Researchers have proposed another explanation as to why women on average live longer than men: twice the genetic fitness. The mechanism for this doubled genetic fitness is related to how women contribute not only nuclear DNA to their offspring, but also mitochondrial DNA. Therefore the pressures of natural selection would favor more genetically fit females but leave males to accumulate harmful mutations. Over time this would lead to an overall increase in the life-span of only women.
The phenomenon of women overall outliving men is a curiosity that analysts have puzzled over for decades. This trend has become particularly more discernible in the last half-century during which rates of pregnancy-related mortality, previously a major cause of pre-mature death in women, plummeted. In fact, a longer-lived population of women is heralded as a marker of increased affluence and better access to healthcare. In 2006 the British Journal of Medicine estimated that in nearly every nation on the globe, excluding Maldives and Qatar, the average woman will outlive the average man. In more progressive nations such as Sweden, England, and Denmark, this trend dates back as early as 1751.
Why does the average woman live longer than the average man? A number of different explanations have been offered. Some researchers hypothesize that the iron-deficiencies more commonly found in women reduce the number of DNA-damaging free-radicals in an individual’s body. Still others suggest that women are on average more health-conscious and less likely to engage in high-risk behaviors such as aggressive driving, smoking, and drinking.
But another hypothesis suggests that there may be genetic differences between the sexes relating to the fundamental selection pressures that have influenced how men and women evolve. Specifically, women have had to contend with twice the evolutionary selection pressures that men have had—pressures that select for good genes within their nuclear DNA and also their mitochondrial DNA.
Upon conception, both father and mother contribute nuclear genetic information to their offspring. For the father, this is the full extent of his contributions to his offspring. On the other hand, the mother donates everything else to the egg, including mitochondria.
Mitochondria are the energy-producing organelles that are a necessary part of every single cell in the human body. Back before the evolution of the modern eukaryotic cell, mitochondria arose out of small bacteria cells that were engulfed by larger cells. These ancient bacteria survived within the larger cell, producing energy in abundance in exchange for a safe environment in which to live and replicate.
The mitochondria found in the cells of a modern human bear only a limited resemblance to their ancient free-living bacterial ancestors. But these mitochondria still have their own DNA which is independent of an organism’s nuclear DNA. And because mitochondria are passed down to an offspring exclusively from the mother, mitochondrial DNA is subject to the pressures of natural selection on the maternal line only.
Based upon this knowledge, researchers hypothesized that one of the reasons why women on average live longer than men is that deleterious mutations in the mitochondria would over time accumulate to affect only men. If the mitochondria in a woman’s eggs develop a mutation that negatively affects a woman’s health, such as increasing her risk of ovarian cancer, those negative effects will impact all of her female offspring. Those daughters and their daughters would pass down these mutations, experiencing reduced genetic fitness. Over time such deleterious mutations would be weeded out by the influences of natural selection.
But if a woman experiences a mitochondrial DNA mutation that negatively affects the health of men—perhaps increasing his risk of prostate cancer—there would be no correctional selection pressure. Such a mother would pass on this mutation to her son, but he in turn would not be able to pass it on to his offspring. If a daughter receives a mutation for increased risk of prostate cancer, she would presumably not experience any reduction to her genetic fitness, but pass it on to both her daughters and her sons. In this way, mutations disfavoring health and longevity in men would accumulate.
Research into this explanation for increased female longevity indicates that this is a trend that can be found across species. In particular, studies with fruit flies show that sex-specific mutations against male aging are liable to accumulate within the mitochondrial genome.
In all likelihood, the answer for why women on average live longer than men will probably stem from a number of different explanations. With time, perhaps researchers will discover how to increase male longevity to equal that of women. Certainly correcting “nurturing” influences such as stress management and leading a healthy life style would be the easiest way to increase anyone’s longevity, male or female. But with today’s advancements in biotechnology, it is possible that therapies for male-specific mutations to mitochondrial DNA might also one day become available.
By Sarah Takushi