(host) Roughly 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 that lived before all modern mammals did.

So 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 than we thought.

But despite its long evolutionary history in mammals and their relatives, a coat of thick fur is one thing that we humans don't have.

In fact, we're the only primate without it.

So there must be a really good reason for why we roam around basically naked and unusually sweaty.

It turns out that this small change in our appearance has had huge consequences for our ability to regulate our body temperature.

And ultimately, it helped shape the evolution of our entire lineage.

Despite what you may have heard or thought, fur and hair are actually the same thing.

We just use a different word to describe the fur that we have.

But they're both the same kind of pellage, or hairy 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 super old, first thought to have evolved as a way for animals to keep warm by trapping air against the skin, which prevents heat loss to the surrounding environment.

And 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, make animals appear larger when they're feeling threatened, and certain coloring patterns can even help keep bugs away.

But fur can also make it more difficult to cool down.

One way many animals lose heat is through panting or taking short forceful breaths.

When animals pant, heat from the insides of their mouths evaporates into the surrounding air, which cools down the blood in the veins of 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 some of the body heat with it, increasing the animal's ability to lose heat.

But lots of sweating comes at the cost of losing more water, not really what you want if it's already hot.

Plus, if sweat soaks the fur, then heat can't really escape anymore.

So for most mammals, fur combined with panting, shade, and a bit of sweating usually cools them down just fine.

But why don't we have fur?

Why did our lineage develop super sweaty bare skin exposed to the elements, only to cover ourselves up again anyway?

The answer might actually lie in another aspect of human oddness, the evolution of bipedalism.

And often, one big change can lead to another.

When our early relative, Australopithecus, came onto the scene in East Africa some 4 million years ago, we started to see major changes in how hominids moved around.

Fossils of hip bones, femurs, and foot bones show that these hominids were able to walk on two legs, but the bones of the fingers and shoulders show that they also spent some time in the trees.

It wasn't until our genus, Homo, emerged around 2 million years ago that we became fully committed to walking bipedally.

And around 1.8 million years ago, Homo erectus took it a step further.

Its tall stature, long limbs, and bowl-shaped pelvis, which we humans have today, gave Homo erectus a more ideal running body compared to those shorter, stockier hominids that had come before.

And some scientists think this ability to run allowed Homo erectus to hunt using an uncommon method called persistence hunting, or chasing prey until it collapses from exhaustion.

In fact, by calculating the amount of water that humans lose when they engage in persistence hunting, a group of anthropologists recently found that Homo erectus could have hunted for over five hours straight without needing a water break.

And we carry on that legacy today.

We're the only living primate that can engage in persistence hunting.

And I say we loosely because I definitely can't do that, but it is possible.

Now, for Homo erectus and later hominids who may have been persistence hunting on the open savannas during the Pleistocene Epoch, they could have been at risk of overheating.

And 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.

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, though, these experts found themselves in a chicken and the egg scenario.

Did we lose the fur first, or did we start running first?

Was Australopithecus hairless, or was Homo erectus still hairy?

This is where fossils aren't much help anymore.

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 have still been necessary for survival.

Remember that fur is a great insulator.

Even mammals living in hot climates have fur, which comes in handy when the temperatures drop at night.

By looking at the environments Australopithecus lived in 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.

Without controlled fire, which doesn't show up in the fossil record until millions of years later, they just wouldn't have been able to generate enough heat to keep up with 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 around 2 million years ago.

This means that extensive fur loss occurred at some point within our genus, Homo, and DNA evidence from our own skin can help us pinpoint when that happened.

Human skin comes in a variety of shades, which are thought to 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 with ancestry from farther away.

Now, 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.

This means that if a hominid 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, suggesting that at this point in our history, hominid skin was adapted to intense sun exposure.

And who was already walking around Africa 1.2 million years ago?

Good old Homo erectus.

The individuals with naturally thinner fur would have been better able to cool down, 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 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.

But when and why did we become so sweaty?

Like fur, sweating is an ancient feature of mammals.

All mammals have two types of sweat glands, apocrine glands and eccrine glands.

Apocrine glands produce a thick, oily type of sweat and cover most mammals from head to toe.

They also produce pheromones, which are chemicals that signal important information about an animal's emotional and physical state.

Apocrine glands aren't very effective in cooling most mammals down, but since most mammals don't rely on sweating much anyways, 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, helping 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 totally sure why this change occurred, but it may have to do with the need to cool off better as their ancestors moved into hotter and drier habitats some 30 million years ago.

And humans are the sweatiest primate of all.

A group of scientists actually 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 in total, ten times more than chimpanzees have.

But we're actually just as hairy as chimpanzees.

We pretty much have the same number of hair follicles as chimpanzees, which, as it turns out, aren't really that hairy compared to other primates.

The difference between our hair and a chimp's 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 tiny, sweat evaporates very close to the skin's surface, transferring body heat to the atmosphere very effectively.

The combination of having a lot of sweat glands and vellus hairs all over our bodies has led us to become very good at cooling down.

We're actually capable of producing up to 3.7 liters of sweat per hour under 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 our loss of fur and increased sweating, which, in turn, made us even more efficient runners and hunters.

As the climate shifted, African primates found themselves faced with new thermoregulatory challenges.

Those with more eccrine glands were able to sweat more.

And as upright running became an important way of getting food, those with less fur were able to maximize the amount of heat they lost from sweating while chasing prey around.

This ultimately led to naked, sweaty persistence hunters.

Great.

And more efficient hunting means more meat, and more protein means a lot of things.

Over time, it could have led to an increase in brain size, more advanced tool use, cooperation, and even speech.

But although we lost most of our fur, it didn't just disappear.

Along with tiny vellus hairs, we still have thick hair on parts of our bodies.

Having hair on the tops of our heads protects our scalps from solar radiation and keeps our brains cool, while pubic and armpit hair may have remained as a way to broadcast sexual maturity.

So as gross as it sounds, and it sounds really gross, it looks like our ability to sweat a lot ultimately shaped the evolution of our lineage.