The emergence of upright walking put the buttocks of our ancestors in a new position—both posturally and evolutionarily.
Other great apes such as chimpanzees have small, hairy, flat rumps with tough skin patches on which they sit. But once our hominid ancestors started walking upright around 4.2 million years ago, the legs and buttocks were re-engineered. Much larger, stronger muscles evolved for powering the leg backwards so that it could propel the body forwards. These muscles are what give the human buttocks their basic rounded shape. Beyond this increased muscularity in both sexes, females evolved larger deposits of fat on the buttocks, hips, and upper thighs. Like breasts, these probably evolved through male mate choice as indicators of youth, adequate fat, and perhaps developmental stability.
We are the only species of primate with permanently protruding hemispherical buttocks, and the only species where this protrusion is permanently amplified in adult females by the addition of fat deposits. Buttock size and shape is a unique human feature and shows substantial sex differences. Buttocks are also age-specific, with almost no differences between the sexes before puberty, followed by a rapid accumulation of fat in female buttocks, hips, and thighs over a few years. Buttock size and protuberance normally peaks in young adulthood, around the time of peak female fertility, and then gradually diminishes relative to the rest of the body's fat reserves. Buttocks also show differences between human populations. In southern African Koi-San populations, female buttocks evolved through male mate choice to be especially prominent.
The sex difference in buttock size and shape is hard to explain through natural selection. Because female breasts and buttocks are composed primarily of fatty tissue, it has been suggested that they evolved to provide adequate fat reserves, to protect against the unpredictability and seasonality of food. However, almost all female vertebrates have evolved to store fat reserves, and only female humans have such an unusual distribution of fat. Gorilla females store plenty of fat inside their abdomens, as do human males. Why did human females deviate from this normal primate pattern to store fat below their hips?
Evolutionary psychologist Dev Singh has suggested that the female human distribution of fat evolved as an indicator of youth, health, and fertility. He found that men around the world generally prefer women who have a low "waist-to-hip ratio": a relatively narrow waist and relatively broad hips. Young, fertile women who are not pregnant have waist-to-hip ratios of around 0.7. This ratio would result from a waist circumference of 24 inches and a hip and buttock circumference of 36 inches, for example. Men almost always have a waist-to-hip ratio of at least 0.9, as do prepubescent girls and women past menopause. Obviously, pregnant women have even higher waist-to-hip ratios. Women with various health problems that impair fertility also tend to have higher than average waist-to-hip ratios. Indian temple sculptors have traditionally depicted Hindu goddesses with waist-to-hip ratios as low as 0.3, to symbolize their supernatural fertility and sexuality. In European fashion, corsets and bustle skirts have been used to lower waist-to-hip ratios deceptively. If male hominids have preferred low waist-to-hip ratios for many generations, this may explain why human females have such narrow waists, such broad hips, and such fleshy buttocks.
Women's breasts and buttocks did not evolve because hominid men happened to develop some arbitrary fixation on hemispheres as Platonic ideals of beauty They evolved as reliable indicators of youth, health, fertility, symmetry, and adequate fat reserves. Starving, sickly women cannot maintain large breasts and buttocks. They need to burn up their fat reserves to stay alive, not keep them hanging around in the hope of attracting a mate. Because starving women tend to turn off ovulation, women without fleshy breasts and buttocks are usually women without fertility. Female longdistance runners, ballerinas, and anorexics who lose most of their body fat tend to have much smaller breasts and buttocks, and often stop menstruating and ovulating. Buttocks, like breasts, reveal the importance of male mate choice in human evolution.
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