An increase in sweat glands, many more than other primates, also kept early humans on the cool side

One popular idea that has gone in and out of favor since it was proposed is called the aquatic ape theory. But during the dry season, they would move to oases and lakesides and wade into shallow waters to collect aquatic tubers, shellfish or other food sources. The hypothesis suggests that, since hair is not a very good insulator in water, our species lost our fur and developed a layer of fat. The hypothesis even suggests that we might have developed bipedalism due to its advantages when wading into shallow water. But this idea, which has been around for decades, hasn’t received much support from the fossil record and isn’t taken seriously by most researchers.

A more widely accepted theory is that, when human ancestors moved from the cool shady forests into the savannah, they developed a new method of thermoregulation. Losing all that fur made it possible for hominins to hunt during the day in the hot grasslands without overheating. The development of fire and clothing meant that humans could keep cool during the day and cozy up at night.

But these are not the only possibilities, and perhaps the loss of hair is due to a combination of factors. Evolutionary scientist Mark Pagel at the University of Reading has also proposed that going fur-less reduced the impact of lice and other parasites. Humans kept some patches of hair, like the stuff on our heads which protects from the sun and the stuff on our pubic regions which retains secreted pheromones. But the more hairless we got, Pagel says, the more attractive it became, and a stretch of hairless hide turned into a potent advertisement of a healthy, parasite-free mate.

For instance, a baby whose skin looks a little green or blue can indicate illness, a pink blush might indicate sexual attraction, and a face flushing with red could indicate anger, even in people with darker skin tones

One of the most intriguing theories is that the loss of hair on the face and some of the hair around the genitals may have helped with emotional communication. Mark Changizi, an evolutionary neurobiologist and director of human cognition at the research company 2AI, studies vision and color theory, and he says the reason for our hairless bodies may be in our eyes. While many animals have two types of cones, or the receptors in the eye that detect color, humans have three. Other animals that have three cones or more, like birds and reptiles, can see in a wide range of wavelengths in the visible light spectrum. But our third cone is unusual-it gives us a little extra power to detect hues right in the middle of the spectrum, allowing humans to pick out a vast range of shades that seem unnecessary for hunting or tracking.

Changizi proposes that the third cone allows us to communicate nonverbally by observing color changes in the face. “Having those two cones detecting wavelengths side by side is what you want if you want to be sensitive to oxygenation of hemoglobin under the skin to understand health or emotional changes,” he says. But the only way to see all TGPersonals Online of these emotional states is if humans lose their fur, especially on their faces.

In a 2006 paper in Biology Letters, Changizi found that primates with bare faces and sometimes bare rumps also tended to have three cones like humans, while fuzzy-faced monkeys lived their lives with just two cones. According to the paper, hairless faces and color vision seem to run together.