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  pelage, 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. Fur is 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  seem 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 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,

which 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 some body heat with it,

increasing the animal's ability to lose heat. But, heavy sweating comes at the cost of

losing more water – not really what  you want when 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 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 hominins moved around. Fossils of hip bones, femurs, and foot bones

show that these hominins were able to walk on two  legs, but the bones of the fingers and shoulder

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 hominins that had come before.

And some scientists think that 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've hunted this way 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. I say “we” loosely because  I definitely can't do that. But it is possible.

Now for Homo erectus and later hominins,  who may have been persistence hunting on the

open savannahs during the Pleistocene epoch,  they could've 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.

Hominins 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 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've 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 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'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  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 further 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 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, suggesting that at this

point in our history, hominins' skin  was adapted to intense sun exposure.

And who was already walking around Africa 1.2  million years ago? Good ol' 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 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,

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 a 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,

10 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, 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 that 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.

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

Over time, it could have led to increases  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.

Hopefully not sweaty high fives  to this month's Eontologists:

Sean Dennis, Jake Hart, Annie & Eric Higgins,  John Davison Ng, and Patrick Seifert!

Become an Eonite by supporting  us at patreon.com/eons

because Eonites get perks like  submitting a joke for us to read!

This episode's joke is from Jared Jordan. "Any  humor found in the evolution of bipedality in

early humans, is technically stand  up comedy."

I'll be here all week you guys

Also if you want more Eons content, then 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 studio. Be sure

to subscribe at youtube.com/eons to discover  more about the evolution of life on earth.