Will menopause, which plagues mammals, disappear after hundreds of thousands of years?

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Menopause, also known as menopause, generally refers to the period in a woman's life when she will never have menstruation again, and she will also permanently lose her fertility. Before that, perimenopause or premenopause refers to the period before a woman enters menopause and completely stops menstruating. According to the North American Menopause Society, perimenopause will last 4 to 8 years. The Center for Menstrual Cycle and Ovulation Research believes that perimenopause will last 6 to 10 years.

For a long time, researchers have been puzzled by human menopause. How did this abnormal phenomenon in mammalian species come about?

Having children has long been a young person’s game. Although there are few ancient records, researchers estimate[1] that for most of human history, women typically conceived their first child in their late teens or early twenties and stopped having children soon after. But in recent decades, people around the world, especially in wealthy, developed countries, have been starting families later and later[2].

Since the 1970s, the average age of childbearing for American women has risen from 21 to 27, while the average age of childbearing for South Korean women has surpassed 32. With more women having children in their 40s, the average age of childbearing for women in most high-income countries is now over 30 or is rapidly approaching 30.

Rama Singh, an evolutionary biologist at McMaster University in Canada, thinks that if women continue to delay childbearing, the basic reproductive phase may change: Women may enter menopause later. Currently, that age is around 50, a number that some researchers believe has remained unchanged since the beginning of our species.[3] But Singh believes that there is no ironclad biological law that prevents women from having children beyond this threshold. He told me that if women decided to continue having children at an older age, then in theory, menopause could disappear entirely over the next few hundred thousand years.

© Hyphen Online

Singer’s view is not mainstream in his field. But changes in human reproductive behavior are not the only reason for changes in menopause. In general, humans now live longer and are in many ways healthier than our ancient ancestors. In the past few decades in particular, researchers have made leaps in technology that allow them to tweak how the human body functions and ages in unprecedented ways. These factors may work together to alter the timeline of menopause.

This is a grand experiment in human reproduction, and scientists don’t yet know what the outcome will be. So far, they have only a small amount of evidence that the age of menopause has begun to change.[4]

Several studies, mostly tracking trends in recent decades, have noted a one- to two-year shift in the age at which women enter menopause in some Western countries, including the United States and Finland. But Singh thinks this may just be the beginning.

Menopause can occur anywhere from a person’s late 30s to their late 60s, and its timing appears to be strongly influenced by genetics.[5][6] This variation suggests there’s some evolutionary wiggle room. If older parents have children, “I’d probably see menopause happening later and later,” Megan Arnot, an anthropologist at University College London, told me.

Singer's argument assumes that menopause is not necessary for survival in humans or any other animal. If a species' primary goal is self-perpetuation, then it would be paradoxical for it to live far beyond its reproductive period. Researchers have found[7][8] that only a few other organisms extend their lifespan beyond their reproductive period, including five species of toothed whales and wild chimpanzees.

However, human women spend one-third or even half of their lives in menopause, which is absolutely rare among mammals. For humans, menopause occurs when the number of eggs in the ovaries is less than about 1,000, ovulation stops, and the level of hormones such as estrogen in the body drops sharply. But it is not biologically inevitable that women's reproductive ability will be lost after the age of 50.

Each female is born with approximately 1 to 2 million eggs [this number drops to approximately 300,000 to 400,000 by puberty. After menarche, a female releases one egg per month, and loses approximately 1,000 (immature) eggs. The number of eggs a female loses each month is not affected by any controllable factors. Although it may seem that many eggs are lost each month, there is no need to worry about it; it is how the female body functions. Editor's note] This is comparable to the number of eggs estimated by researchers for elephants, which can remain fertile until 60 or even 70 years old.[9] Furthermore, animal eggs do not appear to have an inherent shelf life. For example, some whales have been recorded as still reproducing at over 100 years old.[10]

© Matt Kenyon

This has led some researchers to argue that menopause is an unfortunate evolutionary accident. Perhaps, as some have argued, it is a byproduct of rapidly increasing lifespans, so that the ovaries have not been able to keep up with evolution. Singer has suggested[11] that menopause is a side effect of men’s preference for mating with younger women, which has led to the accumulation of fertility-impairing mutations in older women. (If women had only sought out younger men, he told me, men would have evolved their own menopause, too.)

Others disagree: Arnott told me that, if anything, many men today prefer younger women because fertility declines with age, not the other way around.

© Gracey Zhang

But there is overwhelming evidence that menopause is beneficial to evolving species, including us, Francisco Úbeda de Torres, a mathematical biologist at Royal Holloway, University of London, told me. Samuel Ellis, a biologist at the University of Exeter, also pointed out that menopause is undoubtedly so important that it has occurred many times in evolutionary history—at least four times in whales alone.

One of the most prominent and well-supported theories about menopause is the “grandmother hypothesis”[12]. Perhaps menopause evolved to relieve older women of the burden of childbearing, freeing up their time and energy to help their children raise their own children. Across the world, grandmothers’ involvement clearly increases the survival of the younger generation; the same seems to be true in orcas and other toothed whales[13].

Kristen Hawkes, an anthropologist at the University of Utah, believes that the influence of menopausal grandmothers is so profound that it helped us develop larger brains and shaped the family structure of modern society.[14] This, she told me, is enough to explain human menopause and what makes us who we are today. Some researchers suspect that menopause may have other benefits.[15]

Kevin Langergraber, an ecologist at Arizona State University, has noted that some groups of chimpanzees do just fine even after menopause, even though their populations don’t actually need grandmothers to take on the nurturing responsibilities. In chimpanzees and some other animals, he told me, menopause may help reduce competition for resources between mothers and children as they simultaneously try to raise young offspring.

Every woman goes through the hormonal changes of menopause in midlife. © Elizabeth Dalziel

Whatever the specific cause, menopause may be so ingrained in our lineage that it might be difficult to tweak or eliminate it. Michael Cant, an evolutionary biologist at the University of Exeter, told me that after so many years of premature ovulation, there may be “no single timing factor” switch that can extend human female fertility.

However, the timing of menopause may still change—not over hundreds of thousands of years, but over generations. For example, malnutrition and smoking are associated with premature cessation of menstruation, while the use of birth control pills may delay the onset of menopause, perhaps because of their hormonal perturbations.[16]

Moreover, menopause tends to occur earlier in women with lower socioeconomic status and education levels. Accordingly, interventions as simple as improving childhood nutrition might be enough to raise the average age of menopause in some parts of the world, Lynnette Sievert, an anthropologist at the University of Massachusetts Amherst, told me.

Experts told me that a bigger shift might require medical innovations to slow, stop, or even reverse the premature aging of the ovaries and maintain a woman’s previous levels of estrogen and other reproductive hormones. Kara Goldman, an obstetrician-gynecologist and reproductive scientist at Northwestern University, told me that one key to the “ovarian fountain of youth” would be to find drugs that would preserve the structure of the immature egg, keeping it in a dormant, early state.[17] Other researchers see hope in restoring the health of the egg. Some are creating cells and hormones in the lab to try to replenish what women’s bodies naturally lose as they age.

Deena Emera, an evolutionary geneticist at the Buck Institute for Research on Aging in California, thinks the best inspiration might come from species that retain fertility late in life. Bowhead whales, for example, can still reproduce after they are 100 years old and do not seem to develop cancer.[18] Perhaps, Emera told me, they are particularly good at repairing DNA damage in both reproductive and non-reproductive cells.

Goldman and Emera are most concerned about how to reduce the health toll of menopause. Studies have repeatedly linked menopausal hormones to a loss of bone density; some studies have also pointed to cardiovascular and cognitive problems. Entering menopause can bring years of symptoms such as hot flashes, urinary incontinence, vaginal dryness, insomnia and low libido.

Delaying the onset of all these symptoms might extend the period of healthy life supported by reproductive hormones indefinitely. Extending the life of the ovaries won’t necessarily reverse or even mitigate the adverse effects of menopause, Stephanie Faubion, director of the Mayo Clinic Center for Women’s Health, told me.

In addition, it may bring additional risks associated with pregnancy later in life and increase women's chances of breast cancer, uterine cancer, blood clots and stroke. Delaying menopause may also mean longer periods and contraception, a prospect that may make many women hesitate, said Nanette Santoro, an obstetrician-gynecologist and reproductive scientist at the University of Colorado School of Medicine.

But the researchers think some adjustments are worth trying. Even if menopause helped our species survive, Goldman says it’s “hard to imagine” that this still applies. Evolution may have given us a weird mismatch in the lifespan of our ovaries and the other organs that live with them, but it also gave us the smarts to work around those limitations.

By Katherine J. Wu

Tempura

Proofreading/Rabbit's Light Footsteps

Original article/www.theatlantic.com/health/archive/2024/05/menopause-timing-evolution-technology-reproduction/678279/

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