When Hairy Met Sally II

This is the first post of a series on the documentary What's The Problem With Nudity?
All other primates and most mammals are covered with fur. The fur coat provides protection against water, cold and the sun’s rays. This is one of the greatest achievements of mammalian evolution. Why did humans abandon an advantage that tool millions of years to acquire?

Charles Darwin’s theory is our ancestors simply preferred less hairy sexual partners as they seemed more attractive. As an experiment, pictures of men’s torsos were shown to volunteers, and they had to rank them according to how attractive they looked. (No faces were shown.) The participants were not told that some torsos appeared twice, once in their natural state and once after having been completely shaved. But all shaved torsos were ranked in the top half, while the more hairy ones were in the bottom half. When asked to comment, the women expressed a preference for little to no hair.

But scientists say sexual selection cannot be the only explanation. Humans and their most closely related living species, the chimpanzee, branched off from a common ancestor about six million years ago. Chimpanzees still have fur, so it stands to reason that the common ancestor would also have been a furry animal. Our earliest ancestors would selected mates thick, glossy fur and would have stayed away from those with less fur. This is part of evolution’s strategy to weed out weaknesses and disease. Before humans could prefer less hair, this would have to have become an advantage.

To see what this might be, two volunteers, one naked, the other wearing loose clothing, are placed in a room where they are subjected to the intense heat of industrial radiators. At first, the clothed volunteer has the advantage because his clothes protect him from the intense heat and keep the covered parts cooler. This is what fur does to protect the body from the sun's heat.

However, the two volunteers soon begin to sweat profusely. The clothed one sweats just as much as the naked one, but his clothes start absorbing the sweat. The effect of cooling air on the clothed body is considerably reduced as evaporation of sweat is impeded by the clothes. Once again, the clothes are mimicking what fur would do. The naked man retains less heat because the sweat evaporates more easily.

Humans have the largest number of sweat glands of any animal and can produce about one litre of sweat per hour. The surface of our body is a high performance cooling system which no other animal has developed so far. How would this have been necessary or beneficial?

Penn State University professor Nina Jablonski presents the case of the Patas monkey, which lives in the African savannah, in an open environment much like that of earlier human species. Its body has similar proportions to that of our ancestors, including long limbs for covering great distances. The Patas’ range is the largest of all primates.

Like other primates, the Patas is covered with hair. Unlike other primates, the hair is thin and coarse, and it sweats profusely. Its evolution may well be echoing our own. Despite this, the Patas will likely never be as bare as we are. It still walks on all fours, so too much of its body is exposed to the sun.

Our ancestors with less hair and more productive sweat glands had an advantage over others. As generations went by, the only place where hair was preserved for protection is on the head, the part of the body most directly exposed to sunlight. (We will discuss the underarm and pubic hair in another post.) Thanks to their efficient cooling system, humans became the first primates capable of exploiting the resources of the savannah.

To sum up, researchers says the absence of fur in humans is due to three main events:

1) at some point, our ancestors began to stand upright to get from one place to another;

2) they became very active by covering great distances on the savannah;

3) they came to sweat profusely.

Essentially, humans found a better way to control body heat and fur became redundant.

The documentary also shows that the human brain constantly produces about 20 watts of heat. This may seem like very little, but put a lit 20 watt bulb in a box the size of a skull, close it, and you will see just how quickly it overheats.

It doesn’t take much overheating to impair brain function, and just a tad more can be fatal. In addition to protecting the brains of our more primitive ancestors, the efficient cooling system has allowed the brain to grow in size. Without this highly efficient cooling system, humans could never have developed a large brain, nor the intelligence that comes with it.