The last time the Earth was hotter than it is today was at least 125,000 years ago, long before anything that resembled human civilization appeared. Since 1970, the Earth’s temperature has spiked faster than in any comparable forty-year period in recorded history. The eight years between 2015 and 2022 were the hottest on record. In 2022, 850 million people lived in regions that experienced all-time high temperatures. Globally, killer heat waves are becoming longer, hotter, and more frequent. One study found that a heat wave like the one that cooked the Pacific Northwest in 2021 is 150 times more likely today than it was before we began the atmosphere with CO2 at the beginning of the industrial age.
Just look at the events of this year: wildfire smoke from Canada turned the skies on the east coast an apocalyptic orange; sea ice in Antarctica hit a record low; all-time temperature records were shattered in Puerto Rico, Siberia, Southeast Asia, Mexico, and Texas (I live in Austin, where, as I write this in late June, it’s 106 degrees F). In the North Atlantic ocean, sea surface temperatures in late June are the highest ever recorded.
The truth is, extreme heat is remaking our planet into one in which large swaths may become inhospitable to human life. One recent study projected that over the next fifty years, one to three billion people will be left outside the climate conditions that gave rise to civilization over the last six thousand years. Even if we transition fairly quickly to clean energy, half of the world’s human population will be exposed to life-threatening combinations of heat and humidity by 2100. Temperatures in parts of the world could rise so high that just stepping outside for a few hours, another study warned, “will result in death even for the fittest of humans.”
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Life on Earth is like a finely calibrated machine, one that has been built by evolution to work very well within its design parameters. Heat breaks that machine in a fundamental way, disrupting how cells function, how proteins unfold, how molecules move. Yes, some organisms can thrive in higher temperatures than others. Roadrunners do better than blue jays. Silver Saharan ants can run across superhot desert sands that would kill other insects instantly. Microbes live in 170-degree hot springs in Yellowstone National Park. A thirty-year-old triathlete can handle a 110-degree day better than a seventy-year-old man with heart disease. And yes, we humans are remarkable creatures with a tremendous capacity to adapt and adjust to a rapidly changing world.
But to understand the dangers of extreme heat today, it helps to understand how we have lived with heat in the past. Among other things, we evolved clever ways to manage the heating and cooling of our bodies that gave our ancestors an evolutionary edge over competitors. To tell you about it, though, I have to go way back, because you can’t separate heat from the beginning of things.
Fourteen billion years ago, the universe was compressed into a stupendously hot, incredibly dense nugget, which then rapidly expanded. This nugget cooled as it swelled; its particles gradually slowed their frenzied motion and aggregated into clumps, which over time formed stars, planets — and us. How exactly life emerged out of the hot mess of the universe is only dimly understood. The most widely accepted theory is that life began around the volcanoes that rose above the ocean shortly after the earth formed, probably within the first hundred million years. The volcanoes were surrounded by hot geyser‑fed ponds and bubbling hot springs, which were loaded with organic compounds from the asteroids and meteors that bombarded the planet. Volcanoes acted as chemical reactors, creating a hot volcanic soup. Somehow, RNA molecules grew, eventually growing longer and more complex and folding into true proteins and double‑stranded DNA. They formed microbes that floated in thick mats on the volcanic ponds. When the ponds dried out, winds picked up their spores and spread them for miles. Rains eventually washed microbes into the ocean. “Once they reached the sea,” science author Carl Zimmer writes, “the whole planet came alive.”
Evolution’s next trick was developing a way for animals to deal with temperature fluctuations. In the long arc of evolution, two strategies have emerged: one is to let your body’s temperature change with the temperature around it, which is what creatures did for the first three and a half billion years or so. If necessary, these animals warm themselves by basking in sunlight or sitting on warm rocks. This heat management strategy survives today in fish, frogs, lizards, alligators, and all the reptiles and amphibians. Scientists call them ectotherms; you and I call them cold‑blooded.
But around 260 million years ago, a new heat management strategy emerged. Some animals found a way to control their own internal temperature that was not dependent on the temperature of their environment. In effect, it turns their bodies into little heat engines, allowing them to operate independently of the world outside — as long as they can maintain a steady temperature inside. This heat management strategy remains alive and well in animals that scientists call endotherms but that everyone else calls warm‑blooded: dogs, cats, whales, tigers, and virtually every other mammal on the planet, including us. Birds, which are flying dinosaurs, are also warm‑blooded. (“Birds are not like flying dinosaurs,” a scientist once corrected me. “They are flying dinosaurs.”)
The birth of warm‑bloodedness was an evolutionary leap, and one that scientists still don’t fully understand. For one thing, the traits of warm‑bloodedness do not transfer well to fossils, so you can’t just look at the bones of a long‑ago creature and determine whether it was warm‑ or cold‑blooded. For another, the transition from cold‑bloodedness to warm‑ bloodedness didn’t happen with a single quick jump. Many species — especially dinosaurs — had attributes of both.
At first glance, cold‑blooded creatures seem to have it easy. Because they cannot regulate their body temperature internally, they spend thirty times less energy than warm‑ blooded creatures of the same size. So, while mammals and birds are constantly investing their calories in maintaining a high, stable body temperature, reptiles and amphibians can just search for a warm spot in their surrounding environment if they want to get cozy. But if cold‑bloodedness is so great, why did mammals and birds develop a different strategy?
There are a lot of theories for why warm‑blooded animals evolved high, stable body temperatures. To name a few: a stable body temperature aids physiological processes, such as digestion and the absorption of nutrients; it helps animals maintain activity over longer periods of time; it enables par‑ ents to take care of precocial offspring. Warm‑bloodedness also allowed more precise and powerful functioning of certain cells in the nervous system, as well as in the heart and muscles.
Resistance to disease may have been another advantage. Insects bask in the sunlight to superheat their bodies and cook invading organisms; humans do the same by running a fever. But cold‑blooded creatures depend on external sources of heat to kill invaders. If it’s not hot out, a grasshopper can’t fry the dangerous microbes in its body. And if that grasshopper goes looking for a spot of sunshine, it might venture into new places and get picked off by a predator. Warm‑blooded animals don’t have that risk. They can rev up the heat engine wherever they are.
Warm‑blooded animals also move faster. John Grady, a biologist at the University of New Mexico, thinks the evolution to warm‑bloodedness was accelerated by the competitive advantage that comes with being a speedy predator. Higher body temperature equals higher metabolism, which equals quicker reactions and more active predation. “Imagine an iguana the size of a cow,” Grady told me. “These things existed. But they won’t exist in today’s world, because they are too slow. The closest thing we have are giant tortoises, and they have a strategy of just being armored. They don’t have to be fast. When you are big, being fast is important. I think getting killed is a real problem if you are big and cold‑blooded.”
Whatever the particular advantages of warm‑bloodedness may have been, it served mammals well. For the last seventy million years or so, they have spread across the globe, each creature a biological dynamo carrying its own fire inside. Their success gave rise, eventually, to a two‑legged primate that developed a big brain and an even more sophisticated heat management system to go with it. To get a glimpse of this remarkable creature, just look in the mirror.
In 1974, a pile of bones was found in the Awash River valley in Ethiopia by Donald Johanson, who, at the time, was a professor at Case Western Reserve University in Ohio. The bones belonged to a female human ancestor who lived about 3.2 million years ago. Judging from her intact wisdom teeth and the shape of her hip bone, Johanson determined that she was a teenager when she died. He named her Lucy, after the Beatles’ song “Lucy in the Sky with Diamonds,” which Johanson and his team had been listening to in camp when she was found.
It was a remarkable discovery, rewriting the story of human evolution. Even at the time, Lucy was not the oldest human ancestor ever found, but she filled an important gap in the evolutionary tree from early hominins (that is, all our human ancestors since we diverged from chimpanzees about seven million years ago) to modern humans. She was also remarkably well preserved for a girl who had been buried for more than three million years. She had a spine, pelvis, and leg bones very similar to those of modern humans. She did not yet have the brain size of a modern human, but she was positively, indisputably bipedal.
It took a while for our ancestors to learn to stand up. From the structure and shape of the fossils they’ve left behind, paleontologists know that early hominins hung out mostly in trees. On the ground, they moved on all fours, not unlike the way chimps do today.
But Lucy was different. The shape of her lower femur, as well as the development of her knee, indicate that she walked upright at least part of the time. But she wasn’t like us: she had wide hips and short legs. She was an evolutionary toddler just learning to venture out of the cover of the trees and onto the savanna.
The question is, what made Lucy stand up and start walking? It’s a much‑disputed subject among paleoanthropologists.
Some argue that it allowed our ancestors to carry tools better. Others believe that it helped them reach fruit high in trees. Still others suggest that bipedalism was the basis for monogamy and family, in that it allowed male hominins to go out and get food, which the female chimps would reward with companionship and sex.
Or standing up may have been a way of keeping cool. It allowed Lucy to catch breezes and help body heat to dissipate more easily. It also got her up off the ground, which is always significantly warmer than the air a few feet above it.
Whatever her motivation may have been, Lucy walked.
And it changed everything.
To understand the power of heat, you have to think of it not just as a change in temperature, but as an evolutionary hurdle. Heat management is a survival tool for all life on Earth, and the strategies to deal with it are as diverse and colorful as the animal kingdom itself.
Elephants are particularly fascinating. They spend a lot of time in the sun. To cool off, they seek shade and water. (In Botswana, I once watched a young elephant frolic in a muddy watering hole like a six year‑old kid at summer camp). Their thin hair and flapping ears help with heat dissipation. More importantly, as temperatures rise, their hides become more permeable. Their skin effectively opens up, allowing them to perspire, even though they don’t actually have sweat glands. Koalas hug trees with bark that is cooler than the air temperature. Kangaroos spit on their arms to wet them and cool off. Some squirrels use their bushy tails as parasols. Hippos roll in mud (water evaporates more slowly from mud, keeping them cool longer). Lions climb trees to get off the hot ground. Rabbits send blood to their big ears, using them as radiators. Vultures and storks defecate on their legs. Herons, nighthawks, pelicans, doves, and owls cool themselves with gular fluttering, a frequent vibration of their throat membranes, which increases airflow and thus increases evaporation. Giraffes’ beautifully patterned skin functions like a network of thermal windows. They direct warm blood to the vessels at the edges of the spots, forcing heat out of the animals’ bodies.
Other animals build structures to cool themselves, in some ways not so different from the way humans construct air‑conditioned buildings. Termites build an elaborate sys‑tem of air pockets within their mounds. Bees harvest water when they’re on their travels, then return to the hive and pass it by mouth to hive bees, which spread the droplets on the honeycomb. Other bees fan the water with their wings to cool the hive.
There aren’t many people who have thought more about heat as an evolutionary force than Jill Pruetz. For the past twenty years, she has spent a good part of every year in Senegal, near the village of Fongoli, where she has been studying chimpanzees that live in a hot environment. Pruetz has a way of talking about being among the chimps that suggests she knows them better than many people know their own children.
Pruetz and I met on a sunny spring day at a restaurant in Bastrop, Texas, near where she lives on a five‑acre farm. She grew up in south Texas and became fascinated with chimpanzees shortly after college, when she went to work at a chimpanzee center that bred chimps for biomedical research. She is now a professor of anthropology at Texas State, and runs the Fongoli Savanna Chimpanzee Project, where about thirty‑two chimps live in a 100‑square‑kilometer area, out‑ side the national park.
Pruetz and I sat at a wooden picnic table above the Colorado River and ate pizza while we talked. “I study chimps for a lot of reasons,” she told me. “But mostly because they are our closest living relative, and we can learn a lot about early human development by looking at how chimps behave and react to different kinds of stress in their lives.”
For the Fongoli chimps, heat is extremely stressful. During the hot dry season in Senegal, which peaks in March and April, the temperature can hit 120 degrees. “The heat is like a slap in the face,” Pruetz said. Trees are leafless. Water is scarce. Fires burn across their territory. These chimps live in the hottest, most arid place that chimps are known to exist. It is a brutal, apocalyptic landscape that is totally unlike the lush forests and jungles that every other chimp on the planet inhabits.
The chimps have been living on this piece of turf for a very long time. “Millennia,” Pruetz told me. Over time, the chimps gradually evolved a catalogue of strange behaviors — ones rarely if ever seen in others. Forest chimpanzees get enough water from the fruit in their diet, so they need less drinking water and can wander in search of food. The Fongoli chimps, by contrast, require daily drinking water and anchor themselves to reliable water sources in the arid landscape.
And while forest chimpanzees are active throughout the day, Pruetz found that the savanna chimpanzees rest for five to seven hours. Pruetz could often find them lurking in small caves in the dry season, and when the rainy season arrived, the chimpanzees would slip into newly formed ponds and bob there for hours. Forest chimpanzees typically spend all night in nests they build in trees. But at Fongoli, the research team noticed that the chimpanzees often made a late‑night racket.
“During the hot season, the chimps totally change their behavior,” Pruetz told me. They stare at the sky, waiting for the rain they know is coming. At Fongoli, there are few trees, and the ones that are there don’t have many leaves for shade. On a hot day, Pruetz watched an adolescent chimp hiding in the shadow of a single tree trunk. As the day passed, the chimp moved with the shadow, trying to escape the heat.
Pruetz has also noticed something else, something that was perhaps key to the whole human story: in the heat, the Fongoli chimps spend more time standing up and walking around than chimps that live in cooler places.
Lucy lived in a rapidly changing world. It was nowhere near as rapidly changing as ours is today, but in evolutionary terms, it was on the move. The climate of East Africa was growing hotter and drier. Rain forests gave way to woodlands, and as the landscape opened up, the savanna emerged. “Over the past three to four million years, the scenery of East Africa shifted from the set of Tarzan to that of The Lion King,” writes Lewis Dartnell in Origins: How Earth’s History Shaped Human History. Ethiopia’s Rift Valley became a very complex environment, with woods and highlands, ridges, steep escarpments, hills, plateaus and plains, valleys, and deep freshwater lakes on the floor of the rift, which was gradually widening. Meanwhile, volcanoes like Mount Kilimanjaro were spewing pumice and ash across the whole region. New species like zebra were emerging from under the trees and appearing in the grasslands.
In this dynamic new world, Lucy had to be nimble. Water supplies were drying up and filling again with each passing rainstorm. Leopards and lions lurked in the ravines — she was both predator and prey (we think of the world that she lived in as so different from ours, but in fact, the creatures that made up East Africa at that time were similar to what is there today — lions and hyenas and elephants were all more or less the same). If the behavior of chimps today is any indi‑ cation, these early hominins weren’t exactly nimble — afraid of open ground, wary, fleeing back into the safety of trees whenever they could. The changing terrain, and the need to navigate through it, meant that the most vulnerable were killed by predators. But the most adaptive ones survived and thrived and learned new skills, including hunting with tools, which helped them shift away from a diet of fruit and termites and small forest creatures to a more meat‑centric diet, including gazelle and zebra, which they might have hunted in groups.
Kevin Hunt, a professor of anthropology at Indiana University who studies human evolution, believes bipedalism likely evolved gradually, over a million years or so. Lucy was an example of the first phase — she may have stood up both to escape the heat and to help her reach for fruit. The second phase, marked by the arrival of Homo erectus, had elongated limbs that allowed them to walk and run faster, a more slender body that better dissipated heat, and a more carnivorous diet.
But to take the next step in human evolution, to really allow our ancestors to range widely in the newly warmed world, they still needed one more key evolutionary innovation. They needed to learn how to sweat.
In our human ancestors, the evolution of the sweat gland is even more complex than the evolution of bipedalism. Bipedalism can be deduced from fossil bones. Sweat glands can’t. What is known about them can only be inferred by hints of behavior patterns found in other ways, and by the evidence we see in our own bodies and in the bodies of other animals.
What is clear is that as Lucy and her generation made their way out of the trees and into the savanna, they had to contend with heat in a way that they never did when living in the trees. In both cases, our ancestors came up with important innovations that still have big implications for how we live today.
First, there was sunlight to deal with. As they wandered out from under the trees, our ancestors were exposed to more and more ultraviolet radiation, which damages the cellular structure of skin and can harm DNA. So Lucy and her ancestors evolved the ability to produce melanin, the dark‑brown pigment that acts as a natural sunscreen. For several million years, our ancestors were all dark‑skinned. It was only after they migrated out of Africa and settled in more northern climates, and in high latitudes, that dark skin became an evolutionary disadvantage because it limited the sunlight getting through to trigger the production of vitamin D. So in regions where the sunlight was less intense, lighter skin had an advantage.
Dealing with heat was more complex. In warm‑blooded animals, more sunlight means more heat. More activity means more heat too. How far you can chase a wounded antelope in the heat depends on how well you manage heat. On the African plain, if you overheat, you go hungry. In addi‑ tion, our ancestors’ brains were evolving, and getting bigger. But big brains require a lot of cooling, and so developing a robust cooling system was important to advancing other skills, such as toolmaking.
The solution that evolution came up with was to build what amounts to an internal sprinkler system that douses our skin with water when we get too hot. As the water evaporates, it carries the heat off with it, cooling off our skin and the blood circulating just below it. When that cooler blood circulates, it brings down the temperature in our bodies.
If you’ve ever ridden a horse on a hot day, you know that other animals sweat. Horses, as well as many other mammals, have a particular kind of sweat gland that is part of their hair follicles called an apocrine gland. It sends out a thick, milky white liquid. You see it most clearly on racehorses, which sometimes finish a race looking like their necks are covered in shaving cream (thus the origin of the phrase “get in a lather”). Many furred mammals have apocrine glands, including camels and donkeys, as well as chimpanzees. These glands help with heat management, but they can’t really dis‑ sipate a lot of heat fast.
Humans have some apocrine glands in our armpits and pubic areas, which are evolutionary leftovers from an earlier time. They respond to nerves as well as heat, and are why your armpits sweat in an interview, and also why your sweat has a particular odor. Some anthropologists think that smell is an ancient sexual attractant; that it’s one of the ways we got to know one another.
But while our ancestors were wandering around in the heat on the African savanna, chasing down antelopes, they also perfected a much better heat management tool, which is the eccrine sweat gland. Instead of creating a lather, it is basically a mechanism to squirt water on your body, which will then evaporate and cool you off. It’s simple but brilliant. Hominins didn’t invent the eccrine gland. Old World monkeys like macaques have equal parts eccrine and apocrine glands. Our closer relatives, chimpanzees and gorillas, bear roughly two eccrine for every one apocrine gland. But beyond the apocrine leftovers in our armpits and pubic areas, human sweat glands are all eccrine.
Today, you and I have about two million of these sweat glands on our body. The glands themselves are like little coiled tubes buried in your skin. They are tiny, the size of a cell — you need a microscope to see them. They are not evenly distributed on your body: you have the most sweat glands on your hands, feet, and face, and the least on your butt. Sex differences are small. Women often have more sweat glands in any given area than men, but men often have a higher maximum sweating rate. The liquid the glands secrete is 99.5 percent water — its only function is to wet your skin. In hot weather, most people can easily sweat one quart per hour or 12 quarts a day, which is about ten times more than a chimp sweats.
To make our sweat glands even more effective, however, Lucy’s offspring made another evolutionary adjustment: they lost their body hair. For the evaporative sweat to really work, hair (or fur, which is just another name for hair on nonhuman animals) gets in the way, matting down when wet and interfering with the efficient transfer of heat away from your body. The only place we still have significant hair is on our heads, and that’s because our brains are so sensitive to heat, and in this situation, hair works as a sunshade to help keep our brains cool. (It also adds cushioning in a fall.)
The loss of hair on our bodies and the development of eccrine sweat glands were important evolutionary events, perhaps as important as the use of tools or fire. Other animals on the African savanna had developed heat stress strategies — the simplest of which is panting, as dogs do. But for a predator, panting is not a great strategy. A lion can move very fast for short distances, but it can’t pant while it runs. In the heat, it has to stop, rest, pant, and recover its thermal equilibrium. Humans figured out a way to keep cool in motion. We don’t have to stop and pant. We sweat as we go. In the story of human evolution, this was a very big deal. By managing heat, humans were able to go farther from water holes, begin long‑distance travel, and expand their hunting range.
Humans became excellent hot‑weather hunters. They could venture out in the heat of the day when other animals couldn’t, giving them a predatory advantage. By the time Homo erectus appeared about two million years ago, our ancestors were on their way to becoming endurance athletes, with long legs, nimble feet, and strong leg and hip muscles. With their superior heat management systems, they could literally run down an animal until it has heatstroke. This practice continues today. In the Kalahari Desert in southern Africa, modern hunter‑gatherers are able to kill a kudu, a kind of antelope that is far faster than humans over short distances, by chasing it for hours in the middle of the hot day, until it literally collapses of heat exhaustion.
But the heat management strategies of humans, like all living things, has been optimized for the Goldilocks Zone we have been living in for the last 10,000 years or so. Now, as we move out of that world, the job of managing heat gets much more complicated – and much more dangerous.
If we can send photos through the air and drive a rover around on Mars, we can design new ways to live in hot places. You can see it happening right now in Paris and Los Angeles and many other cities around the world, where shade trees are being planted and streets painted white to deflect sunlight. Plant geneticists are developing new strains of corn and wheat and soybeans that can better tolerate high temperatures. Air conditioning is becoming cheaper and more widely used. Communication from public health officials about how to protect yourself during a heat wave is improving. Clothing companies are developing new high-tech fabrics to reflect away sunlight and dissipate heat more quickly.
But even for the wealthy and privileged, adaptation to extreme heat has its limits. And the notion that eight billion people are going to thrive on a hotter planet by simply cranking up the air-conditioning or seeking refuge under a pine tree is a profound misunderstanding of the future we are creating for ourselves. In western Pakistan, where only the richest of the rich have air conditioning, it’s already too hot for humans several weeks a year. Planting a few thousand trees is not going to save them. In India, I talked with families who live in concrete slums that are so hot they burn their hands opening doors. Holy cities like Mecca and Jerusalem, where millions gather on religious pilgrimages, are caldrons of sweat.
In a world of heat-driven chaos, heat exposes deep fissures of inequity and injustice. Poverty equals vulnerability. If you have money, you can turn up the air conditioning, stock up on food and bottled water, and install a backup generator in case there’s a blackout. If things get bad enough, you can sell your house and move to a cooler place. If you’re poor, on the other hand, you swelter in an uninsulated apartment or trailer with no air conditioning or an old, inefficient machine that you can’t afford to run. You can’t move somewhere cooler because you’re afraid of losing your job and you don’t have the savings to start over. “We’re all in the storm, but we’re not in the same boat,” Heather McTeer Toney, the former mayor of Greenville, Mississippi, said during testimony before the US Congress. “Some of us are sitting on aircraft carriers while others are just bobbing along on a floatie.”
Adapted from Jeff Goodell’s new book, The Heat Will Kill You First: Life and Death on a Scorched Planet, published by Little, Brown on July 11
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