The theory of evolution has profoundly transformed our comprehension of humanity. However, despite Darwin’s groundbreaking ideas revolutionizing biology over a century ago, the female aspect of our species continues to be largely enigmatic.
When juxtaposed with the broader animal kingdom, human females exhibit remarkable distinctiveness, the reasons for which scientists are still actively investigating. While each individual’s physique is unique, numerous women’s lives are significantly influenced by these particular characteristics of female physiology.
Although distinct concepts, sex and gender often see scientific discourse surrounding female bodies predominantly focusing on the reproductive biology tied to the female sex – encompassing the physiological processes of gestation, menstruation, and menopause.
A segment of the research community now posits that these evolutionary peculiarities are not merely incidental outcomes of human evolution; rather, they might represent pivotal forces that contributed to the very shaping of our species.
Human parturition presents an unusually perilous challenge
From its inception, human existence is marked by an unusually hazardous undertaking: the process of childbirth. In comparison to other primates, human labor extends for an astonishing duration, often spanning many hours or even days, and complications and risks are frequently encountered.
The pelvic canal of our species is notably convoluted, necessitating that a baby’s head execute a nearly 90-degree rotation during vaginal delivery, akin to fitting a foot into a boot. Frequently, maternal assistance from others is required for successful parturition.
While direct comparisons across the entirety of human history and contemporary life are challenging, obstructed labor is estimated to directly account for as much as 30 percent of maternal morbidity in developing nations today.
The underlying reasons for the significant risks associated with human birth for the mother remain an unresolved question.
Relative to other primates, humans are born with the smallest brain in proportion to their eventual adult size, suggesting that the sheer volume of the cranial capacity isn’t the sole determinant.
Given that we are the solely extant mammal to habitually ambulate on two legs, certain scientific hypotheses propose that a narrower pelvis facilitates upright locomotion or posture.
This biological compromise between intricate anatomical structure and the ease of delivery is termed the ‘obstetrical dilemma.’ It is a disputed hypothesis that has been subjected to considerable skepticism and challenge recently, on biomechanical, metabolic, and biocultural grounds. Some contend that this dilemma fails to adequately address the complexities inherent in anatomy, dietary habits, genetic makeup, hormonal influences, or medical practices.
At present, the sole definitive conclusion derivable is that human childbirth is exceptionally intricate and curiously defies simple explanation – thus, it is prudent to acknowledge and appreciate the women who facilitated our arrival.
Human Menstruation is Profuse and Conspicuous
As the female form progresses through maturity, its enigmatic nature intensifies. Human menstruation appears to be more substantial in volume and more outwardly noticeable than that observed in any other mammal. In fact, upwards of 98 percent of mammalian species do not undergo menstruation at all.
Therefore, the pertinent question arises: why do humans, during their reproductive years, experience monthly menstruation accompanied by widespread physiological and neurological alterations? What is its adaptive significance, if discernible?
Over the years, numerous hypotheses have been proposed to elucidate these queries, yet each possesses inherent limitations, and the domain remains, by and large, comparatively under-investigated.
There exists a possibility, for instance, that the approximately 85 mammal species known to menstruate (including chimpanzees, bonobos, and ourselves) do so to prepare their uterine lining for embryonic implantation.
The mechanism of implantation in human embryos, for example, is considerably more invasive compared to observations in laboratory mice, potentially necessitating a thicker, more differentiated tissue that the body finds more challenging to reabsorb.
The uterine endometrium may even attain a level of maturation and differentiation sufficient to ‘determine’ whether to accept an embryo, although that concept continues to be a subject of contention.
Analogous to humans, menstruating bats and elephant shrews also face the risk of their placenta embedding excessively deeply into the uterine wall, indicative of a highly sophisticated and extensive system.
It is conceivable that menstruation evolved simply as a secondary outcome of the uterus preparing itself for potential offspring. However, for the present, our understanding remains speculative.
Menopause represents an Unexplained Phenomenon
Even after the cessation of menstruation, the evolutionary puzzle that is the female form persists.
Humans belong to a select group of species worldwide that experience menopause. This life stage is exceptionally uncommon in the animal kingdom, and to this day, the rationale for its existence remains largely unknown.
The majority of other mammals maintain reproductive capacity throughout their adult lives, whereas humans can persist for decades beyond their final menstrual period.
Toothed whales, such as orcas and pilot whales, are among the few species with whom we share this phenomenon, and unraveling the commonalities could unlock the secrets of menopause.
One of the most prevalent explanations for the evolutionary development of menopause in certain species is termed the ‘grandmother hypothesis.’ In essence, older females cease producing their own offspring and redirect their energy and resources toward assisting in the upbringing of their grandchildren.
Some evolutionary scientists contend that this phenomenon occurs in toothed whales – they evolved menopause to extend their lifespan without prolonging their reproductive years.
To substantiate this adaptation in humans, researchers would need to demonstrate the existence of a significant number of women in pre-agricultural societies who lived beyond their personal reproductive capacity and contributed to the care of descendants.
However, concrete evidence for this is scarce. Historically, certain models derived from contemporary hunter-gatherer groups have failed to reveal a sufficient benefit that would “offset the evolutionary cost of ceasing reproduction,” according to evolutionary theorists.
One scientist even proposes that menopause is explained not by the matriarch but by the patriarch. The patriarch hypothesis suggests 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 these longevity genes were located on the X chromosome rather than the Y chromosome, it would also lead to increased lifespans in females, potentially resulting in the expenditure of all their viable eggs within a lifetime.
In other words, menopause might have been a consequence of extended life expectancies in males. This explanation does not account for the observation that female animals, on average, appear to live considerably longer than males. It also relies on an unproven assumption: that crucial longevity genes are not present on the Y chromosome.
Without menstruation and pregnancy, human existence would be impossible; and without menopause, it is plausible that our species might not have achieved such a degree of success.
More than 150 years after the theory of natural selection was first articulated, the evolutionary trajectory of the female body continues to represent one of the most profound and incomplete scientific mysteries.
