Wikipedia and the scientific community
Animals almost always continue to reproduce until they die. There are just three exceptions that we know of: humans, short-finned pilot whales, and killer whales. In all three species, females lose the ability to have children at an average middle-age, but continue living for decades after. Human females go through menopause when they are around 45-50. Female killer whales go through menopause their 30s or 40s but can live for up to a century. But what sense does it make in nature for a species to live maybe half their lives without being able to pass on their genes?
One theory about why killer whales experience menopause  is that it provides the older generation of matriarchs a better chance to look after their existing children and grandchildren. Orcas, like humans, invest a lot of time and energy into rearing single offspring that take many years to fully mature. In the sea, it is dangerous to leave a young whale unattended while the parents hunt, so the larger the family group or pod, and the more elder females who aren't breeding there are in that pod, the better the likelihood of the young whales surviving to maturity.
But what about humans? There is no scientific concensus about why we alone of all the primates should go through menopause, although it is argued that it is difficult to observe if and when menopause may occur in wild animals.
So, it's clear that menopause helps the young survive, especially in humans when one pair of hands is needed per child. All the other primates seem to manage foraging and finding food quite well with their infant on their back and their toddler close by.
But they are on land. In the water, even babies that float and toddlers that can swim need an adult keeping a close eye on them. An early human ancestor who spent most of her day in the water, foraging beneath the surface for shellfish, would not be able to do this. This is when a non-breeding grandmother comes in very useful! 
|Website: F. Mansfield, 2015|
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