Mark Pagel, head of the evolutionary biology group at the University of Reading in England and editor of The Encyclopedia of Evolution, fills us in:
We humans are conspicuous among the 5,000 or so mammal species in that we are effectively naked. Just consider what your pet dog or cat (or, for that matter, a polar bear) would look like, and how it might feel, if its furry coat were shorn.
Scientists have suggested three main explanations for why humans lack fur. All revolve around the idea that it may have been advantageous for our evolving lineage to have become less and less hairy during the six million years since we shared a common ancestor with our closest living relative, the chimpanzee.
The aquatic-ape hypothesis suggests that six million to eight million years ago apelike ancestors of modern humans had a semiaquatic lifestyle based on foraging for food in shallow waters. Fur is not an effective insulator in water, and so the theory asserts that we evolved to lose our fur, replacing it, as other aquatic mammals have, with relatively high levels of body fat. Imaginative as this explanation is—and helpful in providing us with an excuse for being overweight—paleontological evidence for an aquatic phase of human existence has proven elusive.
The second theory is that we lost our fur in order to control our body temperature when we adapted to life on the hot savannah. Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated. The body-cooling idea seems sensible, but even though lacking fur might have made it easier for us to lose heat during the day, we also would have lost more heat at night, when we needed to retain it.
Recently, a colleague and I suggested that ancestors to modern humans became naked as a means to reduce the prevalence of external parasites that routinely infest fur. A furry coat provides an attractive and safe haven for insects such as ticks, lice, biting flies and other "ectoparasites." These creatures not only bring irritation and annoyance but carry viral, bacterial and protozoan-based diseases such as malaria, sleeping sickness, West Nile and Lyme disease, all of which can cause chronic medical problems and, in some cases, death. Humans, by virtue of being able to build fires, construct shelters and produce clothes, would have been able to lose their fur and thereby reduce the numbers of parasites they were carrying without suffering from the cold at night or in colder climates.
Human lice infections, which are confined to the hairy areas of our bodies, seem to support the parasite hypothesis. Naked mole rats, animals that can be described as resembling "overcooked sausages with buck teeth," also seem to support the theory: They live underground in large colonies, in which parasites would be readily transmitted. But the combined warmth of their bodies and the confined underground space probably negate the problem of losing heat to cold air for these animals, allowing them also to become naked.
Once hairlessness had evolved this way, it may have become subject to sexual selection—being a feature in one sex that appealed to another. Smooth, clear skin may have become a signal of health, like a peacock's tail, and could explain why women are naturally less hairy than men and why they put more effort into removing body hair. Despite exposing us to head lice, humans probably retained head hair for protection from the sun and to provide warmth when the air is cold. Pubic hair may have been retained for its role in enhancing pheromones or the airborne odors of sexual attraction.
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5 Comments
Add CommentM. Pagel: "Fur is not an effective insulator in water, and so the [aquatic-ape] theory asserts that we evolved to lose our fur, replacing it, as other aquatic mammals have..."
Reply | Report Abuse | Link to thisIt seems that arctic seals, sea lions, polar bears and sub-arctic otters and beavers may disagree with this theory that having fur is a disadvantage in water. Seals, of course, abound in both arctic and temperate waters, and have long been prized by humans for their furs. So the idea that people lost their fur to be better food gatherers in shallow seas seems insubstantial.
The heat management, insect control, and sexual attraction theories also seem lightly grounded in that of the thousands of other species of mammals that lived in the same environs as early humans, none developed the same adaptation.
The question is adaptation to what? Here are some factors to consider: most of the time that humans have existed has been during ice ages; big brains have to be fed by enormous intakes of protein; advancing ice concentrates game.
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Edited by deepspringer at 05/12/2008 12:42 PM
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Edited by deepspringer at 05/27/2008 5:15 PM
Comment part 2:
Reply | Report Abuse | Link to thisAs advancing ice pushed food animals southward that our hominid ancestors depended on for the high protein their big brains needed, these prey were likely concentrated in a band just below the ice. So the big brains with the best hunting tools tended to stay just ahead of the ice in the best hunting grounds, therefore in a cold climate. To fight the cold, they wore the furs of their kills to better retain their own body heat. In parallel evolutions, their manual dexterity, abstract imagination, and cooperative living advanced to enable them to design and make effective clothing from animal skins. From wearing animal skins, they lost most of their own fur and evolved the perspiration system for better body heat management. Thus, wearing animal skins greatly improved the hominids' ability to adapt to seasonal climates and to extend range. Eventually the greater intelligence and dexterity required to make clothing enabled farming and its more reliable sources of protein demanded by even bigger brains.
So here we are.
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Edited by deepspringer at 05/12/2008 12:41 PM
Comment part 3:
Reply | Report Abuse | Link to thisSolving the mystery about why Neanderthals became extinct while our ancestors survived about 30,000 years ago may hinge on discovering whether or not Neanderthals wore animal furs as clothing and had thereby lost their own fur. It has been reported that Neanderthals had somewhat larger brains than homo sapiens, but that the cultural evidence they left behind indicates that their minds lacked the imagination of homo sapiens - an imagination that could have become crucial for adaptive survival in a sudden change of environmental circumstances.
Imagination is a prerequisite for designing and making even crude types of clothing, especially clothing for a cold environment. And once early humans became dependent on clothing, the more natural selection would have favored the development of greater imagination in order to create more durable, more adaptive clothing that would have given them a greater edge for survival. Dependence on clothing would have caused a parallel dependence on housing -and of course fire - to better protect us against the bitter night and vicious weather that even the wearing of skins couldn't withstand. And the abstract imagination developed for making clothing could easily be applied to making shelter.
It was an imagination Neanderthals apparently lacked, though they may have had fur instead.
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Edited by deepspringer at 05/12/2008 12:40 PM
Comment part 2:
Reply | Report Abuse | Link to thisSee above. (This stub was added in error.)
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Edited by deepspringer at 05/12/2008 12:40 PM
Why does hair growth continue on the head while it is short on the arms etc?
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