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In order to have a proper theory of evolution, we must be able to explain any feature of a living creature, doing so solely in terms of the interplay between mutation and natural selection. We must look at the interaction between a species and its environment and discern how features of the environment shape that species by taking advantage of random mutations.
The modern theory of evolution claims that humans evolved from a creature that, six to seven million years ago, was also the ancestor of the chimpanzees. Like the other apes, it was completely covered with thick hair, likely short like the hair on the pygmy (common) chimpanzee, which it closely resembled. But humans are almost completely hairless. We are what Desmond Morris called "The Naked Ape". That fact makes us very different from the other apes, so the theory of evolution must explain it. The theory must also explain the hair that humans do have.
We know that apes live in forests and woodlands. They are rarely far from trees, so when danger appears, they donít have far to run to climb to safety. But six to seven million years ago East Africa began to get drier and the forests and woodlands slowly became sparser, beginning the gradual process of being replaced by savannah, open grassland with few trees. Like the famous (and deceased) frog in a slowly heating pan of water, the proto-chimpanzees didnít notice the danger creeping up on them. As the forests shrank and meadows grew large, the distance to safety grew longer. Gradually, running for the trees became more of a contest, with the loser becoming leopard food.
In that circumstance any mutation that gave an individual a better ability to run would be preferentially reproduced into the population. Of course, such a mutant might still die before they could reproduce, but the mutation improves the odds of their survival and, thus, improves the odds of its being passed on. Over time the mutation would spread until every member of the population carried it. Some obvious mutations that we would expect to see are those that would lengthen the proto-chimpanzeeís stride and improve its balance so that it could run on its hind legs. But the ability to run great distances requires something else as well.
Muscles contract when the chemical bonds in certain molecules get broken. The energy thus released drives the force of the contraction and then turns into heat. That heat must be eliminated in order for the muscle to continue working: the muscle cannot be allowed to get too hot. Most animals eliminate waste heat in the breath: the inhaled breath gets warmed in the lungs and water is also evaporated into the exhaled breath. But a long distance runner needs more powerful cooling. Panting is insufficient by itself. In proto-humans evolution produce something else.
We know that individuals in a population differ from each other in a multitude of ways. Natural selection plays on those differences to promote some traits and demote others in the population. Stress and danger in the environment bring natural selection into play, determining who lives long enough to reproduce and who doesnít. Thus, when running becomes a matter of life or death, natural selection promotes the evolution of longer legs and more efficient cooling of the body.
Among the proto-chimpanzees who lived in East Africa six million years ago, some members of the population had sparser hair than average and some had more sweat glands per square inch than average. Mammals originally evolved hair in part to prevent excessive heat loss when air moved over their bodies. So thinner hair enabled increased heat loss and the evaporation of water spread over the skin yielded even greater heat loss. In the life or death contest to run longer distances, natural selection promoted mutations that enhanced those traits, probably combining them by evolving empty hair follicles into sweat glands. Thus, over thousands of generations the proto-chimpanzees of East Africa became the long-legged, hairless proto-humans.
But humans are not completely hairless. We still have patches of hair on our heads, in our armpits and crotches, and, in adult males, on the face. If hairlessness helps us in long-distance running, then why do those patches remain?
Long, thick hair on the tops and sides of our heads clearly serves as a parasol. It prevents direct sunlight from overheating our scalps and thus overheating our brains. An individual who had sparse hair on their head would suffer from overheating and, thus, be less likely to reproduce. Because of that fact, the proto-chimpanzees of East Africa gained thicker, longer hair on their heads while losing hair over the rest of their bodies.
Medium-long hair in our armpits offers a different kind of protection. It prevents the skin in those areas from chafing against itself and thereby opening a way for infection to enter the body. Pubic hair, growing in after the onset of puberty, performs the same function with respect to the chafing that might otherwise come from sexual intercourse. We can see in that idea why children lack pubic hair: they donít need it.
The facial hair Ė beards and mustaches Ė that appears on adult males doesnít have an easy explanation. We have no obvious life-or-death reason why men have it and women donít or why adult males have it and children donít. Facial hair doesnít seem to provide the male with a reproductive advantage.
In fact, facial hair appears to be detrimental. A beard and mustache (especially if the hair is long and thick like the hair on the top of the head) accumulate juices, bits of food, and other debris that escape from the mouth and nose. They become hotbeds of bacterial growth and a ready source of infection. We should expect that bearded men would get sick more frequently and more intensely than beardless men do.
In the conventional understanding of evolution by means of natural selection, we are tempted to infer that all humans should lack facial hair. Someone who gets sick often would seem to be at a reproductive disadvantage. But another factor enters into the calculation when we remember that the body is not merely a passive sufferer of disease. A subset of cells within the body, comprising the immune system, actively fights disease.
Body systems grow with use. That phenomenon prevents unused systems from taking up resources. A bearded manís immune system gets used more than a beardless manís does, so we should expect the bearded man to have the stronger immune system. Thus, the bearded man has the better chance of surviving a truly catastrophic disease.
That fact benefits the bearded man, certainly. Better able to face disease (because of the "practice" that his immune system gets), he has the better chance of surviving long enough and well enough to reproduce the traits he inherited from his line of descent, including, of course, facial hair. But it also benefits his tribe, through epigenetic descent.
Genetic change, which accounts for the presence or absence of beards (among many other things), consists of changes to the base pairs that make up DNA. It is changes to the rungs of the spiral ladder that some call the double helix and those changes are relatively permanent. Epigenetic change consists of certain partial molecules (such as methyl groups) binding loosely to the DNA like barnacles on a whale. Those add-ons prevent certain portions of the DNA from participating in the creation of proteins and thereby change the form of the organism to which the DNA belongs. These changes are less permanent, lasting perhaps only a few generations.
We assume that frequent activation of the immune system results in epigenetic changes that correlate with a more easily activated immune system. Is there any way to test this hypothesis? We might try to see what happens when a population with bearded men (strong immune systems) meets a population with beardless men (less strong immune systems).
In the Sixteenth Century, when Europeans (bearded men) encountered Native Americans (beardless men), European diseases, such as influenza, nearly wiped out the Native American population but American diseases had very little impact on Europe. On the other hand, when Europeans encountered the native people of India and Australia, European diseases did not decimate those populations. But, then, the native men of India and Australia have beards.
Epigenetic changes are inherited with the DNA on which they ride, so the change in the immune system due to men having beards can also benefit women and children of the next generations. But the add-ons to the DNA fall off after several generations have elapsed, so they must be continuously renewed by men getting sick frequently. Thus, thereís an evolutionary pressure for men to keep their beards, rather than to lose them to improve cooling of their bodies.
That being said, we must now ask why Native American men typically lack beards. What happened during the Great Migration through Siberia, across Beringia, and into North America that favored men with the least facial hair?
One possibility involves Vitamin D. That vitamin is necessary to humans to ensure proper uptake of calcium by the body and thereby prevent bone-softening diseases such as rickets or osteomalacia. The production of Vitamin D in the human body necessitates exposure of the skin to ultraviolet radiation from the sun: that fact explains why the people of Europe evolved pale skin.
During the Ice Age northern Siberia, Beringia, and Alaska were cold the whole year. Cold winds blowing off the glaciers would have kept the world uncomfortable for humans. As a consequence, people rarely went outside their shelters without being completely bundled up in clothing, with only their faces exposed to the environment. If the face is the only skin available for ultraviolet absorption, then men with heavy beards will be at a distinct disadvantage relative to men with less facial hair. Over time bone disease eliminated bearded men from the population and thereby also flushed the coding for beards out of the gene pool of the people who settled North America.
Thus we have a plausible explanation for the distribution of hair on the human body. Of course, it is, at best, merely an hypothesis. More evidence is needed before we can promote it to the status of a theory. Until such evidence shows up, we need to maintain a certain scepticism regarding the veracity of the story just told. Thatís how proper science works.
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