When Hairy Met Sally

The February 2010 issue of Scientific American has an interesting article: The Naked Truth: Why Humans Have No Fur. Here is a brief summary:

Among other things, fur is used to keep warm, to protect against abrasion, moisture, and harmful parasites and microbes. Also, it can be used for social displays, like when a dog raises the hair on its neck and back, a clear signal to stay away.

Most mammals have at least some hair. In some cases, the hair is so sparse and fine that it serves no function. Naked mole rats huddle to keep warm and since they can’t see each other, hair for social display would be useless. Whales and dolphins have no fur because it would cause drag when swimming long distances or when diving. Elephants, rhinoceroses and hippopotamuses are pretty much as naked as we are because they would otherwise be at a constant risk of overheating due to their size. Meanwhile, sea otters have fur that allows them to be buoyant in the water and float more easily, while playing its usual protective role on dry land. Of all primates, humans are the only ones largely without hair.

Humans live neither underground nor in the water, nor are they as large as elephants and rhinos. Why are we so naked, then? It turns out even smaller animals risk overheating in some circumstances if their cooling system can’t cope.

The human way of cooling off is to sweat profusely. There are primarily three types of sweat glands: sebaceous, apocrine and eccrine. In most mammals, the sebaceous and apocrine glands are the dominant sweat glands, and they are found near the base of hair follicles. The secretions mix to coat animals with an oily, sometimes foamy, mixture. While this can cool an animal, its capacity to dissipate heat is limited because evaporation occurs at the surface of the fur, not at the surface of the skin itself. The more the furry animal sweats, the less effectively heat is evaporated, because the fur becomes matted and evaporation is hampered.

Humans, in addition to being without fur, have more eccrine sweat glands than animals with fur. Instead of clustering near hair follicles, eccrine glands are relatively close to the surface of the skin and sweat comes out through tiny pores. The combination of naked skin and the watery sweat produced mainly by the eccrine glands allows humans to cool down more easily. A human being can produce up to 12 litres of this thin watery sweat each day.

Why did humans turn out this way? The theory is there was a global cooling period, which led to drier conditions in East and Central Africa, where the earliest ancestors of humanity are believed to have lived. As the climate became drier, the plant food (fruit, leaves, tubers, seeds) and sources of fresh water became scarcer, and the forest gave way to grasslands. Our ancestors had to give up leisurely foraging and travel greater distances to find edible plants and water. The need for extra calories eventually led to eating meat. Studies have shown that if the greater presence of eccrine glands and the loss of fur hadn’t occurred, these ancestors of ours would have continually been in danger of overheating due to their increased physical activity.

Since humans walked upright, the only area where hair had to be preserved was on the head, as protection against the sun. The dense head hair creates a barrier of air between the sweating scalp and the heat at the surface of the hair.

Hair in the armpits and in the groin area may combine with sweat to produce pheromones, as well as to provide lubrication in those areas when traveling from place to place. Everywhere else, hair generally disappeared, or became so thin and sparse as to be ineffective, while the skin became more waterproof and scuff-resistant.

The more efficient cooling system also allowed the brain to expand over time. Australopithecines had a brain of an average size of 400 cubic centimetres, about the same size as that of a chimpanzee. Homo ergaster’s brain was twice that size. The homo sapiens brain had expanded yet another 400 c.c. Each size increase means extra heat is being produced by the brain. If the brain’s heat has nowhere to go in a hurry, the danger of overheating and seriously damaging the brain is very high. Without a highly efficient cooling system, humans could never have evolved to have such a highly developed brain, nor the intelligence that comes with it.

Another part of the article on why humans don’t have fur concerns how long we have existed BOTH without fur and without clothes. The answer is to be found in lice.

Unlike head lice, body lice feed on blood but live in clothing. Analyses of gene sequences in lice show that while head lice have always been with us, body lice evolved much later. The best estimate at this point is that body lice appeared about one million years after our fur disappeared. This means that for one million years, we were truly naked.