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You might think you know a lot about yourself, what you look like in a mirror, your shoe size, all those annoying headaches you get when you're stressed out, but human bodies are full of mysteries.  One of the least understood phenomena is menopause, when someone's period ends, usually around the age of 50, give or take a few years for genetics and lifestyle.  This means they lose the ability to have babies way before the end of their life, and from a gotta pass my genes on standpoint, that seems kind of like really unhelpful.  In fact, humans are one of only three known mammals that go through this kind of menopause, which makes it even harder for evolutionary biologists to explain, but they do have a couple of guesses.

People with ovaries are born with around a million immature eggs called oocytes and after puberty, around once a month, an oocyte matures and moves to the uterus.  Assuming there's no close encounters of the sperm kind, the oocyte gets flushed out with a bunch of extra tissue and blood.  That's menstruation.  This monthly cycle is regulated by hormones, including FSH from the pituitary gland, which signals the oocytes to mature and estrogen and progesterone secreted from within the ovaries, but starting around age 35, getting pregnant and staying pregnant becomes harder and there's also a higher chance of complications like chromosome abnormalities.  Hormone cycles start to get messed up because there are fewer oocytes and less estrogen being produced and the pituitary gland bumps up FSH production to try and compensate. 

Eventually, around age 50, periods get irregular and then stop.  No more maturing eggs means no more pregnancies and that's menopause.  Changing hormones can come with all kinds of side effects, as anyone who has gone through puberty knows.  In this case, there's a cut-off in fertility, and people usually live long after.  It's a natural phase of human life, which is not the case for most mammals, only killer whales and short-finned pilot whales.  

So for a long time, evolutionary biologists have tried to explain why menopause might be a thing.  A relatively early idea is that menopause was just a side effect of humans suddenly living longer, maybe our bodies have a built-in limit on fertility.  This is considered a non-adaptive theory, since it suggests that menopause didn't happen because of natural selection, it just sorta happened.

Now, it's true that humans today have higher life expectancies than every before, but it does not explain the full picture, a picture that has two whale species splashing around in it.  As far as we know, these species haven't had a rapid jump in longevity, yet they still have menopause a lot like us.  

There's also the mother hypothesis, which suggests that menopause is an adaptation driven by the risks of being an older mom.  We mentioned the increased risk of complications with older pregnancies, but there's also the unavoidable fact that older people are just more likely to die, whether they can't find food like they used to or are more susceptible to disease, and if they leave behind an infant, it probably won't survive either.  But opting out of passing on your genes at all just because of an increased risk doesn't really fit with how we understand evolution.  This hypothesis also doesn't address its elephant in the room: actual elephants.  These mammals live for decades but manage to have babies well into old age without complications.  

Now, maybe the most promising idea is called the grandmother hypothesis, and it has to do with family dynamics, whether children leave the family group once they grow up.  Scientists have compared three social long-lived animals: killer whales, elephants, and early humans.  In all of them, older animals helped their group with a lifetime of knowledge about things like food or danger, but only two experience menopause.  In wild killer whale pods, both sons and daughters tend to stick around so older killer whales become more related to their family group over time.  It's been observed that if older moms and daughters have babies at the same time, the kids of the daughter do much better.  So it's possible that an older female killer whale could better boost her genetic legacy by helping lots of her grandkids survive rather than having more kids of her own, hence, grandmother hypothesis.  

Elephants, on the other hand, have matriarchal societies where sons leave the herd and join new ones, so family groups aren't as closely related as killer whales, so older female elephants might keep needing to pass on their genes because they don't have a bunch of nearby descendants to care for.  

Lastly, many anthropologists agree that ancestral human women typically joined a new family group to have kids.  They became increasingly more related to that group, just like killer whales.  This could kind of explain why we both have menopause, because both of us had more of a reason to stop making babies and take care of them instead.  

Now, this is not super conclusive and the answer may be some combination of these ideas, plus we still have a ton of questions about how human bodies changed and when menopause showed up in our history.  For now, though, it is probably a good policy to just celebrate grandmas anyway, so thanks, grandmas, by which I mean my grandmas and also all of grandmas.  You're great.  

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