Weird Science | Having kids doesn't keep you younger

The same genetic mutations that enhance reproduction early in life are likely to lead to ageing in later years, meaning that fertile individuals may age faster

The older one grows, the less likely they are to have offspring. That is, of course, intuitively known. What may appear counterintuitive, however, is that the more reproductive an individual is today, the faster they are likely to age tomorrow.

For centuries, scientists have been pondering why people age and die(Pixabay)

In a study published in Science Advances earlier this month, scientists studied a database of 276,000 people and found evidence that fertility is indeed inversely correlated with longevity.

For evolutionary biologists, this is not really a revolutionary idea. The theory dates back to 1957, coming from the American evolutionary biologist George Williams. The same genetic mutations that enhance reproduction early in life, Williams speculated, are likely to lead to ageing in later years.

This is known as the antagonistic pleiotropy theory of ageing. It might appear to run contrary to the idea of natural selection, which aims to ensure the survival of the fittest and enhance their reproductivity. If those genetic mutations will eventually cause ageing, and then death, why should natural selection allow that?

Williams offered his explanations in a paper in the journal Evolution in 1957. Among various theories about why organisms age, his ideas are the most widely accepted today. And now, there is genetic evidence on a large scale.

Why we age

 

For centuries, scientists have been pondering why people age and die. The ideas accepted today, however, began to take hold only after Charles Darwin.

Around the middle of the 20th century, a number of biologists, including Williams, proposed that the older an individual grows, the less effective natural selection becomes in maintaining the individual’s fitness. This is called the force of natural selection, which reduces with age.

Among those scientists was the British-Indian biologist JBS Haldane, who used the example of Huntington’s disease to illustrate the idea. This incurable neurodegenerative disease usually affects people over the age of 30. Haldane noted that many ancient humans would already have died before reaching that age, and speculated that they were therefore spared this affliction. As a result, the genetic mutations that cause the disease would have gone unnoticed by natural selection, and were passed on to the next generation, eventually reaching modern humans.

The Brazilian-British biologist Charles Medawar, too, agreed that the force of selection declines with age. If a genetic mutation occurs relatively early but expresses its harmful effects late in life, natural selection (now weaker) is unable to act against these effects. By that time, reproduction might have stopped, but the mutation would already have been passed on to the next generation. Over time, such mutations, being harmless early in life, would accumulate in the population, and ageing would evolve.

When Williams wrote his landmark paper in 1957, he took these ideas further. Some genetic variations enhance reproduction early in life, and some have harmful effects later. Williams proposed that these are the same mutations, causing different effects now and later. This is how ageing evolved, as a late byproduct of selection for reproductivity during youth.

Over the years, there have been individual case studies that back up the pleiotropy theory of ageing. What the theory lacked, however, was evidence on a large genomic scale.

Until now.

The new evidence

 

The UK Biobank, a large biomedical database that stores genomic information from half a million participants, is accessible to researchers with approval. For the new study, evolutionary biologist Jianzhi Zhang of the University of Michigan and his colleague Erping Long of the Chinese Academy of Medical Sciences tapped into the Biobank for the genetic information on 276,406 individuals.

Indeed, they found that genetic mutations that enhance reproduction do tend to shorten lifespan, just as Williams had predicted over six decades ago. Also, if someone carries mutations that promote high reproductive rates, they are less likely to live up to age 76 than someone who carries mutations that are associated with lower reproductive rates, the study found.

The researchers examined the genes for reproduction as well as counting the number of actual offspring of the individuals covered, Zhang said.

The authors, however, underlined a caveat. Reproduction and lifespan are affected not only by genes but also by the environment. And today, environmental factors play a greater role than before. These include increased use of contraception, abortion, and medical advances that increase lifespan.

“My paper studies the change in longevity over time,” Zhang said. “In the last few decades, environmental changes (mainly through improved healthcare) have played a much bigger role than genetic changes in driving the change of longevity. The reason is that, compared with genetic changes, the environmental changes have been extremely rapid.”

Kabir Firaque is the puzzles editor of Hindustan Times. His column, Weird Science, tackles a range of subjects from the history of inventions and discoveries to science that sounds fictional, but it isn't.