How Humans Lost Their Fur

About 165 million years ago, a squirrel-like creature called Megaconus ran around what is now northeastern China. But Megaconus was not a squirrel, or even a mammal; It belonged to a group of mammalian relatives that lived before all modern mammals. So when scientists found impressions of fur (a defining characteristic of mammals) around the fossil remains of Megaconus, they knew that fur must have a deeper history than we thought. But despite its long evolutionary history in mammals and

their

relatives, a thick coat of hair is something

humans

don't have. In fact, we are the only primate without it. So there must be a very good reason we're wandering around...basically naked and unusually sweaty.

It turns out that this small change in our appearance has had enormous consequences for our ability to regulate body temperature and ultimately helped shape the evolution of our entire lineage. Despite what you may have heard or thought, skin and hair are actually the same thing. We just use a different word to describe the fur we have. But they both have the same type of fur or furry body covering. And fur is one of the defining characteristics of mammals. And while we're not sure exactly how it evolved, we do know that it's very old. Fur is thought to have evolved as a way for animals to stay warm by trapping air against the skin, preventing heat loss to the surrounding environment.

And today it also has many other functions. In warmer climates, the coat blocks the sun's heat and ultraviolet radiation from reaching the skin. It can also act as camouflage, making animals appear larger when they feel threatened, and certain coloration patterns can even help keep insects away. But fur can also make cooling difficult. One way many mammals lose heat is by panting or taking short, hard breaths. When animals pant, heat from inside the mouth evaporates into the surrounding air, which cools the blood in the veins of the tongue and cheeks. This cooled blood prevents

their

brains from overheating.

But in hot climates, panting can't always keep up with the heat, so many mammals have to rest in the shade during the hottest hours of the day. To avoid this, some mammals also sweat a little. As sweat evaporates from the surface of the skin, it takes some body heat with it, increasing the animal's ability to lose heat. But intense sweating comes at the cost of losing more water, which isn't really what you want when it's already hot. Additionally, if sweat soaks into the fur, the heat will no longer be able to escape. So for most mammals, fur (combined with panting, shading, and a little sweating) usually cools them down very well.

But why don't we have fur? Why did our lineage develop super sweaty, naked skin exposed to the elements, only to cover us up again anyway? Actually, the answer might lie in another aspect of human weirdness: the evolution of bipedalism. And often one big change can lead to another. When our first relative, Australopithecus, appeared on the scene in East Africa about 4 million years ago, we began to see major changes in the way hominids moved. Fossils of hip bones, femurs, and feet show that these hominids could walk on two legs, but finger and shoulder bones show that they also spent some time in trees.

It wasn't until our genus Homo emerged, about 2 million years ago, that we fully committed to bipedal walking. And about 1.8 million years ago, Homo erectus took another step. Their tall stature, long limbs, and bowl-shaped pelvis, which

humans

have today, gave Homo erectus a more ideal running body compared to the shorter, stockier hominids that had existed before. And some scientists think that this ability to run allowed Homo erectus to hunt using an unusual method called persistent hunting, or chasing prey until it collapses from exhaustion. In fact, by calculating the amount of water humans lose when engaging in persistent hunting, a group of anthropologists recently discovered that Homo erectus could have hunted this way for more than five hours straight without needing a water break. .

And today we continue that legacy: we are the only living primate that can engage in persistent hunting. I say "we" loosely because I definitely can't do that. But it is possible. Now, Homo erectus and later hominids, who may have been persistently hunting in open savannahs during the Pleistocene epoch, could have been at risk of overheating. And it is this connection between the way we move and how warm we are that has led many scientists to suggest that our locomotion was linked to the loss of thick fur. Hominids with less fur could sweat more efficiently, which would cool them down much faster without having to take breaks in the shade and lose valuable hunting time.

Until recently, however, these experts found themselves in a chicken-and-egg scenario: Did we lose our fur first or start running first? Did Australopithecus have no hair or did Homo erectus still have hair? This is where fossils are no longer of much help. So instead of trying to figure out when it would have been beneficial to not have hair, a group of scientists tried to figure out when hair would still have been necessary for survival. Remember that fur is a great insulator. Even mammals that live in warm climates have fur, which comes in handy when temperatures drop at night.

By looking at the environments Australopithecus lived in and how many calories they likely consumed and

lost

in a day, these scientists discovered that they could not have survived without hair at night. Without controlled fire, which doesn't appear in the fossil record until millions of years later, they simply wouldn't have been able to generate enough heat to make up for what they would have

lost

without fur. So this tells us that Australopithecus probably still had a considerable amount of fur until they disappeared from the fossil record about 2 million years ago. This means that at some point in our genus Homo, extensive hair loss occurred.

And DNA evidence from our own skin can help us determine when that happened. Human skin comes in a variety of shades that are believed to reflect genetic adaptations to the sun's ultraviolet radiation. Darker skin is better protected from this radiation than lighter skin. That's why many people with ancestry from places near the equator, where the sun hits the Earth at a higher angle, have darker skin than people with more distant ancestry. Now, this protection would not have been necessary if we had hair, because hair acts as a barrier against UV rays. We can even see this in other primates.

Underneath their fur, their skin is lightly pigmented. But skin that is regularly exposed to the sun becomes darker over time. This means that if a hominid species had dark skin, it must have already lost its fur. A study published in 2004 showed that a genetic variant associated with dark skin, called MC1R, already existed at least 1.2 million years ago, suggesting that at this point in our history, hominid skin was adapted to intense sun exposure. And who was already walking in Africa 1.2 million years ago? The good Homo erectus. Individuals with naturally thinner fur would have been able to cool down better, allowing them to run (and hunt) for longer without needing to rest as frequently.

And these more successful hunters would have passed on their genes more frequently. Over time, fur would have become less common, until eventually the species became naked. Therefore, bipedal running and hair loss are closely related. Both allowed us to become successful persistent hunters, leading to further fur loss. But when and why did we become so… sweaty? Like fur, sweating is an ancient characteristic of mammals. All mammals have two types of sweat glands: apocrine glands and eccrine glands. Apocrine glands produce a type of thick, oily sweat and cover most mammals from head to toe. They also produce pheromones, which are chemicals that send important information about an animal's physical and emotional state.

Apocrine glands aren't very effective at cooling most mammals, but since most mammals don't rely much on sweating, it works. The other type is the eccrine gland. Eccrine glands produce watery sweat and are usually only found on the bottom of the hands and feet, helping animals grip things through friction. But the monkeys and apes of Africa and Asia show a different pattern. Much of its body is covered in eccrine glands, with apocrine glands only in certain places, such as the armpits. Scientists are still not entirely sure why this change occurred, but it may have to do with the need for better cooling when their ancestors moved to warmer, drier habitats about 30 million years ago.

And humans are the sweatiest primates of all. In fact, a group of scientists sat down and counted how many eccrine sweat glands and hair follicles we have compared to other primates. They discovered that we have between 2 and 5 million eccrine glands in total, 10 times more than chimpanzees have! But in reality we are as hairy as chimpanzees. We have practically the same number of hair follicles as chimpanzees, which, it turns out, are actually not that hairy compared to other primates. The difference between our hair and that of a chimpanzee is the type of hair we have. Instead of thick fur, humans are covered in fine, almost microscopic hairs called vellus hairs.

Because these hairs are so small, sweat evaporates very close to the skin's surface, transferring body heat to the atmosphere very effectively. The combination of having lots of sweat glands and hair all over our bodies has led us to become very good at cooling ourselves off. We're actually capable of producing up to 3.7 liters of sweat per hour in really extreme conditions, but we average around 1 liter per hour, which is still pretty sweaty... and gross. So our ability to run directly contributed to fur loss and increased sweating, which in turn made us even more efficient runners and hunters.

As the climate changed, African primates faced new thermoregulatory challenges. Those with more eccrine glands were able to sweat more. And as running upright became an important way to get food, those with less hair were able to maximize the amount of heat they lost by sweating while chasing prey. This eventually led to persistent, naked, sweaty hunters. And more efficient hunting means more meat, and more protein means many things. Over time, it could have led to increased brain size, more advanced use of tools, cooperation, and even speech. But even though we lost most of our fur, it didn't just disappear.

In addition to tiny downy hairs, we still have thick hair on some parts of our body. Having hair on the top of our head protects our scalp from solar radiation and keeps our brain cool, while pubic and armpit hair may have remained a way to convey sexual maturity. So, as gross as it sounds (and it sounds really gross), it seems that our ability to sweat... a lot... ultimately shaped the evolution of our lineage. Let's hope we don't get a sweaty high-five from this month's eontologists: Sean Dennis, Jake Hart, Annie and Eric Higgins, John Davison Ng, and Patrick Seifert!

Become an Eonite by supporting us at patreon.com/eons because Eonites get perks like sending us a joke to read. The joke in this episode is from Jared Jordan. "Any humor found in the evolution of bipedality in early humans is technically comedy." I'll be here all week. Also, if you want more Eons content, be sure to follow Eons on social media! You can find us on Instagram, Twitter and Facebook. And you can join me on Instagram at fossil_librarian. And as always, thank you for joining me in the Konstantin Haase study. Be sure to subscribe at youtube.com/eons to discover more about the evolution of life on Earth.