Humans are very unusual amongst animals in that female humans have an extended period of life after the end of fertility. Average life expectancy in the UK is now around 81 so, as menopause most often occurs around the age of 50, many women spend about a third of their lives post menopause.
From an evolutionary perspective, the aim of living creatures is to pass their genes onto the next generation so the usual strategy is to have as many offspring as possible. For this reason, most animals remain fertile until the end, or near the end of their lives.
There must therefore be some evolutionary benefit to a prolonged period of life during which time women are not fertile. The Grandmother Hypothesis is a possible explanation.
This hypothesis proposes that grandmother’s have always had a vital role in helping their grandchildren survive by helping with foraging, agriculture, childcare and emotional support. Because their grandchildren share some of their genes, by helping to ensure the survival of their grandchildren, they also ensure that their genes are passed onto future generations.
Lots of studies have looked at the type of support grandmothers provide and have found that this varies widely dependent on culture, environment, and need, but it is interesting that some studies suggest that maternal grandmothers provide more support than paternal grandmothers. Why should this be? Possibly, it is because maternal grandmothers can be very confident that their daughter’s child is definitely a direct descendant, whereas paternity is always more uncertain. A paternal grandmother runs the risk of investing in a child who carries none of her genes.
One obvious whole in the theory is that human women have not, in fact, always had long post menopausal lives. Routinely living beyond the age of 50 is quite a recent phenomenon in the scheme of human history. But, whether you subscribe to the theory or not, I think it is interesting that there are still fundamental aspects of our lives that remain a mystery to us.