International Women's Day: The Evolutionary Mysteries of The Female Body

Evolutionary theory has transformed our understanding of human beings - but more than a century after Darwin’s work reshaped biology, the female form in our species is still, in many respects, poorly understood.

Set against the wider animal kingdom, human women are strikingly unusual, and researchers are still piecing together the reasons. While no two bodies are identical, many women’s lives are influenced by these distinctive aspects of female anatomy.

Although sex and gender are different ideas, much of the research on women’s bodies concentrates on reproductive biology associated with the female sex - the mechanisms behind pregnancy, menstruation and menopause.

A growing number of scientists now suggest these evolutionary oddities are not merely incidental outcomes of human evolution; instead, they may have acted as important pressures that helped mould our species.

Human Birth is Unusually Dangerous for the Female Body

From the outset of life, humans face a peril that is unusually severe: childbirth. In comparison with other primates, human labour can last extraordinarily long - often many hours, and sometimes days - and complications and hazards are frequent.

The human birth canal has a pronounced twist. As a result, during a vaginal birth the baby’s head must rotate almost 90 degrees, rather like sliding a foot into a boot. In many cases, a mother relies on assistance from other people to deliver safely.

Although it is hard to draw direct comparisons between the whole of human history and contemporary life, obstructed labour is thought to be directly responsible for up to 30 percent of maternal morbidity in developing countries today.

Why human birth carries such a high risk for the mother is still not fully explained.

Compared with other primates, humans arrive with the smallest brain relative to the size it reaches in adulthood, so the answer cannot simply be that our heads are enormous.

Because humans are the only living mammal that routinely moves on two legs, some scientists have proposed that having a narrower pelvis supports standing and walking upright.

This proposed compromise between intricate anatomy and giving birth more easily is known as the “obstetrical dilemma”. It is a disputed idea that has attracted substantial scrutiny and scepticism in recent years on biomechanical, metabolic and biocultural grounds. Critics argue that the dilemma can miss subtleties involving anatomy, diet, genes, hormones or medical practices.

For now, the clearest point we can make is that human birth is singularly complicated and remains oddly difficult to account for - so it is worth remembering to thank the women who brought you into the world.

Human Menstruation is Heavy and Overt

As the female body develops, the questions only multiply. Human menstruation appears both heavier and more outwardly evident than in any other mammal. Indeed, over 98 percent of mammals do not menstruate at all.

So why, during our reproductive years, do humans menstruate roughly monthly, alongside wide-ranging changes across the brain and body? And if there is an advantage, what is it?

Over time, scientists have proposed many explanations, yet each comes with shortcomings, and overall the topic has received relatively little research attention.

One possibility is that the roughly 85 mammal species known to menstruate (including chimpanzees, bonobos and humans) do so as part of preparing the uterus for implantation.

Human embryos, for example, implant in a notably aggressive way compared with what researchers have observed in laboratory mice, and that may demand a thicker, more specialised tissue that the body struggles to “resorb”.

It is even possible that the uterine lining matures and differentiates to the point where it can, in effect, “decide” whether to accept an embryo - though that suggestion remains contentious.

In a pattern reminiscent of humans, menstruating bats and elephant shrews also face the danger of the placenta attaching too deeply into the uterine wall, which points to a highly developed and extensive system.

Menstruation, then, may have evolved simply as a side effect of the uterus readying itself for potential offspring. At present, however, that remains an educated guess.

Menopause is an Enigma

Even after menstruation ends, the evolutionary puzzle of the female form does not disappear.

Humans are one of only a small number of species that go through menopause. This life stage is exceptionally rare across animals, and even now there is no definitive answer for why it exists at all.

Most other mammals remain reproductively active for their entire adult lives, whereas humans can live for decades after their final period.

Among the few animals that offer a useful comparison are toothed whales, such as orcas and pilot whales; understanding what we share with them might help unlock the biology of menopause.

A widely discussed account of why menopause evolved in certain species is the grandmother hypothesis. In essence, older females stop producing offspring themselves and instead channel energy and resources into supporting the raising of their grandchildren.

Some evolutionary researchers contend that toothed whales follow this pattern - menopause evolved there as a way to increase lifespan without prolonging the reproductive years.

To show the same adaptation in humans, scientists would need evidence that pre-agricultural societies included substantial numbers of women who lived beyond their own childbearing and helped their descendants.

That kind of evidence is difficult to obtain. In the past, some models based on modern hunter-gatherer groups have not demonstrated a large enough benefit to “offset the evolutionary cost of ceasing reproduction,” according to evolutionary theorists.

One researcher has even suggested that menopause is better explained not by the matriarch but by the patriarch. The patriarch hypothesis argues that “once males became capable of maintaining high status and reproductive access beyond their peak physical condition, selection favored the extension of maximum life span.”

If the genes involved in longevity sat on the X chromosome rather than the Y chromosome, lifespan would rise in females as well, potentially enabling them to use up all their viable eggs over the course of a longer life.

Put differently, menopause might be a consequence of increased male life expectancy. What this does not clarify is why female animals, on average, appear to outlive males by a considerable margin. It also depends on an assumption that has not yet been proven: that crucial longevity genes are absent from the Y chromosome.

Without menstruation and pregnancy, humans would not exist; and without menopause, it is possible our species might not have been anything like as successful.

More than 150 years after natural selection was first proposed, how the female body evolved remains one of biology’s most significant unresolved questions.