Losing our body hair most likely helped early humans cope with heat
Even the hairiest human still shows a lot of skin compared to the average primate. Despite striking similarities in much of our anatomy, humans don’t have anywhere near as much hair as chimpanzees or gorillas. Since that hair seems to have been working pretty well for most primates for the last 65 million years, we have to assume members of the genus Homo gained some advantage by going bald, but figuring out what that was isn’t terribly easy, even though we’re still presumably living with that advantage today.
A variety of ideas have been suggested to explain why humans bare so much skin. Everything from spending a lot of time wading in water to having an easier time fending off parasites have been considered, and mostly dismissed. Mammals like sea otters show that fur isn’t a problem in the water, and as much as picking lice off a second grader may make parasites feel like a very compelling argument, too many primates seem to get by with bugs and hair for that to really explain the whole story.
Shedding hair and heat
The current front-runner for humanity’s less-fuzzy look is heat loss. By 3.2 million years ago, Australopithecus was walking upright, but was still probably hairy. This shift would have been crucial when a warming climate pushed some of our ancestors out to hotter, open savannahs. Standing tall would not only make you a smaller target for mid-day sun, but help you hunt animals during the day as well. Once our ancestors started bipedal running, being able to easily shed body heat would become a big advantage, as it lets us run for longer distances than many quadrupeds.
While being covered in dark hair makes a notable difference in who much heat animals shed in the sun, our naked skin wasn’t the end of our transformation. By 1.2 million years ago, a split in the lineage of lice indicates that our tummies were relatively bare, forcing the bugs to specialize in either head or pubic hair. All that available skin wasn’t simply bare though— it was home to our increasing numbers of sweat glands. Studies of the key genes we share with mice have shown that there may be a direct trade-off between hair and sweat glands. When activated, animals grow less hair and can be sweatier, quickly pushing humans to become the sweatiest things on the planet.
Baldness benefits our brains
The gains from all this may have been substantial. By sweating more and shedding more heat, our ancestors could probably run farther, possibly catching more big game to feed their families with. These bigger meals may have provided crucial calories to power our increasingly hungry brains, which would then allow us to invent clothes and antiperspirant, undoing millions of years of hard work. At least we still walk upright.
Source: Our weird lack of hair may be the key to our success by Melissa Hogenboom, BBC Earth