Every individual carries approximately 100 genetic mutations not present in their parents. If reproduction occurs, roughly half of these mutations are passed to offspring, who will then possess their own 100 new mutations. This generational accumulation of genetic alterations raises a critical question: are humans progressively accumulating harmful mutations, leading to a deterioration in our collective physical and mental capabilities?
Some researchers have voiced concerns about this possibility. In 2010, geneticist Michael Lynch posited that industrialized societies could anticipate a significant decline in human fitness over the coming centuries. Concurrently, several studies reported a decrease in IQ scores in various countries, including the United Kingdom, Australia, Denmark, Sweden, and Norway. These findings initially suggested a direct correlation with a perceived decline in human intelligence.
The concept of human degeneration formed the theoretical basis for the ethically reprehensible eugenics movements of the 20th century. These historical episodes are marked by prejudiced ideologies. However, unlike the speculative foundations of early eugenics, modern genomic sequencing technologies now provide the means to directly measure mutations and observe their actual prevalence.
Human Mutation Rates: A Comparative Look
Current genetic analysis reveals that humans exhibit a relatively high mutation rate when compared to most other animal species. A primary contributor to this rate is paternal inheritance. While females are born with a complete set of eggs, males continuously generate sperm from stem cells that undergo mutations over time. This prolonged reproductive lifespan in males allows for a greater accumulation of mutations than observed in species with shorter lifespans.
The majority of the roughly 100 new mutations present in any individual has no discernible effect, largely because a significant portion of our DNA is non-functional. However, a subset of these mutations can be detrimental. These may occur within protein-coding genes, leading to the production of impaired proteins, or within regulatory sequences, thus altering gene expression.
The Role of Natural Selection in Genetic Load
Severe mutations typically result in individuals failing to survive or reproduce. Conversely, mutations with only minor harmful effects can be inherited through generations. The question then arises: what mechanism prevents the unchecked buildup of increasingly deleterious mutations within a population?
The prevailing genetic theory posits that random chance determines the distribution of harmful mutations among offspring. Individuals who inherit a disproportionately high number of these mutations are more likely to face reduced reproductive success, either by dying before reproduction or by being unable to reproduce. This process, though seemingly harsh, serves to stabilize the “genetic load” of harmful mutations at a particular level.
Relaxed Selection and Mutation Accumulation
However, this equilibrium is not static. Historically, nearly half of all children did not survive to adulthood. In higher-income nations, however, near-universal survival is now common, largely attributed to advancements in vaccination, nutrition, and healthcare. Geneticist Michael Lynch suggested that this relaxation of natural selection is contributing to an accumulation of harmful mutations, potentially leading to a decline in human fitness—as much as 1% per generation, and possibly up to 5%.
Mammalian Mutation Studies and Fitness Reduction
If true, such a decline would represent a significant concern. Notably, some of the research informing Lynch’s hypothesis involved animal models such as flies and worms. To investigate mutation accumulation in mammals under relaxed selection, Peter Keightley’s team at the University of Edinburgh, UK, conducted a study. They bred 55 lines of mice over 21 generations under conditions of favorable, meaning relaxed, selection.
The findings, published in 2024, indicate a fitness reduction in humans that would equate to less than 0.4% per generation. Keightley identified several factors suggesting the real-world impact would likely be even smaller.
Natural Selection’s Continued Influence
Natural selection continues to operate on human populations. For instance, approximately one-third of all conceptions result in miscarriage. “There’s always selection,” notes Joanna Masel from the University of Arizona.
Fitness: Not Always a Desirable Trait
Furthermore, evolutionary fitness is not uniformly advantageous. Infectious diseases were a significant cause of high child mortality in the past and remain a threat in some regions. However, gene variants conferring resistance to these diseases can carry substantial drawbacks; a classic example is the sickle cell trait, which offers protection against malaria but causes sickle cell disease. As Masel explains, “If there’s no malaria, you really don’t want them.”
Similarly, starvation and malnutrition were major causes of mortality in earlier times. Yet, gene variants that protect against these conditions might prove maladaptive in environments with abundant food supplies.
Compensatory Mechanisms in Human Evolution
Masel proposes that while evolution can effectively eliminate most detrimental mutations in organisms with small genomes and vast populations, this is less feasible for humans. “Our genomes are monstrously bloated with all kinds of parasitic elements,” she states. “There’s more deleterious mutations coming in than we can get rid of. But we have ways of compensating for them.”
Instead of individually correcting each genetic flaw, organisms have evolved systems akin to “sewage systems” for managing multiple issues simultaneously. A critical, often overlooked, factor is that rare, highly beneficial mutations can compensate for a multitude of minor detrimental ones. It is important to recall that rare mutations with significant harmful effects are rapidly eliminated.
Deleterious Mutations as Drivers of Complexity
This perspective carries significant implications. Masel suggests that “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.” For example, the accumulation of junk DNA within genes prompted cells to develop mechanisms for excising these segments from RNA transcripts.
Intriguingly, simulations conducted by Masel’s team indicate that an increase in mutation rates can lead to a faster accumulation of beneficial mutations than harmful ones. “You’re actually improving the garbage disposal system faster than you’re creating more mess,” Masel observes. This outcome, she notes, emerged counterintuitively from their calculations.
Conclusion: No Cause for Alarm Yet
If these findings hold true, the elevated mutation rate in humans may not be the significant concern many biologists have presumed. The studies reporting declining IQ scores could potentially be attributed to chance variations. While the scientific consensus is not yet definitive, current evidence suggests there is no immediate cause for panic regarding widespread human degeneration, particularly given the absence of an easy reversal mechanism.
Masel emphasizes that more pressing issues warrant our attention. “I think there are things out there, like climate change, where the science is settled and we should be panicking,” she concludes. This sentiment is widely shared.