When did human bodies evolve to sweat? We don't know exactly when

RACHEL MARTIN, HOST:

All summer, NPR's science desk has been looking at sweat. Humans are covered head to toe with millions of sweat glands. But it wasn't always so. NPR's Pien Huang tells us how human bodies evolved so we could sweat.

PIEN HUANG, BYLINE: Sweating is essential to humans.

HEATHER DINGWALL: During the heat wave, I was sweating just, like, walking to work. And I think it probably kept me alive (laughter).

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HUANG: That's Heather Dingwall, a human evolutionary biologist. And she was walking to her job as a postdoc at UPenn. There she studies organs in the skin, like hair follicles and, of course, sweat glands. Sweat glands are actually coiled tubes that bring water from inside your body to the surface.

DINGWALL: You're effectively moving the heat from your body to that water droplet.

HUANG: As the water droplet evaporates, it takes the heat off your skin. Now, a lot of animals don't sweat, but humans do. And we don't know exactly when it started. There's no fossil record of ancient sweat glands because the soft tissue of the skin falls apart. But Dingwall says it's an adaptation that likely came around a long time ago.

DINGWALL: Probably around the time when the environment was getting drier and warmer. And so that would be around, like, 1 1/2 to 2 1/2 million years ago.

HUANG: This was in East Africa, where modern humans emerged. Before then, early humans lived in the cool shade of the forest. They had short bodies covered in thick hair to stay warm. Then the climate and landscape changed. Grasslands replaced forests. And humans spent a lot more time in the sun, where they had the opposite problem. They needed to stay cool. So having a body covered in sweat glands became an advantage.

DINGWALL: So you might be able to forage for longer, hunt for longer, collect more food. Or even if it's just - it could be as simple as, like, not dying of heat stroke.

HUANG: Whatever benefits humans got from sweat must have been huge because sweating requires you to keep drinking lots of water.

DINGWALL: And if you lose too much water, then you become dehydrated. And also, you will die.

HUANG: Humans, of course, haven't died off. Instead, we've thrived thanks to our sweat. It's core to who we are, even more so than our giant brains, which evolved much later. There's nothing more human than being sweaty.

Pien Huang, NPR News.

(SOUNDBITE OF ST. MIC'S "FEELS LOOPING") Transcript provided by NPR, Copyright NPR.