Cat Bohannon on evolution, menopause and why she was happy to donate her eggs to a friend

In an extract from her new book about how the female body drove human evolution, the writer ponders ovaries, pregnancy and why sperm and eggs behave so differently

How women drove evolution: Cat Bohannon on her radical new history of humanity

Here’s a modern love story for you: a friend of mine recently asked if I would be willing to donate my eggs. He and his wife, both professors at Harvard, wanted to have a child. But like many accomplished women with challenging careers, my friend’s wife was already in her early 40s before she could seriously consider getting pregnant, and, as it turned out, she didn’t have a healthy egg left. As far as I’m concerned, this is about the most flattering thing a person could ask you: “Say, friend, would you mind giving us your gametes? We’re hoping there’s a remote chance that our child could end up like you.” I said yes.

There were hoops to jump through, including a rather extensive health questionnaire, involving information on all the possible genetic issues that may run in my family. The IVF clinic needed to check my egg reserve because it turns out that there is no date-specific switch that triggers menopause. Rather, our ovaries just slowly run out of eggs. We actually start losing egg follicles – those little, fluid-filled sacs in the ovaries that harbour our eggs until they properly develop – before we’re even born. If we do have an innate ovarian expiration date, it must be set in the womb.

Call it the “empty basket” theory. While men keep making new sperm until they die, a woman is born with all the eggs she’ll ever have. Or rather, all the egg follicles. Each month, as she moves through her ovulatory cycle, the pituitary gland cooks up a batch of follicle-stimulating hormone. In response, her ovaries begin “ripening” a handful of egg follicles. Typically, only one of these will go on to become a fully mature egg and make its way down the fallopian tube. It’s a kind of in-house competition. Only the best survive.

This is presumably what happened to my friend’s wife. Like nearly every woman on the planet, she was born with roughly a million immature egg follicles. But every year since, thousands of her follicles died off and were reabsorbed by her body. By the time she became a teenager, she had only about 300,000 to 400,000 follicles left. From then on, she lost about a thousand of them every month. If she started ovulating at age 13, she was destined to run out of eggs somewhere in her early 40s. Which is precisely when most women stop being able to get pregnant without medical assistance.

Some women lose a few more egg follicles a month than the average, and some women lose fewer. And for whatever reason, some women in their 30s and 40s retain more higher-quality eggs, while others seem to have more “bad” eggs left: eggs with more chromosomal malfunctions, eggs with buggy mitochondria, or eggs that are just, for whatever reason, no longer up to the task. But we don’t have a clue as to why our bodies have evolved to discard so many eggs in the first place.

I did worry if donating my eggs to my friends would threaten my own chances of having babies later. Happily, no – women who donate eggs don’t seem to have any lessened chance of becoming pregnant themselves. But no one could say whether donating eggs would make me go into menopause sooner than I would otherwise. (The data did suggest that I wouldn’t.) Still, why do we burn through so many follicles every month? Why not lose a hundred instead of a thousand? How does the body know which eggs to save? Do good eggs become damaged over time, or are there only ever about 400 good follicles out of the million we’re born with?

In other words, are most of a woman’s eggs duds? For nearly half a century, the scientific community figured that mammalian eggs may have an expiration date. That would help explain human menopause at least a little: maybe it helps prevent genetic disorders. My friend’s body might have discarded so many of her egg follicles before she reached her 40s because the eggs had major flaws in their genetic blueprints, such as more “double-strand breaks” in their DNA. There may be something wrong with the 1,000 eggs that most women get rid of every month, probably a result of the fact that eggs are just so much harder to make than sperm, so there’s more opportunity for screwups.

While half of your DNA came from your dad and half from your mum, most of your mitochondria and cytoplasm came from your mother. Sperm are basically an information delivery system that dumps the father’s DNA into the egg, whereas eggs have to provide all the construction materials to build that embryo. And that’s the major reason eggs are vastly larger than sperm: they’re not just half a set of blueprints; they’re half a set of blueprints plus the entire factory.

Given that sperm don’t require that much material, testicles don’t have to work that hard or long to make their gametes. Ovaries, on the other hand, have to exert more effort, over a lot more time, to help an egg mature – remember, the human foetus builds its egg follicles while still in the womb.

The longer a cell lives, the more chances it has to be damaged by accumulating waste and free radicals. There are mechanisms in place to repair damage, but those mechanisms get less reliable over time.

For the same reason, older women have more early miscarriages. So maybe ancient humanlike bodies somehow anticipated those problems, discarding all those egg follicles to avoid giving birth to disabled babies. Since most mammals don’t live as long as we do, maybe they don’t have to deal with genetic damage to old eggs. There are some outliers, though, and they kind of punch a hole in that theory. Elephants give birth into their 60s, without any increase in genetic mishaps. Some whales do, too.

Maybe we didn’t evolve to have menopause. Maybe it wasn’t selected for. Maybe, instead, it was a natural side-effect of our extending lifespans. In principle, bodies do just about everything they can to avoid death. So, it’s not hard to imagine evolution selecting for traits that helped us dodge the grave. But in social species, it can also be useful to have the elderly around. That can put further pressure on selecting genes that extend lifespan and, in women, lead to menopause.

The point of menopause isn’t that we stop ovulating. It’s that we keep living past our predicted – and biologically tuned – expiration date. We made it normal to grow old. That means what’s interesting about menopause may not be menopause at all, but how human beings manage to stave off death.

Eve: How the Female Body Drove 200 Million Years of Human Evolution by Cat Bohannon is published by Hutchinson Heinemann. To order a copy, go to guardianbookshop.com. Delivery charges may apply.

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