The humanlike species that evolved during the first three million years after the split with chimps probably looked far more apelike than human. Their bodies were of ape proportions, with long arms, and doubtless covered with hair from head to toe. Not until ergaster's arrival on the scene did the lineage's physical appearance assume a more recognizably human form. Ergaster, unlike its predecessors, possessed an external nose. This acquisition was its most prominent adaptation to hot, dry climates, the role of a nose being to conserve water by cooling and condensing moist air from the lungs before it leaves the body. After the differences in body proportions, humans differ most strikingly from their ape cousins in the distribution of body hair. Over most of the body people are essentially hairless, possessing just vestigial hair that is largely invisible. Nakedness is a complex issue, for which there may be several layers of explanation. Hairiness is the default state of all mammals, and the handful of species that have lost their hair have done so for a variety of compelling reasons, such as living in water, as do hippopotamuses, whales and walruses, or residing in hot underground tunnels, as does the naked mole rat. With humans, the prime cause may have been the need to sweat. Ergaster may have been the first of the human line to shed its fur in favor of naked skin, in the view of the paleoanthropologist Richard Klein.^^ His inference is based on the idea that if ergaster were living in dry, hot places, it would need to have evolved a way of cooling the body and its larger brain. Sweating, an efficient way to do this, requires a naked skin. Besides, humans must have lost their hair at some time, and the most plausible period is when they traded the shade of the trees for the heat of the savanna. Another, perhaps secondary, reason for human nakedness may have to do with sexual preferences. Darwin, who first suggested the idea, gave the matter serious attention in his book of 1871, The Descent of Man. "May we then infer that man became divested of hair from having aboriginally inhabited some tropical land?" Darwin asked. (He had already assumed that humans originated in Africa, because that is where the great apes are found, but there was then no fossil evidence to confirm the idea.) Yet that couldn't be the whole story because other primates in tropical countries have retained their hair. Perhaps shedding the body hair freed humans from the burden of parasites like lice, fleas and ticks. But that didn't seem a decisive enough advantage to Darwin. "The view which seems to me the most probable," he concluded, "is that man, or rather primarily woman, became divested of hair for ornamental purposes." 15 Darwin believed that sexual selection was an important factor in evolution because it determined mating success. Sexual selection arises in two distinct forms, intersexual and intrasexual. The first is the way that men and women choose each other as mates; the second is the competition within each sex, between men for women and, sometimes more discreetly, between women for men. Hairlessness would have been favored, in Darwin's view, if men and women had preferred partners with less hair. Two biologists, Mark Pagel and Walter Bodmer, have recently reinvoked Darwin's idea of sexual selection as the driver of human hairlessness. They suggest that lack of hair was favored among early humans because it was a sure signal that no parasites were lurking in their fur.
The date proposed by archaeologists for when humans lost their hair is based on the guess that it coincided with the emergence of Homo ergaster. But an actual fix on the date has been supplied by geneticists. Their research illustrates the wealth of information that can be extracted from a single gene if the right questions are asked. The gene in question is one that makes the melanocortin receptor, a protein that helps determine skin color. It does so by controlling the proportions of different-colored melanin pigments that are synthesized in a person's skin cells. Some versions of the melanocortin receptor produce black skin and hair, others generate ginger or brown or yellow. Rosalind Harding, of the University of Oxford in England, recently analyzed the order of the DNA units in the melanocortin receptor gene possessed by people from Africa, Europe and Asia. She and her colleagues found that all Africans had essentially the same version of the gene but that people outside of Africa possessed many different 17
An obvious explanation for the receptor gene's constancy in Africa is that it is under fierce selective constraint there, meaning that natural selection prevents any significant change. The African version of the gene is set to produce maximum blackness; any change in its DNA sequence is likely to make the skin lighter and its owner more vulnerable to the sun's ultraviolet radiation, which destroys an essential nutrient known as folic acid. (Ultraviolet radiation can also cause skin cancer, but it is the destruction of folic acid that is more likely to reduce fertility and hence to shape the evolution of the gene.) Anyone with a changed melanocortin receptor gene is likely to leave fewer or no descendants, and the variant gene will in time be eliminated from the population. Hence everyone living under the African sun has the same version of the gene, no deviations allowed.
Before human ancestors lost their hair, however, their skin was almost certainly pale, according to Nina Jablonski, an expert on the evolution of human skin color.^^ This can be inferred from the skin color of chimpanzees, the reliable surrogate for the joint human-chimp ancestor.
Beneath their dark hair, which protects them from the sun, chimpanzees have light skin. They too have a melanocortin receptor gene, but it exists in many different versions, as if natural selection does not mind letting it vary, and all produce pale skin. (Chimps have dark-skinned faces, but that is from tanning of the pale faces they have at birth.) Reading Harding's article, Alan Rogers, a population geneticist at the University of Utah, wondered how African populations had all acquired the same version of the gene. The process must have started, he supposed, when the human lineage first started to lose its apelike hair, dangerously exposing the pale skin beneath. Any mutation in the melanocortin receptor gene that led to a blacker, more protective skin would have conferred a great advantage on its owner. In several generations the new version of the gene would sweep through the population. Genetic sweeps can often be dated because after a must-have gene has become universal it starts to accumulate what are known as silent mutations, ones that don't alter the structure of the gene's protein and so are not eliminated through natural selection.^ Since the silent mutations accumulate at a known rate, the number of them is a measure of the time that has elapsed since the new version of the gene swept through a population.
Rogers realized that from the silent mutations in the African version of the melanocortin receptor gene, he could calculate the date at least of the gene's most recent sweep. He estimates that this event took place about 1.2 million years ago. 19
There may have been several earlier such genetic sweeps, each one producing a progressively more effective version of the melanocortin receptor gene. The gene, after all, had to make a very significant transition, from producing the pale skin of the joint human-chimp ancestor to the black skin that protected the newly hairless body from the sun in the scantly shaded African savanna. If the first of these sweeps had started several thousand years before, that would fit well with the archaeological evidence for the emergence of Homo ergaster 1.7 million years ago.
Outside of Africa, to jump a little ahead in the human story, the melanocortin receptor gene became free once more to collect mutations and become less efficient at triggering the black, radiation-protective form of melanin. That could well have been an advantage for people living in cold northern climates since they require extra exposure to the sunlight that is needed to help synthesize sufficient quantities of vitamin D, the lack of which causes misshapen bones and the disease known as rickets.
In every population of the world, women's skin color is 3 to 4% lighter than men's, perhaps through sexual selection by men, and perhaps
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