Researchers believe ancient humans moved out of the trees as the climate changed under the hot scorching savanna. One study published in the current journal anthropology around 1.2 million years ago, to cope with the heat, lost human fur and started sweating as a uniquely efficient way to cool off.
One study published in the Royal Society Proceedings suggests that we lost most of our hair through natural selection and sexual selection. The researchers believe that humans like naked partners not for obvious reasons but because it helps them see whether they have parasites.
Why does human fur/hair evolve?
Why don’t humans have fur? Why did our lineage develop sweaty, bare skin exposed to the elements, only to cover ourselves again anyway? The answer might lie in another aspect of human oddness: the evolution of bipedalism. Often, one big change can lead to another.
For preventing parasites
Parasites like fleas or ticks can cause deadly diseases like typhoid. So bare skin means a healthier mate. But a different study published in Biology Letters suggests we kept some of the body hair to spot some bugs. In this study, researchers shaved a bin pair off volunteers’ arms and put a bedbug on their skin.
Researchers noticed that the volunteers quickly spotted the bug on their hairier parts. The men in the study were better at spotting the bugs than the women. So maybe women selected mates with more hair because they could save parasites on them. These are two competing theories.
There’s another possibility here. Maybe men are hairier because women like it. Some researchers think that men are hairier since hormones like testosterone control hair. Testosterone also increases the size of hair follicles on the face. A thick beard in a mane of chest hair could signify how much testosterone a guy has. Researchers think they work well at keeping sweat out of their eyes which would be helpful to as evolved to thrive in a hot climate. They’re great for nonverbal communication.
For creating attraction
We also can’t talk about body hair without talking about hair in our armpits and pubic areas. Researchers think hair in these areas does double duty. First, moisture wicks away, which helps deal with the Savannah Sun. It also might trap sense which is a good thing. Maybe these hairs evolved to trap the human musk to appeal to potential mates. While human pheromones haven’t been found, they have been theorized.
Some studies show that women prefer the armpit odor of men with compatible immune systems. So armpit hair could trap more of this alluring scent, or some researchers think because pubic hair growth coincides with puberty. Maybe it’s an outward sign of sexual maturity. Meeting and speaking of mating pubic hair might serve as a cushion during sexy times. That protects the genitals from abrasions or micro-tears, whereas protection from things like dirt.
How humans lost their fur?
About 165 million years ago, a squirrel-like creature called Megaconus was scurrying around in what’s now northeastern China. But Megaconus wasn’t a squirrel or even a mammal. It belonged to a group of mammal relatives who lived before all modern mammals. When scientists found impressions of fur, a defining feature of mammals surrounding the fossil remains of Megaconus, they knew that fur must have a deeper history.
But despite its long evolutionary history in mammals and their relatives, a coat of thick fur is one thing humans don’t have. We’re the only primate without it. So there must be an excellent reason humans roam around basically naked and unusually sweaty. It turns out that this small change in appearance has had huge consequences for regulating body temperature. Ultimately, it helped shape the evolution of the entire lineage.
Fur and hair are the same things. We use different words to describe the fur that we have. But they’re both the same kind of pelage or hairy body covering. Fur is one of the defining characteristics of mammals. While scientists are unsure how it evolved, it’s super old. Fur is thought to have evolved as a way for animals to keep warm by trapping air against the skin, preventing heat loss to the surrounding environment. Today, it also has a lot of other functions.
In warmer climates, fur blocks the sun’s heat and UV radiation from reaching the skin. It can also act as camouflage and make animals seem larger when threatened. Certain coloring patterns can even help keep bugs away. But fur can also make it more difficult to cool down.
One way many mammals lose heat is through panting, or taking short, forceful breaths. When animals pant, heat from the inside of their mouths evaporates into the surrounding air. It cools down the blood in the veins in their tongue and cheeks. This cooled blood prevents their brains from overheating.
But in hot climates, panting can’t always keep up with how hot it gets. So many mammals have to rest in the shade during the hottest part of the day. To avoid this, some mammals also sweat a little. As sweat evaporates from the skin’s surface, it takes somebody’s heat with it, increasing the animal’s ability to lose heat. But, heavy sweating comes at the cost of losing more water. If sweat soaks the fur, heat can’t escape anymore. So, for most mammals, fur combined with panting, shade, and a bit of sweating usually cools them down fine.
Human fur evolution by earth timeline
Our early relative, Australopithecus, came onto the scene in East Africa some 4 million years ago. We started to see major changes in how hominins moved around. Fossils of hip bones, femurs, and foot bones show that these hominins could walk on two legs. But the fingers and shoulder bones show that they also spent some time in the trees.
It wasn’t until genus Homo emerged, around 2 million years ago, that we became fully committed to walking bipedally. About 1.8 million years ago, Homo erectus took it a step further. Its tall stature, long limbs, and bowl-shaped pelvis gave Homo erectus an ideal running body compared to those shorter, stockier hominins that had come before. Some scientists think that this ability to run allowed Homo erectus to hunt using a unique persistence hunting method – or chasing prey until it collapses from exhaustion.
In fact, by calculating the number of water humans lose when they engage in persistent hunting. Many anthropologists recently found that Homo erectus could’ve hunted this way for over five hours straight without needing a water break.
Homo erectus and later hominins may have been persistent hunting on the open savannahs during the Pleistocene epoch. They could’ve been at risk of overheating. It’s this connection between how we move and how hot we get. That has led many scientists to suggest that our locomotion was connected to our loss of thick fur.
Hominins with less fur could sweat more efficiently. It would cool them down much faster without having to take breaks in the shade and lose valuable hunting time. Until recently, these experts found themselves in a chicken and egg scenario: did we lose the fur first, or did we start running first? Was Australopithecus hairless, or was Homo erectus still hairy?
So, rather than trying to figure out when being furless would have been beneficial, a group of scientists tried to figure out when fur would still be necessary for survival. Remember that fur is an excellent insulator. Even mammals in hot climates have fur, which is handy when temperatures drop at night. By looking at Australopithecus’ environments and how many calories they probably consumed and lost in a day. These scientists found that they couldn’t have survived being hairless at night.
It doesn’t appear in the fossil record until millions of years later. They wouldn’t have been able to generate enough heat to keep up with what they would’ve lost without fur. So, this tells us that Australopithecus probably still had a considerable amount of fur until they disappeared from the fossil record around 2 million years ago. This means that extensive fur loss occurred sometime within our genus, Homo. DNA evidence from our skin can help us pinpoint when that happened.
Human skin comes in various shades, which reflect genetic adaptations to UV radiation from the sun. 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 strikes the Earth at a higher angle, have darker skin than people from farther away. This protection wouldn’t have been necessary if we had fur because fur acts as a barrier to UV rays.
We can even see this in other primates. Under their fur, their skin is lightly pigmented. But skin that’s regularly exposed to the sun becomes darker over time. It means that if a hominin species did have dark skin, it must have already lost its fur. One study published in 2004 showed that a gene variant associated with dark skin, called MC1R, already existed at least 1.2 million years ago. It suggests that hominins’ skin was adapted to intense sun exposure in our history.
Who was already walking around Africa 1.2 million years ago? The individuals with naturally thinner fur would have been better able to cool down, allowing them to run and hunt for longer without rest as frequently. These more successful hunters would have passed on their genes more often.
Over time, fur would have become less common until, eventually, the species was naked. So, bipedal running and fur loss are closely connected. Both allowed us to become successful persistence hunters, which drove further fur loss.
When & Why did humans become so sweaty?
Like fur, sweating is an ancient feature of mammals. All mammals have two types of sweat glands: apocrine and eccrine. Apocrine glands produce a thick, oily sweat and cover most mammals from head to toe. They also have pheromones, chemicals that signal essential information about an animal’s emotional and physical state. Apocrine glands aren’t very effective in cooling most mammals down. Since most mammals don’t rely on sweating much anyway, it works out.
The other type is the eccrine gland. Eccrine glands produce watery sweat and are usually only found on the undersides of hands and feet. It helps animals grip things through friction. But monkeys and apes from Africa and Asia show a different pattern. Much of their bodies are covered in eccrine glands, with apocrine glands only in certain places, like the armpits.
Scientists still aren’t sure why this change occurred. But it may be due to a need to cool off better as their ancestors moved into hotter and drier habitats some 30 million years ago. Humans are the sweatiest primate of all. A group of scientists sat down and counted how many eccrine sweat glands and hair follicles we have compared to other primates.
They found that we have between 2 and 5 million eccrine glands, 10 times more than chimpanzees! But we’re as hairy as chimpanzees. We pretty much have the same number of hair follicles as chimpanzees, which, it turns out, aren’t that hairy compared to other primates. The difference between a human’s hair and a chimp’s is hair type.
Instead of thick fur, humans are covered in fine, almost microscopic hairs called vellus hairs. These hairs are so tiny that sweat evaporates very close to the skin’s surface, effectively transferring body heat to the atmosphere. The combination of having many sweat glands and vellus hairs all over our bodies has led us to become very good at cooling down.
- Humans can produce up to 3.7 liters of sweat per hour under extreme conditions, but the average is around 1 liter per hour.
So, our ability to run directly contributed to our fur loss and increased sweating, making us even more efficient runners and hunters. As the climate shifted, African primates faced new thermoregulatory challenges. Those with more eccrine glands were able to sweat more. As upright running became an important way of getting food.
Those with less fur could maximize the amount of heat they lost from sweating while chasing prey around. This ultimately led to naked, sweaty, persistent hunters. More efficient hunting means more meat, and more protein means a lot of things.
Over time, it could have increased brain size, more advanced tool use, cooperation, and even speech. But although we lost most of our fur, it didn’t disappear. Along with tiny vellus hairs, we still have thick hair on parts of our bodies.
Hair on our heads protects scalps from solar radiation and keeps brains cool. At the same time, pubic and armpit hair may have remained a way to broadcast sexual maturity. So, as gross as it sounds, it looks like our ability to sweat a lot ultimately shaped lineage’s evolution.
“The evolution of man.” BBC Science & Nature. Retrieved May 6, 2015.
“Becoming Human.” Arizona State University’s Institute of Human Origins. Retrieved May 6, 2015.
“Bones, Stones, and Genes: The Origin of Modern Humans” (Video lecture series).
“Evolution Figures: Chapter 25”. Cold Spring Harbor Laboratory Press. Retrieved May 6, 2015.