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  • Over the past several months, we've taken you on a journey through geologic history, one

  • era at a time.

  • If you haven't been on that trip with us yet, those videos, they're all in a playlist down

  • below.

  • And by now, you're probably tired of hearing us tell you that you're related to all of

  • these bizarre organisms that look nothing like you.

  • Like, in the Mesozoic Era, we introduced you to the Megazostrodon, a little insectivore that

  • lived among the dinosaurs as one of the earliest known mammals.

  • At least that's a mammal, so you can see the connection, right?

  • But what about Dimetrodon?

  • It lived in the era before the Mesozoic, in the Paleozoic.

  • It's not our direct ancestor, but it was a stem mammal, part of the group of animals

  • that descended from reptiles to give rise to mammals.

  • And when you look at it, wellit's not exactly like looking in a mirror, is it?

  • By the time we follow our lineage back even farther, to LECA, the ancestor of all

  • eukaryotes, and LUCA, the single-celled ancestor of everything that's alive today, we're

  • talking about forms of life whose lives and structures we can only speculate about.

  • But, now you've arrived at the Cenozoic Era.

  • And in fact, you've always been there!

  • Because that's the era we're in now!

  • And the Cenozoic is when many organisms took shapes and behaviors that you could actually

  • recognize.

  • Most of the mammals and birds that you can think of appeared during this era.

  • And reptiles went through some surprising changes, but they eventually settled into

  • the ranges they inhabit today.

  • But perhaps more importantly--for us at least-

  • the Cenozoic marks the rise of organisms that look a lot

  • like you and me.

  • OK to be fair, if you traveled back to the start of the Cenozoic Era, 66 million years

  • ago, there would still be a lot that you would not recognize.

  • It was so warm that the whole world was full of tropical and subtropical forests, even

  • at the poles.

  • And for about the first 10 million years of the Cenozoic, the world was still recovering

  • from the K-Pg extinction event that wiped out the non avian dinosaurs.

  • This was the very beginning of the Paleogene Period, and the world waskind of empty.

  • Along with the dinosaurs, almost all other large land vertebrates had vanished.

  • Many terrestrial plants were gone too, and in the oceans, the giant marine reptiles and

  • even most of the plankton had disappeared.

  • Because of this scarcity of life, during the first chapter of this period, known as Paleocene

  • Epoch, there were plenty of open ecological niches, and the surviving forms of life began

  • to fill them.

  • The last remaining dinosaurs -- birds -- had begun to diversify into some pretty familiar

  • forms.

  • For example, around this time, we begin to see the likes of Waimanu, a small, flightless

  • waterbird from New Zealand that's one of the earliest known penguins.

  • Likewise, in New Mexico, the appearance of Tsidiiyazhi tells us that the ancestors of

  • mousebirds, found today all over sub-Saharan Africa, were already on the scene.

  • Meanwhile, on the forest floor, some early, ungulate-like mammals began to take over.

  • At first, these mammals had it pretty easy, because there weren't many predators.

  • But plenty of insectivorous mammals had survived the extinction.

  • And it didn't take very long for some of them to start developing a taste for bigger

  • prey.

  • These were the creodonts, predators that first appeared in North America like the small,

  • kinda dog-like Galecyon as well as, Oxyaena, which looked more like a cat.

  • For a long time, scientists thought that these small meat-eaters were the direct ancestors

  • of today's modern carnivores.

  • But in recent years we've learned that they were actually a separate lineage, one that

  • happened to converge on the same strategies and general body plans of the carnivores we

  • know today.

  • Now, other mammals made their homes in the trees, including some of the first primate-like

  • species: the Plesiadapiforms.

  • They first showed up in Europe and North America, and even though most researchers think they

  • weren't direct ancestors of primates, they can still tell us a lot about what the earliest

  • primates might have looked like.

  • Purgatorius, for example, looks a lot like a rat.

  • But it had long, grasping fingers, useful for life in the trees, and wide teeth for

  • chewing things like fruits and leaves.

  • But like other plesiadapiforms, Purgatorius had claws instead of nails, and it was missing

  • one of the key features of a true primateits eyes didn't face forward.

  • So, by the middle of the Paleocene Epoch, animal life was on the rebound.

  • And then it started to get really warm.

  • About 55 million years ago, the average temperature on land went up by 5 to 8 degrees Celsius

  • in less than 20,000 years.

  • This spate of global warming marks the transition to the next epoch, the Eocene, and it's

  • known as the Paleocene-Eocene Thermal Maximum, or PETM.

  • Remember this one

  • If you've seen our episode on this phenomenon already, then you know that we're not totally

  • sure what caused it.

  • It might've had to do with volcanic eruptions or melting methane ice on the ocean floor

  • that released greenhouse gases into the atmosphere.

  • Either way, as a result, a world that was already warmer today's started to get even

  • hotter.

  • At the poles, lush ecosystems took hold, while parts of western North America became arid.

  • This was great for animals that thrived in the heat, like reptiles.

  • How great?

  • Well, this is when the world saw the biggest snake that ever lived.

  • Titanoboa slithered through South America during this hot spell -- all 13 meters of

  • it, about twice the size of a modern anaconda.

  • It feasted on fish but also crocodilians and, also anything else it could get its jaws

  • around--which was most things

  • Also in the water were reptiles like the giant turtle Carbonemys, which, unlike our shelled

  • friends we know today, were about 3 meters long.

  • And it fed on mammals and other reptiles

  • While all this was going on, some of the first true primates were appearing.

  • The tiny Omomyids for example, had grasping fingers with nails instead of claws, and giant

  • eyes like tarsiers'.

  • Then, about 49 million years ago, this warming trend shifted, and the world began its long

  • journey from a greenhouse to an icehouse.

  • The shift may have been caused, at least in part, by what's known as the Azolla event,

  • where massive amounts of the small, moss-like Azolla fern grew in the Arctic.

  • These plants took up to half of the carbon dioxide in the atmosphereenough that

  • the climate began to cool.

  • And a lot of mammals from the Paleocene couldn't handle the colder, drier weather of the late

  • Eocene, so, by about 40 million years ago, many of them had gone extinct.

  • At least 45 groups disappeared from Asia, like the Eurymylids, a family thought to be

  • closely related to early rodents.

  • In North America and Europe, the changing forests caused problems for mammals that lived

  • in the treesincluding primates.

  • By the late Eocene, practically all of the primates on those continents died out.

  • But at the same time, some modern mammal groups start to show up in the fossil record, like

  • familiar rodents and the odd-toed ungulatesthe group that includes today's horses,

  • rhinos, and tapirs.

  • And it's in the late Eocene that some of the simians, the clade that includes monkeys

  • and apes, begin to appear in the fossil record.

  • By the time the next Epoch, the Oligocene beings around 34 million years ago

  • We start to see the likes of Aegyptopithecus in northern Africa.

  • An early member of the group that includes what are known as the Old World Monkeys

  • and eventually the apes

  • But now the global temperature took an even steeper plunge.

  • With ice sheets beginning to form in Antarctica

  • And soon, another extinction event beganthis one mostly in Europe.

  • It's known as the Grande Coupure, and like most other extinctions, we haven't figured

  • out all the details of what caused it.

  • But we know the drop in temperature would have made it hard for some of the older groups

  • of mammals to survive.

  • It might have also lowered sea levels enough to allow for more migration from Asia and

  • therefore, more competition.

  • Almost all of the tree-dwelling European mammals were wiped out, and all kinds of new creatures

  • moved in, like true carnivores, and artiodactylsthe group of ungulates that includes animals

  • like today's pigs, deer, and cattle.

  • Then, in the cooler, drier climate of the Oligocene, a new habitat appeared: grassland.

  • And this was an enormous deal, because the fibrous grass was much harder to digest than

  • softer vegetation like leaves, which meant that animals had to adapt, or die.

  • Among the herbivores, the even-toed ungulates known as ruminants had the advantage, because

  • they had an extra stomach chamber where grass could be fermented and partly digested, then

  • sent back to the mouth to be chewed again.

  • In the late Oligocene, there was also a major split among the simians.

  • Around 26 million years ago, the first so-called New World monkeys appear in the fossil record,

  • in South America.

  • We're not really sure how they got there, although the lower sea levels might have helped

  • them get around.

  • These New World monkeys retained a lot of the traits of earlier monkeys, like their

  • small size and fruit-based diet.

  • Meanwhile, the larger Old World monkeys in Africa and Asia began to take a different

  • route.

  • Many of these Old World monkeys had a broader diet, and some started to spend more of their

  • lives on the ground.

  • Now, the transition from the Paleogene to the next period of the Cenozoic, the Neogene,

  • is a subtle one, usually recognized by changes in microscopic fossils of things like algae

  • and foraminifera.

  • But the events that unfolded during this period, starting 23 million years ago, were hard to

  • miss.

  • The Neogene opened with the Miocene Epoch as continental plates were on the move, kicking

  • off era of mountain-building that continues today.

  • The Himalayas were forming, as the Indian plate rammed into Asia, while collisions in

  • Europe started to create the Pyrenees and the Alps.

  • Meanwhile, in Africa, another transition was underway, as the first apes evolved from the

  • Old World monkeys.

  • We're not completely sure what the first ape was, but a transitional genus called Proconsul

  • first appeared around the start of the Neogene that may have been close to the start of the

  • ape lineage.

  • The most obvious trait of these animals was their lack of a tail.

  • But they also had more flexible shoulder joints; broad, flat rib cages; and a shorter spine.

  • Among other things, these traits combined to make it easier for apes to swing from the

  • trees andeventuallyto balance on two legs.

  • For the most part, the rest of the Miocene continued the trends that began in the Oligocene.

  • As the world continued to cool, forests began to shrink while grasslands spread.

  • For animals that grazed instead of browsing on trees, they had vast new expanses to disperse

  • to

  • But almost all of the herbivores that couldn't survive on grass -- like browsing horses -- began

  • to disappear.

  • In the oceans, the bizarre marine mammals like Desmostylia had disappeared.

  • But new forms of life start showing up in the fossil record -- like sea otters and other

  • animals that made their homes in the world's first kelp forests.

  • And in the meantime, the apes continued to diversify.

  • Over the next several million years, the ancestors of each of the great apes split off from the

  • ancestors of humans.

  • The ancestors of orangutans diverged from our lineage first, about 13 million years

  • ago.

  • The ancestors of gorillas were next, around 10 million years ago; followed by the ancestors

  • of chimpanzees and bonobos, around 7 million years ago.

  • By the time the Miocene ended 5.33 million years ago, the lineage that would lead to

  • humans was established.

  • In fact, most of the groups of animals around today had evolved.

  • The world was getting closer to something we would truly recognize.

  • Sure, there was still the occasional gomphothere or three-toed horse.

  • But other groups, like canids, bears, and whales, were fully fledged.

  • The transition to the next epoch, the Pliocene, involved a brief period of warming, followed

  • by an even faster drop in temperature.

  • South America, which had detached from Antarctica in the Oligocene, finally bumped into North

  • America, cutting off the Atlantic Ocean from the warm currents circling the equator.

  • And as the climate continued to cool, our ancestorsthe homininswere taking

  • over the expanding grasslands.

  • The first known hominins, Australopithecus, appear in the fossil record around 4 million

  • years ago.

  • Researchers think some of these early hominins were actually able to digest grass, which

  • would have made it easier for them to find food no matter what the climate was.

  • Over time, they became better runners and more skilled hunters.

  • And there's evidence that toward the end of the Pliocene, they had begun using stone

  • tools.

  • By 2.8 million years agojust a couple hundred thousand years before the end of the

  • epoch — a new genus, Homo, appeared on the scene in the form of a lower mandible found

  • in Ethiopia, known as the Ledi Jaw.

  • The end of the Pliocene also marked the end of the Neogene, and the start of our current

  • period of the Cenozoic, the Quaternary, about two and a half million years ago.

  • And this most recent chapter in the history of life is so packed with crucial developments

  • -- from the Last Ice Age to the rise and spread of Homo sapiens -- that we we'll handle them

  • all in a separate episode.

  • But by now you've seen how the events of the Cenozoic truly shaped our worldand

  • us.

  • It began with a world recovering from extinction, with millions of niches for mammals to fill.

  • And it peaked with a warming event that helped spur the rise of the primates.

  • Yes, you might not be the spitting image of Aegyptopithecus, but the Cenozoic Era is when

  • the world as we know it came to be.

  • More than any other era in history, it is our time.

  • Thanks for watching me today!

  • Now, what do you want to know about the story of life on Earth?

  • Let us know in the comments.

  • And don't forget to go to youtube.com/eons to subscribe!

  • As part of us trying to give you things with which to support this show

  • Eons has developed a really kind of beautiful piece of art in the form of a poster

  • that's now available at DFTBA.com

  • I've got mine. Did you get yours?

Over the past several months, we've taken you on a journey through geologic history, one

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从迪诺斯陷落到人类兴起(From the Fall of Dinos to the Rise of Humans)

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    joey joey 發佈於 2021 年 04 月 30 日
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