Life stages of various primates.
My apologies for the crappy graph quality, as I only have access to a crappy pdf scan for this paper. As you can see, human females often live 20+ years after menopause, whereas most other primates only live a few years past the end of their reproductive stage. (You'll also notice that weaning has shifted to the middle of infancy in humans instead of occurring at the border of infancy and childhood.)
One hypothesis for why female humans live so long past their reproductive stage is that perhaps we're just out-living our viable eggs? Historically, humans didn't live much longer than what we now consider 'middle' age, but as I mentioned earlier this week, we've seen a sharp decline in middle-age mortality in the past 200 years. As longevity continues to increase, we are just living that much longer past our reproductive age. The authors state that "Life-expectancy, however, is dominated by early life mortality and obscures the fact that a human female who attains maturity can expect to live well beyond her menopause." I would have liked to see some references to back this up, especially since third world countries are booming with young people of reproductive age, many of which will not live to see menopause.
Rejecting this hypothesis that humans simply outlive their gametes, they explore the idea that menopause might actually be an adaptive trait in humans. Their first hypothesis is based on the fact that as we age, reproduction becomes more and more dangerous for the mother. After a certain age, it makes sense for a woman to cease trying to have new children because of the increased risk of mortality for her and the increased risk of birth defects for her children. Instead, it makes much more sense for the woman to focus on raising her existing children rather than trying to have new children. Remember how I mentioned above that humans are weaned much earlier than other primates? This idea comes into play here, because since infants are relatively quick to be weaned, it is easy for other family members to step in and care for the infant while younger mothers concentrate on having more babies (or going back to work or whatever it is the modern mother does, but for the sake of evolution we'll pretend that her job is now to get pregnant with another child to spread her genes around a bit more). Therefore, similarly to the above, it also makes sense that if a woman doesn't have any children of her own to care for when she reaches menopause, she can step in and help care for her grandchildren. However, this mother/grandmother effect is hard to test empirically.
Grandma takes care of little Sally so that Momma can go get knocked up again. Actually maybe I should have used a photo of the Palin family for this one...? (Source)
Another hypothesis bears on the idea that, historically, women left the home group or tribe and got married to men who lived relatively far away. When they first joined this new community, they would have had no blood relatives. At this point, they would want to focus on only having their own children and not caring for any of the existing children in the community, since she had no genetic stake in their survival. The elder women in the community might help her raise her children because it was their son or grandson that she was having children with, so they would have genetic stake in the survival of her children. The idea is that as the woman gets older and her children and grandchildren were integrated into the society, the benefit of helping out with parenting would continue to rise for that individual, and she would focus less on investing considerable amounts of energy into having her own children and more on investing a bit less energy into caring for her descendants as they begin to fill the new community. There is no evidence that this was the practice in every human culture, though, and it is possible that in other cultures the males were the ones who left the home society, and in others still all of the offspring stayed nearby, so this hypothesis is hard to prove universally.
Now once all these sprog go out and populate the town, Momma can invest some time in helping someone ELSE have kids for a change. (Source)
A third hypothesis rests on the importance of the father. If it is true that a child's father is generally older than its mother (I am skeptical as to the truth of that), and that the mortality rate of males is higher than females, then it stands to reason that children born late in life might lose the father during the course of that child's development. If the above is all true AND the father carries a substantial amount of the burden for caring for the child, then it would be advantageous for females to stop having children past a certain age to diminish the risk of having to raise the child on her own. However, for this theory to uphold, it would have to be proven that fathers are generally significantly older than mothers, and that fathers carry a substantial burden of childcare. As you can see, I don't lend this theory much credence.
Yeah. Right. (Source)
Now that we've talked about life history reasons why menopause might be adaptive, let's talk about physiology for just a moment. What could be the mechanism by which menopause evolved? The authors bring up an interesting relationship seen in rodent models between a cytokine, called LIF, responsible for (among other things) blastocyst implantation and a gene, called p53, responsible for (again, among other things) up-regulating the gene responsible for LIF. p53 function declines as the mice age, but reproduction can be restored in mice with diminished p53 functionality if they are administered LIF. Evolutionarily speaking, p53 is ancient and the gene that codes for LIF is relatively young, and the authors hypothesize that putting LIF downstream of p53 may be responsible for menopause. They think that this might be selectively advantageous because it diminishes the chance of late-life implantation in a model where a late-life pregnancy is less likely to be successful to begin with. If the egg can't implant, the mother doesn't waste any energy on a pregnancy that isn't likely to succeed.
In sum, it becomes less advantageous for women to have children of their own as they get older. The chances of dying during childbirth go up due to aging, and the chances of birth defect go up due to a lifetime of accumulated damage to the mother's oocytes. No matter how you cut it, late life pregnancies are less likely to succeed. After a certain age, it becomes selectively advantageous for women to stop investing energy in getting pregnant and to start focusing on existing children or grandchildren. And because of the young age of weaning in human infants, it is relatively easy for elder women to take over the care for younger women in their family, so that those women can continue to churn out more sprog to populate the planet with.
[As an interesting aside, both searches for 'three generations' and 'father with children' on Google image search bring up pictures of Barack Obama's family on the first page.]
Rashidi, A., & Shanley, D. (2009). Evolution of the menopause: life histories and mechanisms Menopause International, 15 (1), 26-30 DOI: 10.1258/mi.2009.009005