Why is the difference between men’s and women’s height so obvious? Maybe it wasn’t natural selection.

Beijing time july 8 news, male and female one of the most obvious physical differences is the average size. In general, men are higher. Textbook interpretation of this is the result of the natural gender selection and competition between men, the idea was first proposed by Darwin: “There is no doubt that men are generally larger, stronger than women, and have wider shoulders, more developed muscles, more rugged body contours, and more aggressive … During the long barbarian period, these characteristics have been preserved, even reinforced, by the success of the strongest and boldest men in survival and conjugate. “

Why is the difference between men's and women's height so obvious? Maybe it wasn't natural selection.

Fundamentally, all humans today may be about the same size if it weren’t for the fighting of men for mating rights. Evolutionary psychology extends this argument, saying that this biological instinct is why humans produce different behaviors, that men are inherently more aggressive and competitive, and that women are born to be silent and to pick and choose.

It’s important to say that differences in human size are as if this is the most fundamental gender difference. Bioanthropologists point out, “People take this as a fact, and once you raise an objection, they think you’re denying science.” “

But in a recent paper published in Evolutionary Anthropology, science actually points the conclusions in the other direction. There is insufficient evidence that “human size is determined by gender selection”, and medical and anthropological literature may instead provide a better explanation. All this may have nothing to do with competition, but rather with bone development and the different effects of hormones produced by the ovaries and testosterone on the bones.

It’s not that gender selection has nothing to do with the difference in height in humans, it’s that there’s not enough evidence to support this hypothesis, and we haven’t fully evaluated other theories. This debate is not just about evolution, it’s about the nature of who we are.

The answer bone hidden

Differences in human size are often cited as typical examples of gender selection: the evolution of such characteristics is often influenced by differences in reproductive success rates rather than survival rates. But theorists have come to this conclusion a little too quickly, and the available evidence is far from sufficient to suggest that gender selection is the main cause of the body size difference.

“We need a lot more evidence that the fact that there is a difference in size between men and women is not enough.” Louise Barrett, an evolutionary anthropologist at the University of Lethbridge, points out that the gender selection hypothesis is certainly more in line with our intuition, after all, scientists have made the inference that “men are bigger and better fighting than women, so there must be a correlation between the two.” This statement is also in line with the concept of masculinity and femininity in popular culture. “The problem is that the research is not good enough.”

For example, while studies often compare humans to other primates, they do not take into account the extent of overlap between humans and the evolutionary history of these animals. That said, even if gender selection can explain the difference in male and female chimp size, the gender difference in human size may be more relevant than our chimp’s same ancestor, rather than the result of perpetuating this gender selection.

“We need more data to explain why men are taller than women using the gender selection hypothesis.” Mr Barrett said. In fact, we might think in a different direction: “Perhaps dominance and competitive behavior are the result, not the cause, of the differences in the size of the sexes.” “

In studying the literature on bone biology and evolution, it is important to find a more straightforward explanation, especially the association between bone development and hormones, which women are shorter than men, possibly because most women have larger ovaries.

The ovaries are important because they produce much more estrogen than the testes, which help guide bone development. In all human bones, large amounts of estrogen stimulate long bone growth. Before puberty, both men and women grew at the same rate. But after entering puberty, the estrogen secreted by the ovaries gradually increases, stimulating the growth plate of the human bone and making the bones grow. The effect on the long bones is particularly pronounced. Thus, in early adolescence, girls tend to grow taller than boys.

But this does not last long, as high hormone levels can cause the growth plate to close. This is the reason for the difference in height between men and women: the growth rate of the long ovary reaches its peak soon after puberty, while the bones of the long testicles continue to grow for several years before estrogen levels reach their peak, so eventually the height will exceed the former.

This “hormone theme” is very much in line with historical changes in the size of human sushes. For example, after the Black Death that swept Europe in the 14th century, the average height difference between men and women increased by 62 per cent: 9 cm for men and 5.5 cm for women. The increase in male height makes sense because people generally become healthier, more nutritious and more nutritious after a pandemic, and their height in adulthood is greatly influenced by childhood nutrition and health. But women’s height is shorter, does this mean that their health levels have declined after the plague?

Sharon DeWitte, an anthropologist at the University of South Carolina, doesn’t think so. In a 2018 paper, she noted that “the decline in female body size after the Black Death may actually reflect an improvement in diet or health” because healthy health tends to be associated with earlier primary trends.

If so, then the change in the size of men and women has nothing to do with competition. Women after the Black Death do not particularly favour tall men, and men’s preferences for women do not suddenly change. Changes in body shape in men and women may be just one of the byproducts of improved health, and the healthier the body, the earlier the ovaries mature and the earlier the initial wave occurs.

The point of contention

The theory of using estrogen to explain height differences is not recently proposed, but human evolutionary biologists have not given it much attention. Estrogen seems to explain how the height difference sfall between men and women is generated, but it does not explain why it occurs from an evolutionary point of view.

But the claim itself is misleading. How can physiology, endocrinology, and bone development not be part of evolution? Estrogen affects bone growth, which is a direct physiological cause of men being taller than women. Any factors that affect estrogen levels and secretion time inevitably affect the differences in body shape between men and women, even if this is not the result of nature’s choice. Any environmental conditions that lead to early start-up will incidentally cause a relatively small female body size, but it will not change the mating system of human society, nor will it change the level of competition between men.

The mystery of pelvic width

This means that the gender differentiation of human body size may indeed have nothing to do with natural selection. So to understand why men are taller than women, we may first have to figure out why we experience puberty at a specific time and what contributes to the hormonal differences between different primates.

The hypothesis that women’s pelvis is wider than men’s is due to the need to give birth to a larger baby has a brain, as with the development of a long bone, where the pelvic width is largely determined by estrogen levels.

There is no evidence that pelvic width affects fertility success rates. In a species where childbirth is much easier than in humans, the pelvic width of females and males also shows this pattern. For example, the ratio of a chimpanzee newborn to a mother’s birth canal is much smaller than that of a human, but the female chimp’s pelvis is also wide.

Women’s pelvis may be wide not to help give birth, but to leave a complete body space for the entire reproductive system. “The vagina, clitoris, cervix, uterus and ovaries account for a much larger total mass than the male reproductive system.

“As soon as this explanation comes up, people will feel like they’re in a coma, ” Barrett laughs. ” “

Barrett also points out that the difference in pelvic width between men and women can be explained by such simple anatomical knowledge that it has been ignored by the scientific community for so long, suggesting that there are systemic problems with the inclusiveness of the scientific community. “We always think of the male body as the default configuration, and the female body is the result of deviation sourcing on that basis. But if we stop taking the body structure of a particular sex as the default configuration, but study how these physiological processes lead to gender differences, we can better validate these hypotheses. “

Go fake and save the truth

The “competitive hypothesis” for height and the “birth hypothesis” for hip width seem reasonable, so they are deeply rooted in the hearts of the people and have a real impact on our daily lives.

“Gender Selection” points out that men are born to compete, and civilized people have to contend with their “true nature” if they want to be willing to cooperate and be kind to others. It is this idea that leads to all kinds of stereotypes. But the truth that bones tell us is quite different.

We need to better and more rigorously verify all kinds of evolutionary hypotheses, and be especially careful when it comes to humans, and our perceptions of ourselves affect our behavior, so there is no mistake. This is about the evolutionary history of humanity, how we tell our story, so we have to make sure that the story is correct. (Leaf)