字幕列表 影片播放 列印英文字幕 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 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 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 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 to being devoid of life altogether. 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 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 Haikouella – went 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 fish – the placoderms The largest of these armored, jawed fish filled the niches we associate with whales and sharks today – including 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 water – but 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 groups – the 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 Everything” which is Bill Bryson's wry take on… well… 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.