How we got our sweat glands

The following is an excerpt from The joy of sweat: the strange science of sweat by Sarah Everts.

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The joy of sweat: the strange science of sweat

The eccrine sweat glands, millions of which produce our refreshing salty sweat, are actually found in all mammals. But in most other animals, the fluid from the eccrine glands is not used for cooling, but for ensuring adhesion. Most mammals have eccrine pores only in the soles of their feet or hands. In times of stress, salty fluid emerging from the eccrine glands provides additional friction for landing jumps and for climbing. It is normally only released when animals are under stress, such as when they have to escape a predator or grab prey. You can blame your vestigial self when your hands get sweaty in times of stress: Humans may no longer need to climb trees to deal with (most) potential threats, but our sweaty hands during times of stress. anxiety reveal that old habits die hard.

At some point in primate evolution, the eccrine glands began to extend beyond the soles of the feet and the palms of the hands to appear on the torso, face, and limbs of our ancestors. But not in all primates: baboons, macaques, gorillas and chimpanzees have eccrine pores throughout their bodies. Lemurs, marmosets, and tamarins don’t. This ancestral sweaty split likely occurred around 35 million years ago. But it’s a date with caveats: “Sweat pores don’t fossilize,” says Jason Kamilar of the University of Massachusetts at Amherst. So you can’t just look at fossilized specimens of human evolution and say, Presto, we see a sweat gland! So the researchers looked at which primates have eccrine glands throughout their body (Old World monkeys, which scientists call catarrhines) and which don’t (New World monkeys, called platyrrhines) to determine the pivotal point of evolution after which perspiration has obtained a biological effect. promotion.

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Sweat is our biological superpower

Even so, non-human primates aren’t enthusiastic sweaters. Although some of our primate cousins ​​use sweating moderately as a recovery technique, most also rely on other strategies that serve their hairy bodies better: chimpanzees, one of our closest primate relatives, with who we share nearly 99% of our genome, rely heavily on panting in hot weather, possibly because evaporation as a cooling technique isn’t particularly effective on their hairy skin.

One of the hallmarks of humans is that we are a sweaty naked ape. “Naked” doesn’t really mean hairless – most of our body fur has turned into very fine hairs on most of our skin, says Yana Kamberov, a geneticist at the University of Pennsylvania who studies the evolution of sweat glands. . “We look naked but we’re not really naked – we have the same density of hair follicles that monkeys have fur follicles.” But losing the body’s fur in favor of almost invisible, miniaturized hair helped our ancestors to capitalize on the eccrine glands throughout the body.

Human skin is not only much less hairy than that of our primate cousins; we also have many more eccrine glands. “Humans are slightly larger than chimpanzees, but we have ten times the density of eccrine glands,” Kamberov explains. It is clear that at some point in our evolution, after our separation from chimpanzees about 6 million years ago, our predecessors began to lose fur and acquire sweat glands. The question that came first is a long-standing chicken or egg conundrum: Hair doesn’t fossilize any more than sweat glands. Kamberov therefore began to seek an answer to this question in our genome.

When we develop as a fetus in utero, our first sweat glands begin to form on our hands and feet during the first trimester. At halfway, 20 weeks, they develop all over our body. But the stem cells of the skin are inconstant, they have a series of possible fates. They can become teeth, mammary glands, hair follicles, or eccrine sweat glands. Kamberov and his colleagues find evidence that biological signals pushing these precursor cells to eccrine sweat gland fate also inhibit hair formation.

Once again, evolution appears cunning and efficient. Eccrine glands are most useful for temperature control when there is not a lot of thick hair around. Perhaps evolution has miniaturized our hair while simultaneously increasing the production of sweat glands. Kamberov’s work suggests that the chicken or egg conundrum is moot: instead, evolution may have orchestrated a two-for-one sweat.

His preliminary work also suggests that Neanderthals and Denisovans were also sweatier than chimpanzees. I like to imagine our predecessors letting off steam together, sweating.

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Reprinted from “The Joy of Sweat: The Curious Science of Sweat”. Copyright © 2021 by Sarah Everts. Courtesy of the publisher, WW Norton & Company, Inc. All rights reserved.