Why do we grow old and die?
In the 19th century, German biologist August Weismann argued that the machinery of life inevitably wore out over time. Death had evolved “for the needs of the species,” he declared. He eliminated the old and weak individuals so that they could not compete with the young.
That explanation never made sense to George Williams, an American evolutionary biologist. Natural selection acts only on genes that are passed from one generation to the next. What happens at the end of an animal’s life can have no effect on the course of evolution.
It occurred to Williams that aging might instead be an unavoidable side effect of natural selection. In 1957, he proposed A new theory: Genetic mutations that increase an animal’s fertility could also cause damage later in its life. Over many generations, those mutations would create a burden that would eventually lead to death.
a new study, published Friday in the journal Science Advances, bolsters Williams’ theory using a trove of human DNA. The researchers found hundreds of mutations that could increase a young person’s fertility and were linked to bodily harm later in life.
Smaller studies in the past had already offered some support for Williams’ theory. In 2007For example, a team of researchers studying a small worm found a pair of mutations that lengthened the creature’s lifespan while reducing its average number of offspring.
But Jianzhi Zhang, an evolutionary biologist at the University of Michigan, was not satisfied with these experiments. “These are case studies,” he said. “We don’t know if there are many such mutations in the entire genome.”
Dr. Zhang took advantage of the UK Biobank, a database containing genetic material from half a million volunteers in Britain, along with information about their health and life experiences. Biobanking has allowed scientists to discover subtle links between genetic variations and thousands of traits such as high blood pressure, schizophrenia and smoking.
Working with Dr. Erping Long, a medical researcher now at the Chinese Academy of Sciences, Dr. Zhang pored over the database for information on reproduction and longevity. The scientists found that genetic variations related to fertility, such as the number of children a volunteer had, were also linked to a shorter life expectancy.
What’s more, variants that affected reproduction were almost five times more likely to influence longevity than variants that had nothing to do with reproduction. And variants good for reproduction were much more likely to be bad for long life.
Dr. Zhang and Dr. Long also found that volunteers with a high number of reproduction-promoting variants had slightly lower odds of surviving to age 76. Taken together, all of these results suggest that George Williams was right and that aging is essentially a side effect of natural selection’s impact on fertility. “They’re all pointing in the same direction,” Dr. Zhang said.
He and Dr. Long also found evidence suggesting that this evolution did not stop at some point in our distant past. People in the database who were born in 1965 carried a higher number of variants that stimulate reproduction than people born in 1940.
The idea that fertility variants shorten life expectancy may seem like a paradox, given how long people live today. In Great Britain, for example, the average life expectation It is about 80 years, compared to 59 years a century ago.
Dr. Zhang noted that the mutations he and Dr. Long identified each had a small influence on a person’s longevity. As variants have become more common, the environment has changed dramatically: Better foods and medicines have reduced infant mortality and helped more people reach older ages.
Steven Austad, an aging expert at the University of Alabama at Birmingham who was not involved in the study, said detecting the effect of these variants even though life expectancy had increased made the results even more impressive.
“The pattern is so strong that it manifests itself through these important changes in our life stories in modern times,” he said.
