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Archive for the ‘sexual dimorphism’ Category

A recent study of hand prints in prehistoric caves shows that many of the hands were women’s. The media played up this fact as if it were surprising, although why it should be, I’m not sure, considering that we know almost nothing of the cultures responsible for the hand prints. But what caught my attention was a passing comment that a comparison of ancient and modern hand prints shows that the sexual dimorphism of humans was greater 40,000 years ago than today. In other words, men and women look more alike today than they once did.

This comment is interesting to me for several reasons. To start with, it is an example of how short a time is needed for evolution to take place. Forty millennia is a longer time than fifteen, which is about how long humans have been retaining the ability to digest milk into adulthood, but we tend to think of evolution in terms of millions of years – perhaps because establishing that the earth was ancient was a necessary part of proving the fact of evolution in the nineteenth century.

Just as importantly, this tidbit helps to answer those who suggest that civilization has stopped human evolution. Usually, the argument is that, because urban life and medical advances have decreased infant mortality, they have canceled out natural selection, the main mechanism of evolution. However, living to adulthood is only one aspect of natural selection. If nothing else, health and opportunity to reproduce are also part of natural selection.

In fact, it is often forgotten that sexual selection may be as important a mechanism for evolution in its own right. Since culture can determine all these things, it seems more reasonable that it simply because another set of criteria for adaptation, especially since such pieces of information are starting to accumulate to prove that evolution is still shaping humanity.

As to how humans are evolving, the greatest sexual dimorphism usually occurs in polygamous animals. Among gorillas, for instance, males are almost twice the size of females, and multiple mates are the norm for the males. By contrast, species that show little sexual dimorphism are usually monogamous or female-dominated. Given that humans show moderate sexual dimorphism, which seems to be decreasing, the natural conclusion is that we are descended from polygamous species, but evolving towards monogamy or egalitarianism, or perhaps female domination.

(In fact, although a couple of centuries would be an extremely short time for any evolutionary changes to be observable, I sometimes wonder if increased urbanism explains why each of the last few generations of women has been taller than the last, while men’s heights have increased less dramatically. Or perhaps the increased height of women is due to the fact than we have been moving away from societies based on hard labor. In such societies, men are often fed first so that they can continue to work, which opens the possibility that historically women were often underfed or even starved sometimes – an aspect of inequality that, so far as I am aware, has never been acknowledged or studied).

But if humans are becoming less sexually dimorphic, what does that imply for the future? I think I can suggest some answers, because, for much of my adult life, I have lived with a species of parrot that has so little sexual dimorphism that humans can only distinguish male from female reliably by surgical sexing or DNA samples. There are no external sex organs, and even sexual behavior is reliable, since homosexuality does exist.

I like to think that the lives of my parrots are a foretaste of what humans might be becoming. In my parrots, monogamy is the norm, and a hen is as likely to dominate as a cock. The sole exception is male territorial fights, which the hens generally ignore aside from being vaguely supportive of their mates (which amounts to the vague supportive chirp unless another male gets too close to their nests). Egg-sitting is largely, but not entirely the hen’s concern, but most males are supportive spouses and share in the care of chicks, especially immediately after they leave the nest.

The lives of my parrots are not completlye egalitarian, but they’re closer to that goal than anything the living generations of humans can boast. And as a supporter, I am tickled by the idea that feminists can probably state – with much more accuracy than evolutionary psychology usually manages – that evolution appears to be on their side.

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