Placeholder Image

字幕列表 影片播放

  • 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

  • mammalit 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 mammalssurrounding

  • 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, were the only primate without it.

  • So there must be a really good reason  for why we roam aroundbasically

  • 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 theyre both the same kind of  pelage, or hairy body covering.

  • And fur is one of the defining characteristics of  mammals. And while were 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 theyre 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 waternot 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 shorterstockier 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 couldve hunted this way for over five  hours straight without needing a water break.

  • And we carry on that legacy todaywere  the only living primate that can engage in

  • persistence hunting. I sayweloosely because  I definitely can’t do that. But it is possible.

  • Now for Homo erectus and later homininswho may have been persistence hunting on the

  • open savannahs during the Pleistocene epochthey couldve 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 wouldve 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 wouldve 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, homininsskin  was adapted to intense sun exposure.

  • And who was already walking around Africa 1.2  million years ago? Good olHomo erectus.

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

  • allowing them to runand huntfor 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 commonuntil 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 sosweaty?

  • Like fur, sweating is an  ancient feature of mammals.

  • All mammals have two types of sweat glandsapocrine 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, were 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 andchimp’s is the type of hair that we have.

  • Instead of thick fur, humans are covered in finealmost 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 nakedsweaty persistence hunters.

  • And more efficient hunting means more meatand 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 HigginsJohn 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.

Roughly 165 million years ago, a squirrel-like  creature called Megaconus was scurrying

字幕與單字

影片操作 你可以在這邊進行「影片」的調整,以及「字幕」的顯示

B1 中級 美國腔

人类如何失去皮毛(How Humans Lost Their Fur)

  • 7 2
    joey joey 發佈於 2021 年 05 月 03 日
影片單字