Darwin, Menopause, and Humans

My mother recently asked me if there were any evolutionary answers as to why she had to go through menopause. What’s the purpose of the weight gain, the mood swings, and the dry skin? Why, in the course of human evolution, did menopause be selected as “something that should happen?”

According to Darwin, evolution by natural selection is a process of finding traits that most suit the environment of an organism. The more well-adapted that organism is, the more likely that they will live to reproduce and contribute their genes to further generations. Fitness, according to Darwin, all depends on the reproductive fitness of one’s offspring. The more offspring one has, the more likely genes will be perpetuated in a species. So why is it that, in the case of humans, female reproduction halts in the fifth decade of life? What possible benefit does stopping reproduction have for the human species?

            Menopause is the permanent cessation of menstruation, and is universal in all human females occurring, more or less, in the fifth decade of life. Because we have a set number of eggs when we are born, menopause is associated with the depletion of ova (eggs) in the ovaries and the subsequent loss of sex hormones. When the ovarian follicles are depleted, the hormone progesterone and two estrogen hormones: estradiol, and estriol, are no longer produced. Many of the effects associated with menopause (i.e. hot flashes, decrease in bone mass, difficulty losing weight, etc) can be associated with the rapid loss of these hormones (1).

Menopause is notably difficult for American women because of their pathologically high levels of estrogen throughout their lives. Overrefined diets, inactivity, and xenoestrogens (foreign estrogens from plastics and other petroleum-derived products) create unusually high levels of estrogen (2). When these hormones levels radically decrease during menopause, the effects can be extreme. Essentially, American women are on a hormonal rollercoaster, an estrogen flume ride that emphasizes the discomforts of menopause.

            Amusement park rides aside, what does Darwin’s evolution have to say about menopause? There is an age-related increase of birth defects and complications for women past peak fecundity (the mid-20s). A decrease in ova quality is observed with an advance in age. Thusly, reproduction after a certain point would have no fitness benefits. Birth complications associated with age could even leave existing children with no mother, decreasing the likelihood that they will contribute their maternal genes to future generations, another decrease in fitness (3).

However, there are increased fitness benefits to having a maternal grandmother. Children that have both their mother and their maternal grandmother involved in their care have been shown to be healthier, grow faster, and gain more weight than children who were raised by mothers only (3). These children carry some of their grandmother’s genes, so it would be to her benefit that they survive to reproduce and that she survives (avoiding complications from a later birth) to encourage their growth.

            Most female mammals reproduce until they die (2). But their chance of survival before and during reproductive ages is less than that of humans. It could be that the genes that ensure survival before and during reproductive age in humans just so happens to make human females live past follicular depletion and menopause (3). It is also possible that menopause is a by-product of earlier survival and reproductive success. If follicular depletion is necessary for regulation of ovarian cycles at a younger age, then it is possible that menopause helps ensure fitness earlier in life (2).

            So, Mom, focus not on the hot flashes and mood swings, but instead the fact that you’re living a longer life than the average chimp or Basset Hound. And that you have succeeded in producing an amazing daughter who is more than willing to hold the fan while you sweat.

References:

1. http://www3.interscience.wiley.com/cgi-bin/fulltext/77003160/PDFSTART

2. http://csde.washington.edu/downloads/01-04.pdf

3. http://www.tribunes.com/tribune/art98/pett.htm