Placeholder Image

字幕列表 影片播放

  • Thank you to Audible for supporting PBS Digital Studios

  • Lemme tell you: There are no dull moments in the story of life.

  • Sure, within the whole Geologic Time Scale, there may be some episodes that are less exciting

  • than others.

  • But the first era of our current eon, the Paleozoic Era, is probably the most deceptively

  • fascinating time in Earth's history.

  • I saydeceptivelybecause, even though Paleozoic meansancient life”, this span

  • of time -- from 541 million to 252 million years ago -- doesn't have stuff like dinosaurs

  • or sabre-toothed cats.

  • So it's often overlooked as a time of primitive and boring creatures.

  • But the fact is: the Paleozoic Era was truly a make it or break it time for life on Earth.

  • At the beginning of the Paleozoic, living things were extremely simple and not very

  • dynamic.

  • Life was fragile, vulnerable.

  • But the Paleozoic was also easily the most chaotic of the three eras of our eonwith

  • near constant revolutions in life, punctuated by catastrophic extinctions.

  • In fact, by the end of this era, our planet was the closest its has ever gotten

  • to being devoid of life altogether.

  • It's the closest that life ever got to justfailing.

  • So, far from dull, the Paleozoic Era just might be one of the most harrowing chapters

  • in the story of life on Earth.

  • For the first three and a half billion years or so that life existed on this planet, things

  • were simple.

  • Almost all living things were in the oceans, and most of them couldn't move on their

  • own.

  • So, the big, complex ecosystems that we know today -- with active predators and mobile

  • prey -- didn't really exist.

  • Pretty much whatever drifted by you was what you ate.

  • That is, until about 541 million years ago, when the Paleozoic Era began with a period

  • known as the Cambrian.

  • And right from the start, things were dramatic.

  • This burst of evolutionary innovation, which you know as the Cambrian Explosion, was likely

  • the result of a whole bunch of environmental triggers.

  • In the Early Cambrian, thanks to a boom among phytoplankton in the oceans, oxygen levels

  • suddenly ramped up, allowing life to flourish.

  • Meanwhile, changes in the chemistry of the oceans, brought on by erosion, allowed animals

  • to develop things like shells and exoskeletons.

  • And this led to the formation of new body plans that the world had never seen before.

  • Almost every major group of animals that exists today developed within the first 40 million

  • years of Cambrian, along with many game-changing adaptations, like calcified hard parts, flexible

  • limbs, and the very first eyes.

  • For instance, the first large predatory organisms appeared, like Anomalocaris

  • which hunted worms and other soft bodied creatures.

  • And over time, some animals like arthropods developed hard exoskeletons to protect themselves.

  • But, others -- like Pikaia and Haikouellawent in a

  • different direction.

  • They developed the ability to swim, under their own power, with a flexible rod of cartilage

  • to power their tail.

  • These ancient swimmers would become the ancestors of the vertebrates.

  • Major breakthroughs like these resulted in whole new, fantastic ecosystems with complex

  • food webs that life had not experienced before.

  • But, this explosion of life couldn't last forever.

  • The Cambrian ended 488 million years ago in a mysterious mass extinction that saw the

  • disappearance of many trilobite and mollusk species.

  • While this extinction is not well understood, a sudden crash in oxygen levels may have been

  • to blame.

  • And remember that: Because a lot of the pivotal events in the Paleozoic will hinge on the

  • amount of oxygen either in the oceans or in the atmosphere.

  • But the next period, the Ordovician, bounced back with another burst in diversity

  • -- this one known as The Great Ordovician Biodiversification Event, or GOBE.

  • You can think of the the GOBE as a kind of sequel of the Cambrian explosion -- a sequel

  • that was every bit as impressive as the original.

  • Like the Cambrian event, the GOBE was brought on by a host of changes in the environment.

  • For one thing, a burst of geological activity about 470 million years ago meant that the continents

  • were on the move, creating new chains of islands, and with them new, isolated habitats.

  • And this, combined with changes in sea level and another boost of oxygen in the oceans,

  • stimulated a lot of rapid change.

  • For example, this is when the first true fish appear on the scene: ostracoderms

  • which were jawless

  • and covered in bony plates

  • Meanwhile, cephalopods reached new lengths, like the shelled giant Cameroceras

  • which grew up to 6 metres long!

  • But while all this was going in the water, things on land were truly revolutionary.

  • Before the Ordovician, terrestrial life was probably limited to microbes.

  • But early in the GOBE, the very first land plants sprouted on the primordial rocks.

  • Based on their fossilized spores, we know these plants were small, like modern mosses.

  • And if anything, this early greenery might have been too successful.

  • The massive number of plants on land took in so much carbon dioxide that the Earth's

  • temperature plummeted and BOOM: ice age.

  • This cold snap occurred 444 million years ago, just as marine oxygen levels started

  • to drop.

  • And this combination caused the most devastating mass extinction that animal life had faced

  • so far.

  • The Ordovician-Silurian Extinction Event wiped out 86% of marine species, including many

  • kinds of trilobites and cephalopods.

  • As you can tell from the name, it brought an end to the Ordovician, and marked the beginning

  • of the next period, the Silurian

  • Life was able to recover once again in the Silurian, this time as the climate gradually

  • warmed.

  • Plants, for example, started to spread over the land.

  • Fossils of early vascular plants, like Cooksonia, first appear in rocks from

  • this period, as do the first fossils of terrestrial fungi.

  • The jawless ostracoderms were still the most common swimmers in the sea, and they developed

  • all sorts of spines and horns, likely to protect themselves from carnivorous sea scorpions,

  • the Eurypterids.

  • But by the end of the Silurian, 419 million years ago, a new type of fish learned to bite

  • back.

  • Jawed fish, like Entelognathus appeared in the seas, equipped with biting

  • power that allowed them to tackle prey that the jawless fish couldn't handle.

  • And the Silurian drew to a close with its own, albeit minor, series of extinction events,

  • likely brought about by drops in sea level that caused many bottom-dwelling species,

  • particularly cephalopods, to die out.

  • The period that followed, the Devonian, was when fish began their takeover of the

  • seas.

  • During this time, the earliest sharks make their appearance, but the true kings of the Devonian

  • were another kind of fishthe placoderms

  • The largest of these armored, jawed fish filled the niches we associate with whales and sharks

  • todayincluding filter feeders like Titanichthys and apex predators like

  • Dunkleosteus

  • While the placoderms were conquering the seas, things were getting even more crowded on land.

  • Arthropods really started to diversify with the first insects and terrestrial

  • arachnids emerging.

  • Trees also arose at this time, forming a canopy above the arthropods, and these Devonian forests

  • became the first major terrestrial ecosystems on earth.

  • Meanwhile, back at the water's edge, relatives of lobe-finned fish had adapted to spend more

  • time in the shallows, pulling themselves along muddy shorelines.

  • Then 397 million years ago, descendents of these critters finally hauled themselves onto

  • land for the first time.

  • No skeletons of these ancient pioneers are known, but footprints in Poland show evidence

  • of these very first tetrapods, animals with four limbs.

  • By the Late Devonian, 365 million years ago, tetrapods had made their way across the entire

  • globe, from Acanthostega (ah-kan-thoh-steg-uh) in Greenland to Tulerpeton (too-lur-poh-tuhn)

  • in Russia.

  • So the Devonian produced the first complex land ecosystems, while also seeing vertebrates

  • take center stage in the ocean for the first time.

  • But, the period ended in, you guessed it, another mass extinction.

  • This one took place from 375 to 358 million years ago and came in at least two phases.

  • Together these events are known as the Late Devonian Extinctions, and they seem to have

  • been caused by a series of drops in oxygen levels in the seas.

  • By the end of the Devonian, many trilobites and all of the armored placoderms had vanished

  • from the world's oceans.

  • Luckily for us and other land animals, at least some early tetrapods survived into the

  • next period, the Carboniferous.

  • But these creatures lived in a very different world from the Devonian.

  • This time, oxygen in the atmosphere ramped up, and this, along with a humid, warm climate,

  • allowed dense forests and swamps to spread across the continents.

  • All that oxygen was also a plus for the arthropods, which got HUGE in the Carboniferous.

  • But the bigger news was a major change among the tetrapods.

  • Previously, all tetrapods had laid their eggs in waterbut around 340 million years

  • ago, a group of tetrapods called amniotes began laying their eggs with shells that protected

  • them from drying out.

  • And this turned out to be a crucial adaptation.

  • Because it was around this time that the continents began to merge into single supercontinent.

  • Eventually, over millions of years, it would become Pangea.

  • This landmass was so big that it was impossible for moisture from the ocean to reach inland.

  • The result: a severe drop in humidity and temperature that wiped out much of the Carboniferous

  • forests about 305 million years ago.

  • It's known today as the Carboniferous Rainforest Collapse.

  • By the end of this period, about 290 million years ago, much of the forests were replaced by

  • a giant desert at the heart of the continent, with extensive glaciers in the southern hemisphere.

  • These arid wastelands would've been uninhabitable for early tetrapods, but not for the amniotes.

  • And they rapidly split into two major groupsthe reptiles and the synapsids.

  • And both groups spent the last period of the Paleozoic, the Permian, spreading across Pangea.

  • The synapsids were actually stem-mammals, and they were survivors,

  • producing the first large terrestrial herbivores, like Dimetrodon

  • But as the Permian went on, the climate got hotter and drier.

  • Stem-mammals and reptiles alike were forced to adapt to even harsher conditions.

  • On the reptile side, there were cattle-sized herbivores like pareiasaurs (par-ee-uh-soars).

  • Meanwhile, stem-mammals grew even stranger, with the sabre-toothed gorgonopsids

  • hunting hippo-like omnivores with weird head ornaments, like Estemmenosuchus

  • But this Permian safari park came to an especially terrifying end 252 million years ago.

  • It was probably caused by a combination of volcanic activity and climate change, but

  • the result -- the Permian-Triassic Extinction Event -- nearly spelled doom for life on Earth.

  • 96 percent of marine species, including many sharks, fish, and all the trilobites, were

  • wiped out.

  • On land, the stem-mammals and reptiles suffered as well, with a 70 percent loss of terrestrial

  • species.

  • So, it began with an explosion of life, but the Paleozoic ended in near apocalypse.

  • And while life started out in this era as simple, small marine organisms, by its end,

  • life had conquered the oceans, taken the very first steps onto land, and spread to the most

  • inhospitable corners of our planet.

  • So yes, it was not exactly a heyday of charismatic megafauna.

  • But it's no overstatement to say that the Paleozoic Era made life what it is today.

  • Thank you to Audible for supporting PBS Digital Studios

  • And, right now Audible is offering Eons viewers a free 30 day trial.

  • Just go to audible.com/eons to access all their audio titles and programs.

  • This week, I want to tell you about “A Short History of Nearly Everythingwhich is Bill

  • Bryson's wry take onwell… a lot.

  • From the Big Bang to Civilization, Bryson is funny, informative, and insightful.

  • Sometimes science writing can get a little heady and Bryson does a wonderful job of both

  • making it all much more clear and making it fun and funny.

  • You can check out this title and many others by going to audible.com/eons.

  • Make sure you use this link to help us out and to get your membership trial.

  • And if you are really into dinosaurs and other big, scary creatures, then by all means watch

  • for an episode coming soon about the next chapter in life's history -- the Mesozoic

  • Era!

  • But for now, thanks for joining me!

  • And as always, I want to know what you want to learn about!

  • So leave me a note in the comments below!

  • And be sure to go to youtube.com/eons and subscribe.

  • Now, you feeling smarter already?

  • Well continue to flex that brian muscle and check out It's Ok To Be Smart.

  • Their latest episode explores the most extreme life on earth...like my favorite microfauna

  • -- the tardigrades.

Thank you to Audible for supporting PBS Digital Studios

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

从寒武纪爆炸到大死(From the Cambrian Explosion to the Great Dying)

  • 3 2
    joey joey 發佈於 2021 年 04 月 30 日
影片單字