Under the relentless sun in Africa, the birthplace of humanity, every living thing had to find a way to beat the heat. Lions rested in the shade, termites built giant ventilation mounds, and elephants evolved giant ears that could flap like fans. Around 2 million years ago, our ancestors perfected the weirdest technique of them all: pushing water from inside our bodies to outside, a gift for enduring sweltering temperatures. Other animals can sweat a bit, but not like us. Running around in the heat, a person can shed more than two gallons of water each day, draining one of life’s precious resources at a speedy pace. As the body tries to cool down, blood vessels widen, redirecting hot blood from the core of your body toward the surface. In tandem, sweat glands pump water, drawn from that blood, onto your skin. When those tiny beads evaporate, they carry heat off the body and into the air. “It is crucial to being human,” said Yana Kamberov, a geneticist studying the evolution of sweat at the University of Pennsylvania. “It’s something that differentiates us from every other animal on the planet” — right up there with our oversized brains. The average person has between 2 and 4 million sweat glandsin their skin, at 10 times the density of a chimpanzee’s, one of our closest living relatives. For humans, sweat proved even more useful than protective fur; our thick coat dwindled into peach fuzz to allow water to evaporate more efficiently. Our biological sprinkler systems are now being put to the test. This summer was not just the hottest three consecutive months on record, but the hottest on Earth in 125,000 years. Phoenix spent 31 days in a row with a high of 110 degrees F or above. Across the Northern Hemisphere, from continent to continent, heat records fell at an alarming pace, with Morocco and China setting all-time highs above 120 degrees F. The swampy Gulf Coast heat soared as high as 115 degrees F, rewriting records for Houston and New Orleans. Even South America, in the throes of winter, saw unbelievable heat: A town in the Chilean Andes topped 100 degrees F — another all-time high.It is getting to the point that life is dangerous without air-conditioning. If a widespread power outage hit Phoenix during a heat wave and lasted for days, it could kill thousands and send half the city to the emergency room, according to a recent study. And the soupy heat in the Gulf Coast comes with a challenge of its own: Super hot and humid air makes it hard for sweat to evaporate, because the environment is already thick with water molecules, which means more heat stays trapped inside the body, raising the risk of getting cooked from the inside out.  “Dying from a heat wave is like a horror movie with 27 endings that you can choose from,” said Camilo Mora, a climate scientist at the University of Hawaiʻi at Mānoa, who has cataloged27 different waysthat heat can lead to organ failure and death.  As blood gets shunted toward the skin, for instance, it strains the heart and deprives the brain and gut of oxygen, leading to heart attacks and other grisly outcomes from widespread inflammation and clotting. Prolonged sweating can also cause dehydration, sometimes inducing kidney failure. Heat has so many ways to kill you that it’s easily the deadliest of all weather disasters Americans face. In 2017, Mora and colleagues found that 30 percent of the world’s population was already exposed to potentially deadly heat for 20 days or more each year. Given how crucial perspiration is for survival, you’d think that researchers would have the science of sweat all figured out by now, but there are still open questions. Exactly how hot is too hot for the human body? How important is humidity? And why aren’t we more grateful for sweat? Its nasty reputation for making you stink belies the fact that it’s essentially a built-in life jacket to help you ride out record-breaking heat waves.        https://grist.org/culture/sweat-heat-body-climate-change/?utm_source=syndication&utm_medium=rss&utm_campaign=the-looking-forward-newsletter-archive-grist