Evolutionary theory has profoundly transformed our understanding of human biology, yet over a century after Charles Darwin’s groundbreaking work, the intricacies of the female form remain elusive. Human women stand out in the animal kingdom, and scientists are still striving to comprehend why. While individual differences abound, many women’s lives are shaped by distinctive features of female anatomy.
Although sex and gender are distinct, much of the scientific discourse surrounding women’s bodies focuses on reproductive biology, including the physiology behind pregnancy, menstruation, and menopause. Some researchers now propose that these evolutionary peculiarities are not mere byproducts of human evolution but crucial forces that have shaped our species.
The Perils of Human Birth
From the outset, humans face an unusually perilous process: childbirth. Compared to other primates, human labor is remarkably prolonged, often lasting many hours or even days, with complications and risks being commonplace. Our species’ birth canal is notably twisted, necessitating a nearly 90-degree turn of the baby’s head during vaginal birth, akin to slipping a foot into a boot. This often requires assistance from others.
While it’s challenging to compare the entirety of human history to modern life, obstructed labor is estimated to account for up to 30 percent of maternal morbidity in developing countries today. The mystery of why human birth poses such great risks to mothers remains unsolved.
Unlike other primates, humans are born with relatively small brains compared to their eventual adult size, so our large heads alone cannot explain the danger. Some scientists hypothesize that our narrow pelvis, a result of bipedal locomotion, complicates childbirth. This biological trade-off, known as the ‘obstetrical dilemma,’ has faced scrutiny on biomechanical, metabolic, and biocultural grounds. Critics argue that it overlooks nuances in anatomy, diet, genetics, hormones, and medical practices.
At this point, the only certain conclusion is that human birth is uniquely complex and curiously inexplicable – so remember to thank the women who got you here.
The Enigma of Menstruation
As the female body matures, it becomes even more perplexing. Human menstruation is notably heavier and more overt than that of any other mammal. In fact, over 98 percent of mammals do not menstruate at all. So, why do humans menstruate monthly during their reproductive years, with widespread changes to the brain and body? What is the adaptive value, if any?
Over the years, scientists have proposed various hypotheses, but each has its flaws, and the field remains relatively under-researched. One possibility is that the 85 or so mammals known to menstruate, including chimps, bonobos, and humans, do so to prime their uteruses for implantation. Human embryos implant aggressively compared to lab mice, possibly requiring a thicker, more differentiated uterine lining.
The uterine lining may even mature enough to ‘decide’ whether to accept an embryo, although this idea remains controversial. Similar to humans, menstruating bats and elephant shrews also face risks of deep placental attachment, indicating a highly mature system. Menstruation may have evolved as a byproduct of the uterus preparing for offspring, but for now, we can only speculate.
The Mystery of Menopause
Even after menstruation ceases, the evolutionary enigma of the female form persists. Humans are among the few species to experience menopause, a stage of life extremely rare in the animal kingdom. To this day, the reason for its existence remains unclear.
Most mammals remain reproductively active throughout their adult lives, but humans can live for decades beyond their last period. Toothed whales, like orcas and pilot whales, are some of the only animals we can relate to, and understanding what unites us could reveal the secrets of menopause.
One popular explanation is the grandmother hypothesis, which suggests older females stop having offspring and instead help raise their grandchildren. Some evolutionary scientists argue that this occurs in toothed whales, where menopause evolved to extend lifespan without extending reproductive years.
To demonstrate this adaptation in humans, scientists would need evidence from preagricultural societies showing women living beyond reproduction and assisting descendants. However, such evidence is scarce, and models based on modern hunter-gatherer groups have struggled to show sufficient benefits to offset the evolutionary cost of ceasing reproduction.
Another theory, the patriarch hypothesis, posits that menopause is explained by males maintaining high status and reproductive access beyond their peak physical condition, leading to the extension of maximum lifespan. If longevity genes were on the X chromosome, lifespan would increase in females, allowing them to expend all viable eggs in a lifetime.
Without menstruation and pregnancy, humans would not exist; and without menopause, it’s possible our species may not have been nearly as successful.
More than 150 years after the theory of natural selection was first proposed, the evolution of the female body remains one of the greatest unfinished puzzles in science.
