,” he says. “More harmful mutations are coming in than we can get rid of. But we have ways to compensate.”

Essentially, rather than trying to clean up each genetic “mess” individually, organisms develop the equivalent of sewage systems to constantly remove multiple messes, Masel says. From a biological point of view, it has been overlooked that rare beneficial mutations with a large effect can compensate for a large number of mildly deleterious mutations. (Remember that rare mutations with a large deleterious effect are quickly eliminated.)

This idea has profound implications. “Detrimental mutations can be a driver of complexity because they create a mess that needs to be cleaned up at higher levels of complexity,” says Masel. For example, when mutations filled the genes with bits of junk DNA, cells developed a system to cut these pieces of junk out of the RNA copies of the genes.

Interestingly, the simulations her team performed indicated that as the number of mutations increased, beneficial mutations accumulate faster than the harmful ones.

“You’re actually improving the waste disposal system faster than you’re creating more clutter,” says Masel. “To our surprise, the math worked out like this.

If this is true, higher mutation rates in humans may not be the big problem that many biologists thought, and those studies that report declining IQ may just be a fluke. The science isn’t settled, but it looks like there’s no reason to panic about human degeneration—which is just as well, as if there’s no easy way to reverse it.

In the meantime, there are other things we should worry about, says Masel. “I think there are things like climate change where the science is settled and we should be panicking,” he says. I totally agree.

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