The Human Breast
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Hereís another challenge to the theory of evolution. Human women have relatively large, protruding breasts, but the females of other apes do not. The females of chimpanzees, our closest relatives, are flat-chested: looking only at a chimpanzeeís chest, we see no difference between male and female, not like we see in humans. Why does that difference exist? How did it come about? What life-or-death feature of the ancient landscape promoted the mutations that gave hominid women larger breasts?
We find a strong clue in a strange fact about human milk. Compared to the milk of other mammals, human milk is watery: itís a thin liquid. Simply put, human milk consists of 7.1% carbohydrate, 4.5% fat, 0.8% - 0.9% protein, and 0.2% minerals, the remaining 87.4% being water. And women can produce a lot of it.
Itís the need to produce a large volume of watery milk that makes the human womanís breasts bigger than those of a chimpanzee. We can think of them as being analogous to the camelís hump. Like the camelís hump, the breast consists largely of fat, which can readily be metabolized into water: each gram of fat yields more than a gram of water. And like the camelís hump, the human breast evolved its present form as an adaptation to life in a dry environment.
As East Africa began drying out, six to seven million years ago, forest began its slow transformation into savannah. A chimpanzee born into that world would not have noticed any change in her lifetime. Significant changes took centuries, even millenia, to occur. But they did occur and gradually the wet forest gave way to dry grassland. One consequence of the dryout was a sparser distribution of water sources across the landscape.
An adult hominid can go several days without water, just as a human can, so a long walk between waterholes was not fatal. But infants will dehydrate and that stress reduces their chances of reproducing and passing their genes on to the next generation. In any group the females differed from one another: some had slightly larger breasts and could thus produce more milk. The women who produced more milk had healthier babies, who then grew up to pass their mothersí genes on to their offspring. Over time the preferential reproduction of hominids whose mothers had larger breasts made the genetic coding for larger breasts dominant in the population. Mutations that made the breasts even larger were also promoted by natural selection until large breasts on the women came to be one of the characteristic traits of genus Homo.
That evolution also produced the human milk cycle. For the first half week after a woman gives birth her breasts produce colostrum, a thin, yellowish fluid that is rich in proteins and antibodies. The colostrum provides immunity to the baby until its own immune system begins working and it also stimulates the growth and function of the babyís digestive system. Three to four days after the woman gives birth her breasts begin to produce milk that varies from thin, watery, and sweet to thicker and creamier. At the beginning of a feed the baby gets foremilk, which is waterier, lower in fat, and higher in carbohydrates relative to the creamier hindmilk. The foremilk quenches the babyís thirst and may be the only milk that the baby takes in a given feed. If the baby takes all of the foremilk, it then takes the thicker and creamier hindmilk, which satisfies the babyís hunger. Thus the breast evolved to produce milk that prevents dehydration first and then provides the nutrition the baby needs to grow: the baby can take one without taking the other. In essence the human femaleís breasts evolved to serve as canteens for her baby.
There are, of course, other hypotheses that may add their own truths to the eventual complete theory of human evolution. One such hypothesis that warrants our attention claims that womenís breasts became larger due to sexual selection and therein lies a discussion.
Sexual selection denotes the idea that animals evolve to become more attractive to the opposite sex. We see the easiest example of sexual selection in peafowl. A large, fancy tail bespeaks a peacock that can, nonetheless, escape from predators: thatís one strong bird, well fit to his environment. Sure enough, peahens are preferentially attracted to males with big, fancy tails. But the peacocks and peahens didnít start out that way.
Peafowl (genus Pavo) started out looking like their closest relatives, the pheasants. Pheasants are fairly plain, drab birds, but at one time in India a mutation occurred and gave a pheasant a longer, heavier tail. That trait got passed on to the individualís offspring and as it spread throughout the population natural selection went to work on it. The weakest birds were so hindered by the larger tail that they fell easy prey to the hunters of birds. Over time the stronger birds fathered more offspring and the weaker birds fathered fewer or none; thus, the population evolved a connection between big, fancy tails and strength and vigor. Further mutations made the tails bigger and fancier, but the process described so far didnít act alone.
Although the actual genetic code for vigor and tail size spanned a continuum, we can divide the proto-peacocks into four groups: weak/small, weak/large, strong/small, and strong/large. Birds that fall into the weak/large group will reproduce poorly, if at all, and their genetic coding will, over time, be eliminated from the population. Now the hens come into play.
As to every trait in living things, there are variations throughout any given population. In the group that we can call Proto-Pavo the hens would have responded to slightly different sexual signals. Most hens preferred the normal small tail and those hens and their mates didnít change: they and their descendants remained pheasants. But some hens, due to mutations in the brain, had a preference for larger tails. If the large-tail mutation did not occur, then those hens would have settled for the standard pheasant male. But when the large-tail mutation entered the population, those hens mated, insofar as they could do so, primarily with its carriers. That process yielded a positive feedback effect: further mutations for large, fancy tails and mutations for a preference for large, fancy tails reinforced each other and as long as nothing interfered with that reinforcement that population of birds would change away from the original population of pheasants and become a new species, the first true peafowl.
Did something like that process happen with regard to the human female breast? It seems unlikely. Sexual selection works in birds because hens are very picky about who they will accept as a mate and they can actually shake off any unwanted suitors who get too insistent (in the act of intercourse the cock must stand on the henís back, a precarious position from which he can be thrown easily, especially since birds canít grasp each other with any significant force). In hominids, as in the chimpanzees, the women arenít picky about who they will mate with. But some kind of sexual selection was likely at work in the hominids, though it didnít modify the females: it modified the males.
We know that sexual selection did not produce the enlarged breasts of the human female. We already have a solid explanation for that trait in terms of ensuring the survival and health of infants in a dry environment. No, sexual selection did not produce enlarged female breasts because men like them; it produced men who like big breasts because successfully breeding women have them.
That particular feature of human evolution was simply a continuation of a development that had occurred earlier when Genus Australopithecus (proto-humans) separated from Genus Pan (the chimpanzees). Any primate that walks upright on its hind legs as its primary means of locomotion has an invisible estrus in its females. The swollen genitalia that signal sexual readiness in chimpanzees got selected out of Australopithecus so that the female genitalia could lie in the femaleís crotch without chafing. The males changed as well. Those males who needed the old sexual signal in order to gain sexual arousal simply didnít reproduce. Those males who had the brain mutations that let them gain arousal at any time did reproduce themselves and thereby reproduced that tendency to want sex often. That tendency had nothing to do with the size of the femaleís breasts.
Assume the existence of a brain mutation that transforms the old sexual desire for enlarged female genitalia into a desire for big breasts. Men with that mutation will mate preferentially with bigger-breasted women while other men will mate with bigger-breasted women and smaller-breasted women indifferently. As a consequence, the genetic coding for desiring big-breasted women will come to dominance in the population. This explains why modern men are so eager to make drooling fools of themselves over big-busted bimbos.
Again we see how the interplay between mutation and natural selection produces a new trait in an organism. The logic is sound and the hypothesis makes good sense. But science does not rely on logic alone. Hypotheses must be proven and either verified or falsified by physical evidence: the Scientific Method demands that we provide such evidence before we can declare that an hypothesis has been promoted to the status of a theory.
That fact creates a problem for anyone wanting to promote the above hypothesis to the status of a theory. Soft tissues donít fossilize, so nobodyís going to get direct evidence for the hypothesis, unless they can obtain some kind of time machine. But indirect evidence can do the job under the right circumstances. In this case it will have to be DNA extracted from hominid fossils. If biochemists can track the mutations that created larger breasts and correlate the timing of those mutations with climate change in ancient East Africa, then we may regard the hypothesis as proven and verified. For now, though, we will have to wait.
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