What was the evolutionary advantage for Homo sapiens becoming essentially hairless compared to other primates?

Robert Gillespie from Oakville, Ont., asks: "What is the evolutionary advantage for Homo sapiens becoming essentially hairless when compared to all other primates?"

Dr. Andrew Simons, a professor in the department of biology at Carleton University, says it depends on which hypothesis you believe.

The leading hypothesis, known as the "keep cool hypothesis," happened 1.6 to 1.2 million years ago. Prior to that point, our human ancestors were eating seeds and a lot of other vegetation. Climate change was also occurring at that time, which forced our ancestors to dramatically change their lifestyle to become much more active. That's when they started hunting and meat was introduced into their diet. 

Our ancestors would sweat to avoid heat damage to tissues, especially the brain. The evolutionary advantage of losing most of our body hair is because otherwise, it would have been impossible to lose heat quickly and efficiently enough.

Sweat experts

Unlike other mammals, we have a huge number of eccrine sweat glands on the surface of our skin. Other mammals have fewer eccrine sweat glands that were associated with hair follicles. It's not very efficient to lose heat by evaporation at a distance from our skin, as would be the case when sweat glands are associated with hair follicles. Instead, we lose heat directly off our skin.

We are now experts at sweating. Many modern primates are constrained in terms of how far from their water source they can move, because they're relatively less efficient at sweating. To lose the same amount of heat in a particular time period, they have to consume massive amounts of water, which limits the amount of time they can be active. 

That said, we humans still have some hair on our bodies. When humans became bipedal — meaning to stand on two legs — the surface of our body that faces the sun is our head. With other mammals, they still have fur to protect them from UV radiation. 

Underarm and groin hair seems to function as a lubricant in our joints that move. We also communicate chemically via pheromones that are emitted from these regions.