Name: 从寒武纪爆炸到大死（From the Cambrian Explosion to the Great Dying）
Uploaded: 2021-04-30T14:24:04.000Z
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Lemme tell you: There are no dull moments in the story of life.

情態助動詞 B1

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

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

I say “deceptively” because, even though Paleozoic means “ancient life”, this span

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

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

But the Paleozoic was also easily the most chaotic of the three eras of our eon – with

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

It's the closest that life ever got to just … failing.

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

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

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

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

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

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

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 Haikouella – went in a

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

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

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,

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

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

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

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

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

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

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

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

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,

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

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,