We are called the naked ape because we show more skin than any other primate. The gentleman in the picture appears hairless, but we know his hair has been removed for the shot. However, it may surprise you to know that we have just as many hair follicles as a chimpanzee. Our hair is just finer and much shorter. So why do we show so much more skin than any other primate?
There are those who think the answer lies in water. The Aquatic Ape Hypothesis claims we went back to the sea after we separated from our cousins who stayed in the trees about five million years ago. They say we lost our hair in the same way dolphins and whales did. However, they have a vascular adaptation under their flippers that allows them to regulate heat loss. While we may have gone back to the water, as current trends in evolutionary theory are beginning to consider, it was to live on the coast, not directly in the water.
Aquatic Ape Hypothesis
There is also an ectoparasite explanation. Lice and other creepy crawlers are blamed for the hair change, but this is wrong on every level. First is that lice transport themselves through clothing. The oldest known remnant of "fabric" is a 300,000-year-old piece of tanned animal hide. Not nearly old enough for body lice to be responsible. Geneticists put the loss of hair at two million years ago. Secondly, all primates have lots of parasites and they aren't showing skin. Lastly, people still get parasites today. Darwin himself did not think parasites were responsible.
Some scientists have promulgated the concept that we lost hair because we were scavengers of carrion. Vultures and other scavengers lose hair because carrion is full of bacteria, so have lost the hair on the neck and shoulders. We still have hair on our heads and the thought of sticking a hairy head into rotting meat is beyond disgusting. Even if we just used our hands to pull the carrion into bite-size pieces, it wouldn't account for our naked legs.
The most recent theory has us losing hair because we were long distance runners in the savannahs of Africa. By not having thick hair, we were able to cool ourselves down by exposing our sweat to the air using evaporative cooling. If you are furry, once the hair is soaked with sweat, you can't use evaporative cooling. This makes no sense because you would be trading coolness during the day for freezing at night. Other animals run all over the savannah with lots of fur, so what happened to us?
Animals run with hair.
While rummaging through my extreme collection of scientific papers recently downloaded, one beckoned me like a ray of sun on a rainy day. It is called Hair for brain trade-off, a metabolic bypass for encephalization by Yosef Dror and Michael Hopp. It is only available if you are willing to pay an exorbitant fee, so no URL here. Here is a brief interpretation:
There are certain amino acids that are extremely rare. So very rare that an omnivore is the only organism that can produce them. Amino acids are the building blocks of protein. In order to get these, an organism must eat meat, plants, and many kinds of seafood. Hair and brain both need these nutrients. While the body can normally pass amino acids from one organ to another, it can't do it between hair and brain, because hair is dead. Once the amino acids are in hair, they cannot be dislodged. They are locked in. We started losing our hair exactly when the brain took that evolutionary great leap forward, two million years ago. No hairy animal can have a big brain, and no big-brained animal can be hairy.