The Hairy Origins Of Our Sweat Glands

The following is an excerpt from The Joy of Sweat: The Strange Science of Perspiration by Sarah Everts.

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Eccrine sweat glands—millions of which produce our salty, cooling perspiration—are actually found in all mammals. But in most other animals, the fluid from eccrine glands is not used to cool down, but to provide grip. Most mammals have eccrine pores only in the soles of their feet or hands. In moments of stress, the salty liquid emerging from eccrine glands provides extra friction for landing jumps and for climbing. It’s normally only released when the animals are stressed, such as when they need to escape from a predator or catch prey. You can blame your vestigial self when your hands get sweaty under moments of duress: Humans may no longer need to climb trees to deal with (most) potential threats, but our sweaty palms during moments of anxiety reveal that old habits die hard.

At some point in the evolution of primates, eccrine glands began expanding beyond the soles of the feet and palms of the hand to appear on our ancestors’ torsos, faces, and limbs. But not in all primates: Baboons, macaques, gorillas, and chimpanzees do have eccrine pores across their bodies. Lemurs, marmosets, and tamarins do not. This ancestral sweaty split probably happened about 35 million years ago. But it’s a date with caveats: “Sweat pores don’t fossilize,” says Jason Kamilar at the University of Massachusetts Amherst. Thus, you can’t just look at fossilized specimens from human evolution and say, Presto, we see a sweat gland! So researchers looked at which primates have eccrine glands across their whole bodies (Old World monkeys, which scientists call catarrhines) and which do not (New World monkeys, called platyrrhines) in order to figure out the evolutionary pivot point after which perspiration got a biological promotion.

Even so, nonhuman primates aren’t enthusiastic sweaters. Although some of our primate cousins use sweat moderately as a cool- down technique, most also rely on other strategies that better serve their hairy bodies: Chimpanzees, one of our closest primate relatives, with whom we share nearly 99% of our genome, rely heavily on panting in hot weather, probably because evaporation as a cooling technique isn’t particularly effective on their furry skin.

One of the defining traits of humans is that we are a sweaty, naked ape. “Naked” doesn’t actually mean hairless—most of our body fur evolved into very thin hair across the majority of our skin, explains Yana Kamberov, a University of Pennsylvania geneticist who studies the evolution of sweat glands. “We look naked but we are not actually naked—we have the same density of hair follicles as apes have fur follicles.” But losing body fur in favor of nearly invisible, miniaturized hair helped our ancestors capitalize on body-wide eccrine glands.

Human skin isn’t just a lot less furry than that of our primate cousins; we also have a lot more eccrine glands. “Humans are slightly bigger than chimpanzees but we’ve got ten times the density of eccrine glands,” Kamberov says. It’s clear that at some point in our evolution, after our split with chimpanzees about 6 million years ago, our predecessors started losing fur and gaining sweat glands. The question of which came first is a long-standing chicken-or-egg conundrum: Hair doesn’t fossilize any more than sweat glands do. So Kamberov began searching for an answer to this question within our genome.

When we are developing as fetuses in utero, our first sweat glands begin forming on our hands and feet in the first trimester. By the halfway mark, 20 weeks, they’re developing over our entire body. But the skin’s stem cells are fickle—they’ve got a suite of possible destinies. They might become teeth, mammary glands, hair follicles, or eccrine sweat glands. Kamberov and her colleagues are finding evidence that the biological signals nudging these precursor cells toward an eccrine sweat gland destiny also inhibit the formation of hair.

Once again, evolution appears crafty and efficient. Eccrine glands are most useful for temperature control when there is not a lot of thick hair around. Maybe evolution miniaturized our hair while simultaneously dialing up sweat gland production. Kamberov’s work suggests the chicken-or-egg conundrum is moot: Instead, evo-lution may have orchestrated a perspiration two-for-one.

Her preliminary work also suggests that Neanderthals and Denisovans were also sweatier than chimpanzees. I like to imagine our predecessors romping around together, building up a sweat.

Reprinted from “The Joy of Sweat: The Strange Science of Perspiration.” Copyright © 2021 by Sarah Everts. With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.