A Timeline of Life on Earth: 4 Billion Years of History - YouTube

The Earth formed about 4.6 billion years ago, which is an almost unimaginable amount of time. But that's how long it took for the Earth to settle down and for life to develop into the forms we see today. It was the longest miniseries we've ever done, but it was worth it. It turns out the history of our planet is kind of incredible. And now we've collected those videos in this compilation so you can watch the full series in one place.

地球形成于大约46亿年前，这是一个几乎无法想象的漫长岁月。但正是经过了这么长时间，地球才最终平稳下来，生命得以发展成为我们今天所见到的形态。这是我们所做过的最长的迷你系列，但是它是值得的。原来我们的星球的历史相当不可思议。现在我们已经将这些视频汇集在一起，让你可以在一个地方观看整个系列。

Our first episode covers the first few billion years, which was just enough time for life to get going, and for very simple, mostly single-cell organisms to form. Where did life come from? And how did it come to this? The path from bacteria to baseball wasn't simple. In this miniseries, we're going to explore the evolution of life on Earth. It's been a wild journey, with plenty of twists and turns, and there were a lot of times we almost didn't make it.

我们的第一集涵盖了最初几十亿年，这段时间足以让生命开始出现，并形成非常简单的、主要是单细胞有机体。生命从何而来？它又是如何发展至今？从细菌到棒球的过程并不简单。在这个迷你系列中，我们将探索地球上生命的进化历程。这是一段充满曲折和变化的狂野之旅，我们在许多时候几乎都没能坚持下去。

This first episode is about two of the earliest geological eons, the Archaean and the Proterozoic. Eons are the second largest way to divide up Earth's history, after super eons. There was one geological eon before the Archaean, the Hadian, which was 4.5 to 4 billion years ago, but because Earth's rocks are constantly being destroyed and reshaped, literally all of the rocks that old are gone. It's hard to have geology without rocks, so we're going to start with the Archaean.

这第一集讲述了地质学中最早的两个纪元：太古代和元古代。纪元是将地球历史划分的第二大方式，仅次于超纪元。在太古代之前有一个地质纪元叫哈德期，大约在45亿至40亿年前，但由于地球的岩石不断被破坏和重塑，那个时期的岩石都已经消失了。没有岩石很难进行地质学研究，所以我们将从太古代开始。

The Archaean began about 4 billion years ago and ended around 2.5 billion years ago, when the chemistry of rocks began to change and plate tectonics, the way the Earth's crust moves around, started to become more of a thing. That kicked off the Proterozoic eon, which continued until 542 million years ago. These two eons are where life began and developed in complexity, though it took its sweet time about it. It's also the time when life transformed the atmosphere, making it more suitable for some living things, but more dangerous for others.

太古代始于约40亿年前，结束于约25亿年前，当时岩石的化学成分开始改变，并且地壳板块构造运动开始越发显著。这标志着原生代的开始，一直持续到5420万年前。在这两个代中，生命开始并逐渐发展复杂性，尽管这过程需要花费很长时间。这也是生命改变大气成分的时期，使其对某些生物更适宜，但却对其他生物更具危险性。

Earth and the creatures that live on it have a complex relationship. We've influenced each other. The geology of the planet has guided the evolution of life, and life has shaped Earth just by existing. The geology during the Archaean shaped what was probably the first forms of life, but you wouldn't recognize the planet. The atmosphere contained a lot of methane, ammonia, hydrogen, and carbon dioxide, a type of chemical mixture that's called a reducing environment, which means it can make electrons available for chemical reaction. That was probably good news for the molecules that would eventually turn into biological molecules, they were able to react with each other and start becoming more complex.

地球和生活在地球上的生物之间存在着复杂的关系。我们彼此相互影响。地球的地质学引导了生命的进化，而生命的存在也塑造了地球。在太古代，地球的地质学形成了最初的生命形态，但你肯定无法辨认出那个时期的地球。大气中含有大量的甲烷、氨、氢和二氧化碳等化学物质混合物，这种化学环境被称为还原环境，意味着它能够为化学反应提供电子。对于最终会转化为生物分子的分子来说，这可能是个好消息，它们能够相互反应并开始变得更加复杂。

Carbon dioxide and methane are greenhouse gases, so the Earth was much hotter than, even though the sun was younger and fainter. In fact, there's no evidence for polar ice caps or glaciers in the Archaean. It was too hot for ice. Meanwhile, the continents were solidifying. Plate tectonics have shuffled them around a whole bunch since then, but the innermost rocky centers of most of today's continents date to the Archaean. Volcanoes belched carbon compounds and water into the atmosphere. The oceans condensed pretty quickly from that water. And because there was almost no oxygen in the atmosphere, there was no ozone layer. UV radiation would have been pretty intense.

二氧化碳和甲烷是温室气体，所以尽管太阳当时年轻且黯淡无光，地球的温度要比现在高得多。事实上，我们没有任何关于太古宙时期极地冰盖或冰川的证据。那时候实在是太热了，无法形成冰川。与此同时，大陆正在凝固。虽然板块构造在此后进行了很多重组，但大多数今天的大陆地壳核心可以追溯到太古宙时期。火山喷发释放了大量碳化合物和水进入大气层。海洋很快凝结出来了。而由于大气中几乎没有氧气，没有臭氧层。紫外线辐射非常强烈。

If you hopped into the TARDIS and time traveled to Archaean Earth, you might think you were on Venus. That's very different from the temperate climate and nitrogen-oxygen atmosphere we have now. So what changed? Well, life happened. At some point, in the warm oceans and carbon-rich atmosphere, biological molecules that contained the necessary information to copy themselves and had the chemical ability to do so formed from organic compounds. These became encapsulated in an oily membrane that kept them safe from the outside world, the first thing is to resemble a living cell. And eventually, this early life transformed the atmosphere and climate.

如果你跳进时间机器TARDIS，穿越到太古代的地球，你可能会认为自己来到了金星。这与现在的温和气候和氮氧大气非常不同。那么，发生了什么变化呢？嗯，生命发生了。在温暖的海洋和富含碳的大气中，一些具备自我复制所需信息和化学能力的生物分子，从有机化合物中形成。它们被一层油脂膜包裹，使其远离外界环境，成为第一个类似生命细胞的物质。最终，这种早期生命改变了大气和气候。

Some scientists believe the earliest biological molecules were RNA. The molecule RDNA now uses to send messages outside a cell's nucleus. It's called the RNA World Hypothesis. RNA is very similar to DNA, but it's easier to form from simple components. It also stores genetic information the same way DNA does, but it can tangle itself into shapes that make it easier for chemical reactions to happen the way proteins do in our cells now. The world hypothesis would explain why RNA is the go-between for DNA and protein in our cells. But, some scientists argue it would have been too complicated for life to switch over from RNA to DNA and protein, and it's more likely that all three evolved together.

一些科学家认为最早的生物分子是RNA。现在，分子RDNA用于发送细胞核外的信息。这被称为RNA世界假说。RNA与DNA非常相似，但它更容易从简单成分形成。它也以与DNA相同的方式储存遗传信息，但它能够缠绕成形状，使化学反应更容易发生，就像我们的细胞中的蛋白质那样。RNA世界假说可以解释为什么RNA在我们的细胞中成为DNA和蛋白质之间的中介。然而，一些科学家认为，生命从RNA转变为DNA和蛋白质过于复杂，更有可能是三者同时进化。

Whether life started as an RNA world or not, by the time the common ancestor of everything alive today came along, the system was based on DNA.

无论生命是否始于RNA世界，但当今天所有生物的共同祖先出现时，该系统已基于DNA。

The earliest fossils show that life in the form of bacteria existed 3.5 billion years ago, but a recent finding shows that life could be even older than that. A team of researchers from California found evidence of life that is 4.1 billion years old.

最早的化石表明，以细菌形式存在的生命在35亿年前就存在了，但最近的发现表明生命可能比那还要古老。加利福尼亚的一支研究团队发现了41亿年前的生命证据。

Now, I just said that there weren't any rocks older than 4 billion years, and there aren't. Which means there aren't any fossils, either. But, the team found resilient little crystals called zircons, which can be preserved when the rock surrounding them is destroyed. Then they get incorporated into new rocks. The zircons in question, found in Australia, contain traces of 4.1 billion year old carbon.

现在，我刚才说过4亿年之前没有任何岩石，确实是没有的。这意味着也没有化石。但是，团队发现了一种叫做锆石的坚韧小晶体，当它们周围的岩石被破坏时可以保存下来。然后它们被纳入新的岩石中。在澳大利亚发现的这些锆石中含有410亿年前的碳痕迹。

There are lighter and heavier forms of carbon called isotopes, and living things tend to have more of the lighter ones compared to the heavier ones. And so do these zircons.

碳存在着质量较轻和较重的同位素，而生物和这些锆石倾向于含有更多质量较轻的同位素。

For many scientists, it's hard to believe that life could possibly be that old. For one thing, Earth was pelted by asteroids 3.8 billion years ago in what was called the late heavy bombardment. Next time you look up at the moon, check out the craters. A lot of them came from the late heavy of bombardment. And as we can infer from what happened to the dinosaurs, asteroid impacts are not the best for the survival of living things. But some analyses suggest that the late heavy of bombardment would have wiped out only most of life. And there's a lot of it. So if there were any living things that old, some of them could have survived.

对于许多科学家来说，很难相信生命可能如此古老。首先，地球在38亿年前经历了被称为晚期重型轰炸的小行星撞击。下次你仰望月亮时，可以看看坑洞。其中很多是来自晚期重型轰炸的。从恐龙灭绝事件可以推断，小行星撞击对生物的存活并不利。但是一些分析表明，晚期重型轰炸可能只会消灭大部分生物。而生物的数量庞大。所以如果有那么古老的生物存在，其中一些可能幸存下来。

Either way, we're more confident about those fossils that are 3.5 billion years old. They're called stromatolites, and they're made up of layers laid down by films of bacteria. Stromatolites are pretty uncommon today, although you can find them living in Australia's shark bay. But pretty much up until the time grazing animals evolved half a billion years ago and started eating all of the bacteria lying around, they were an extremely widespread form of life. That means they basically ruled the Earth for a solid 3 billion years.

无论如何，我们对那些有35亿年历史的化石更有信心。它们被称为叠层石，由细菌形成的层状结构构成。如今，叠层石相当罕见，尽管你可以在澳大利亚的鲨鱼湾找到它们的生存。但在大约5亿年前食草动物进化并开始吞食四处散落的细菌之前，它们是一种非常广泛存在的生命形式。这意味着它们在整整30亿年的时间里基本上统治了地球。

They may look like slimy lumps of rock, but stromatolites are actually kind of incredible. Even if life only goes back 3.5 billion years, that's still pretty amazing, because it means that Earth was about a billion years old when the first life emerged. That's not a whole lot of time, especially since the continents and oceans and stuff were still forming. So it might not have been that hard for life to get started. The hard part would have been staying alive once there was oxygen everywhere.

它们看起来像是湿滑的岩块，但化石菌石实际上是非常了不起的。即使生命只能追溯到35亿年前，这仍然非常不可思议，因为这意味着第一次生命诞生时地球大约已经有10亿年的历史了。考虑到当时大陆、海洋等的形成过程，这段时间并不算长。因此，生命的起源可能并不那么困难。困难的部分可能在于生命在氧气铺天盖地时如何维持存活。

A few kinds of microorganisms make stromatolites. One of them is cyanobacteria. And cyanobacteria are pretty special. Instead of acquiring energy from their environments, like the other first organisms, cyanobacteria could capture energy from the sun. They could photosynthesize. Photosynthesis seems to have evolved in the arcane, but didn't really kick into high gear until early in the protorozoic. That's because the water cycle needed time to work on the brand new continents. Water weathers continents. It carries sediment into the sea, and that creates a shallow region around the shore, the continental shell. The continental shelf is great for photosynthesis, because it's shallow and gets plenty of sun.

一些微生物可以形成叠层石。其中之一是蓝藻。蓝藻相当特殊。与其他最初的生物不同，蓝藻能够从太阳获得能量，而不是从周围环境中获取。它们能够进行光合作用。光合作用似乎是在早期原生代时演化出来的，但直到晚原生代初期才真正发展起来。这是因为水循环需要时间来作用于全新的大陆。水侵蚀大陆，将沉积物带入海洋，从而在海岸周围形成一个浅水地区，即大陆架。大陆架非常适合进行光合作用，因为它浅且阳光充足。

But photosynthesis has a nasty, chemically voracious, toxic byproduct. Oxygen. One of the most greedy electron stripping elements on the periodic table. It reacts with practically anything. It's where we get the word oxidizing for chemically stealing electron. Once continental shelves formed, the cyanobacteria started pumping out oxygen like there was no tomorrow. And for many of the anaerobic or oxygen intolerant life forms on the planet, there was no tomorrow. They weren't used to oxygen and they couldn't handle it. It was poisoned to them. It reacted with and destroyed them. All that oxygen even changed the rocks. It reacted with iron that had been dissolved in the oceans, laying down bands of iron ore that we still find today. Earth basically rusted.

光合作用产生了一个讨厌的、化学上贪婪的、有毒的副产品 - 氧气。在元素周期表上，它是最贪婪的电子捐赠元素之一。它能与几乎任何物质发生反应。我们称其为"氧化作用"，意指化学上的电子转移。一旦大陆架形成，蓝藻细菌开始无休止地释放氧气。对于地球上许多厌氧或不耐氧生物来说，它们没有未来。它们对氧气不习惯，也无法处理它。对它们来说，氧气是致命的毒药，它与它们反应并摧毁它们。所有这些氧气甚至改变了岩石。它与在海洋中溶解的铁发生反应，形成了我们今天仍能找到的铁矿带。地球基本上生锈了。

But there was another problem. The cyanobacteria also pulled those carbon-containing greenhouse gases out of the atmosphere. Earth's temperature plunged. In the early protorozoic, and again near the end, maybe twice, the planet became a snowball. Life nearly poisoned and then froze itself to death.

但还存在另一个问题。蓝细菌也将大量含碳温室气体从大气中去除掉。地球的温度下降了。在早期原生代和接近末期的时候，可能发生了两次，地球变成了一个冰球。生命差点被毒害，然后冻结至死。

Cyanobacteria changed the atmosphere so much they destabilized the climate and changed the composition of the Earth itself. Eventually, the volcanoes coughed up more greenhouse gases in the snowball thought out. Cold, tolerant forms of life managed to survive the crisis. And some organisms learned not only to put up with oxygen, but to use it to make energy. They became aerobic life forms, but many anaerobic organisms had to either find a place to hide or kick the bucket.

蓝藻细菌改变了大气层，甚至影响了气候并改变了地球本身的组成。最终，火山喷发出更多温室气体，导致地球陷入了"雪球地球"的状态。耐寒的生物形式设法在危机中存活下来。而一些生物体不仅学会忍受氧气，还学会利用它来产生能量。它们成为了需氧生物，但许多厌氧生物则不得不寻找藏身之处或者灭亡。

Life is easy, but surviving is hard. And so is becoming any more complex than a single, simple cell. Life stayed single-celled for 2 billion years or so, and it only changed because something really weird happened. That something weird was endosymbiosis. Sometime in the protorozoic, before 2.1 billion years ago, an anaerobic cell ate anaerobic bacterium, one that used oxygen to produce energy. And the bigger cell never digested the smaller one. The smaller one kept on living, using oxygen and producing energy. And there was so much surplus energy that the bigger cell benefited from having the little guy in there. These are aerobic cells, the ones with nuclei, have the descendants of these aerobic bacteria inside them. They are our mitochondria.

生活很简单，但生存是艰难的。而且要比一个单一简单的细胞更加复杂，也同样困难。生命保持着单细胞状态大约20亿年，直到某件非常奇怪的事情发生。那个奇怪的事情就是内共生。在原生代的某个时候，在21亿年前之前，一种厌氧细胞吞噬了一种厌氧细菌，这种细菌利用氧气产生能量。而那个较大的细胞没有消化掉那个较小的细菌。较小的细菌继续存活下去，利用氧气产生能量。而有如此多的剩余能量，较大的细胞从中获益。这些带有细胞核的有氧细胞，它们的后代内含了这些有氧细菌。它们就是我们的线粒体。

That convenient energy arrangement paved the way for life to become more complex and all because of a weird fluke. Except, maybe it wasn't that weird, because it happened again. The ancestor of plants engulfed a cyanobacterium, which kept right on doing its photosynthesis thing, and now plants have chloroplasts.

那种便利的能量排列为生命的复杂化铺平了道路，所有这一切都因为一个奇怪的意外。除非，也许这并不那么奇怪，因为它再次发生了。植物的祖先吞噬了一株蓝藻细菌，它继续进行光合作用，现在植物拥有了叶绿体。

The earliest possible evidence for multicellular life is 2.1 billion years old. But it could have just been a colony of single-celled organisms. The earliest multicellular eukaryotes we can be fairly confident about are 1.5 billion years old. Animals didn't show up until 600 to 800 million years ago. Late in the protorozoic and into the next geologic time period, animal life established the groundwork for every kind of animal body plan that still exists today. So we've got life. Although it is still fairly simple. But in the next stage of Earth's history, things start to get really interesting, as evolution takes off, and we see a huge explosion in the diversity of life.

多细胞生命可能存在的最早证据距今已有21亿年。但这只可能是一个单细胞生物的群体。我们能相对确信的最早的多细胞真核生物已有15亿年的历史。动物出现在6至8亿年前。在前寒武纪晚期和下一个地质时期，动物生命为现今存在的各类动物体型奠定了基础。所以，生命已经出现了，虽然还相当简单。但在地球历史的下一个阶段，事情开始变得非常有趣，演化起步，生命多样性呈现爆炸性增长。

Next comes the fanarozoic eon, and here we're going to zoom in a bit to the first chunk of the fanarozoic eon, the paleozoic era, which lasted from 542 to 252 million years ago. Right at the beginning of the paleozoic, there was a huge explosion of more complex life. And that's when things really started to get interesting.

接下来是风暴界纪元，我们将稍微放大一点来看风暴界纪元的第一段，即古生代纪，这段时间从5.42亿年前持续到2.52亿年前。正是在古生代的开始阶段，复杂生命迎来了一次巨大的爆发。从那时起，事情变得真正有趣起来。

The paleozoic era is divided up into six periods. And by the first one, the Cambrian multicellular life, including animals, already existed. But it had not done much. Animals were simple things like sponges. They didn't have complex organs, and really, they weren't much more than lumps eating bacteria they strained out of the water. All that changed about 542 million years ago with the Cambrian explosion.

古生代被分为六个时期。而在第一个时期，寒武纪，已经存在了包括动物在内的多细胞生物。但它们并没有做太多事情。动物们像海绵一样简单。它们没有复杂的器官，实际上，它们不过是吃水中细菌的一团团。所有这一切发生在大约5.42亿年前的寒武纪爆发中。

There was an increase in oxygen levels just before the beginning of the Cambrian caused by a boom in life that produced oxygen using photosynthesis. It still debated how big this oxygen event was and when exactly it happened. But it might have made predation the predator-prey relationship possible for the first time. And the filter-feeding lumps got gobbled up.

在寒武纪开始之前，氧气水平的增加是由生命繁荣一并使用光合作用产生氧气而引起的。人们对于这次氧气事件的规模以及确切发生时间仍存在争议。但这可能使掠食和捕食者-被捕食者关系首次成为可能。而滤食的小块肉也被吞噬了。

Predation involves chasing after things you want to eat, and that takes more energy than just sitting on the ocean floor, waiting for food to come to you. To maintain that high-energy lifestyle, predators needed a whole lot of oxygen. Once predators evolved, prey started evolving better defenses and ways to run away, which led to predators getting faster and better at capturing their prey. It was basically an evolutionary explosion. Hard-body tissues like shells and skeletons began to show up in the fossil record just before the Cambrian. It took a lot of energy to produce that tissue, but it was worth it since these animals were less likely to be eaten. Officially, the beginning of the Cambrian was when animals started to burrow under the thick mat of bacteria on the ocean floor to escape predators.

捕食涉及追逐你想吃的事物，这比只是坐在海底等待食物来到更耗费能量。为了维持这种高能量的生活方式，捕食者需要大量的氧气。一旦捕食者进化出来，猎物开始进化出更好的防御和逃跑方式，这导致捕食者在捕捉猎物方面变得更快更强。这基本上是一次进化爆炸。坚硬的体组织，如壳和骨骼，开始出现在寒武纪之前的化石记录中。制造这种组织需要大量能量，但这是值得的，因为这些动物不太容易被捕食。官方上说，寒武纪的开始是动物开始在海底厚厚的细菌覆盖物下挖掘来逃避捕食者。

These predator-prey relationships combined with other factors, like changes in the minerals in the oceans and flooding that opened up shallow habitats. This led to such an enormous boom in diversity that almost every major animal group that exists today evolved during the Cambrian, including arthropods, mollusks, and the chordates that eventually gave rise to vertebrates.

这些掠食者-被食者关系与其他因素相结合，如海洋中矿物质的变化和洪水造成的浅水栖息地的开放。这导致了生物多样性的巨大爆发，几乎包括了今天存在的几乎所有重要的动物群，包括节肢动物、软体动物以及最终演化为脊椎动物的脊索动物。

The second period of the Paleozoic Era was the Ordovician, which started 485 million years ago. This name comes from a Celtic tribe, since many of the best studied rocks from the Paleozoic come from Britain. The Ordovician was when vertebrates first appeared. They were fish without jaws. Then came the Silurian Period, named for another Celtic tribe which started 443 million years ago. Sometime during the Ordovician and into the Silurian, life made the jump to land. Moving to land wasn't as easy as washing up on shore and going about your business. Life began in water, and living in water had some advantages over living in air. Water holds your body up.

古生代的第二纪是奥陶纪，始于4.85亿年前。这个名称来自于一个凯尔特部落，因为许多研究最详尽的古生代岩石来自于英国。奥陶纪是脊椎动物首次出现的时期，它们是没有颚的鱼类。接着是志留纪，它的名字来源于另一个凯尔特部落，始于4.43亿年前。在奥陶纪和志留纪之间的某个时候，生命开始向陆地进化。然而，迁移到陆地并不像上岸后马上可以进行自己的事情那样容易。生命最初始于水中，生活在水中相对生活在空气中有些优势。水可以支撑你的身体。

It helps you with gas exchange. If you release your sperm or eggs into the big blue, it's much more likely that they will meet up with other gametes to reproduce. None of that is true for air. You have to lift your body up, and in order to make the next generation, you have to get physically close to each other. So the earliest land organisms had to evolve. Support structures new respiratory systems, ways to avoid drying out and methods of reproduction that were a little more controlled.

它帮助你进行气体交换。如果你释放精子或卵子到广阔的大海中，它们更有可能与其他配子相遇并繁殖。而空气对此没有任何作用。你必须抬起身体并接近彼此，才能进行繁衍后代。因此，最早的陆地生物必须进化出支撑结构、新的呼吸系统，以及避免干燥和更为控制的繁殖方式。

Extremely simple plants might go back as far as the early Ordovician or even the Cambrian. They were spore forming and had very little in the way of internal support. The oldest fossil we have that isn't a plant spore or a single-celled organism is a fungus called Tortotubus. It's about 440 million years old from around the Ordovician-Silurian cut-off. This fungus was a total game-changer. Tortotubus probably helped pave the way for more complex plants and for animals, too. It lived as an underground network of filaments, much like modern fungi. It might have formed mushrooms to disperse its spores, but we don't know for sure. It fed by rotting the few other organisms on land, likely early plants and microbes. By breaking down nutrients, it helped develop the soil on Earth's surface. That helped complex plants grow and develop soil even further.

极其简单的植物可能可以追溯到早奥陶纪甚至寒武纪时期。它们以孢子形成，并且几乎没有内部支持结构。我们所拥有的最古老的化石，除了植物孢子或单细胞生物外，是一种称为Tortotubus的真菌。它大约有4.4亿年的历史，来自奥陶纪和志留纪的分界线附近。这种真菌是一种彻底的游戏改变者。Tortotubus可能为更复杂的植物和动物铺平了道路。它以地下的菌丝网络为生，类似于现代真菌。它可能形成蘑菇来散布孢子，但我们并不确定。它通过分解陆地上少数其他生物（可能是早期植物和微生物）来获取食物。通过分解营养物质，它有助于发展地球表面的土壤。这有助于复杂植物的生长和进一步发展土壤。

Now, there are doctors who creatures called Silurians, but they are pretty badly named because there were no land vertebrates during the Silurian period, let alone intelligent humanoid reptiles. But there were insects and other arthropods, earthworms and other terrestrial invertebrates colonizing the land. They formed the first simple land ecosystems along with plants and fungi.

现在，有些医生称之为"史莱恩人"的生物，但这个命名不太准确，因为在史莱期间并没有陆地脊椎动物，更别提智能的类人爬行动物了。然而，那个时候有昆虫和其他节肢动物、蚯蚓和其他陆地无脊椎动物开始在陆地上繁衍并建立最早的简单陆地生态系统，它们与植物和真菌一起形成了第一个陆地生态系统。

The fourth period of the Paleozoic called the Devonian started 419 million years ago and ended 359 million years ago. It's sometimes called the Age of Fishes because it's when the first fish with jaws appeared and it was followed by a lot of fish with jaws. These fish, called placoderms, had tough bony armor surrounding their skulls. They were the earliest vertebrates with jaws and jaws were pretty useful for eating stuff, so they were a big success in an evolutionary sense. Plaquoderms showed up in the middle Devonian before the Devonian was over the first tetrapods or four-footed creatures with backbones had already evolved. In fact, there's evidence that tetrapods may go back 395 million years or more smack in the middle of the Devonian. Which means that those fish, the ancestors of birds, mammals, reptiles and amphibians, got around to having legs and crawling out of the ocean almost instantly on an evolutionary time scale. At the beginning of the Devonian, jawless vertebrates were the most complex life around. By the end of the Devonian, there were early amphibian-like land dwellers walking around. That is a gigantic leap. Arthropods and land plants had a huge boom, too, meaning those simple land-based ecosystems from the end of the Silurian were a lot more complex by the end of the Devonian.

古生代第四期被称为泥盆纪，始于4.19亿年前，终于3.59亿年前。它有时被称为“鱼类时代”，因为第一个具有颚骨的鱼类出现在这个时期，随后又出现了很多具有颚骨的鱼类。这些被称为原甲鱼的鱼类，头部周围有坚硬的骨甲保护。它们是最早具有颚骨的脊椎动物，颚骨非常有用于进食，因此在进化过程中取得了很大的成功。在泥盆纪中期，原甲鱼出现了，泥盆纪结束时，第一批脊椎动物的四足生物已经进化出现。事实上，有证据表明四足动物可能追溯到3.95亿年前或更早，几乎在泥盆纪的中期。这意味着那些鱼类，也就是鸟类、哺乳类、爬行动物和两栖动物的祖先，几乎在进化的时间尺度上立马拥有了腿，爬上陆地跳出海洋。在泥盆纪初期，无颚脊椎动物是最复杂的生命形式。到泥盆纪结束时，陆地上已经有类似两栖动物的早期生物在行走。这是一个巨大的飞跃。节肢动物和陆地植物也大幅度增长，这意味着从志留纪末期的简单陆地生态系统到泥盆纪末期的复杂程度有了很大的提升。

Then, 359 million years ago, the fifth period of the Paleozoic began, the Carboniferous Period. You might have heard that fossil fuels are made of dinosaurs, but they are actually much, much older than that. The Carboniferous was when land plants really started to establish themselves. The climate was mild enough for plants to grow year-round and huge forests grew. The word Carboniferous means coal-bearing, and for good reason. Hundreds of millions of years later, we're digging up the remains of those forests as coal. The forests pumped oxygen into the atmosphere like crazy, much more oxygen than there is today, which led to the development of the first big land animals, arthropods, bugs, basically. They grew huge in the oxygen-rich atmosphere. That's right, 350 million years ago, Earth was full of giant bugs. The vertebrates were still fairly small in the Carboniferous, but they did develop one major evolutionary innovation, the Amniotic Egg, which is the reason you can store a chicken egg without it drying out. Amniotic eggs don't need to be kept in water because they have a tough shell and membranes to manage gas exchange without letting the embryo dry out. The reptiles that laid these eggs were less dependent on water than the first tetrapods who still had to return to the water to lay their eggs. The Amniotes could spend their entire life cycle on land, and they got better and better at it. And they got bigger.

然后，359万年前，古生代的第五个纪开始了，即石炭纪。你可能听说过化石燃料是由恐龙形成的，但实际上它们比恐龙古老得多得多。石炭纪是陆地植物真正开始建立自己的时期。气候温和到足以让植物全年生长，并形成了巨大的森林。石炭纪这个词的意思是含煤的，原因很简单。几亿年后的今天，我们正在开采那些森林的遗迹作为煤炭。森林向大气中大量释放氧气，比如现在的氧气含量要高得多，这导致了第一批大型陆地动物的发展，也就是节肢动物，也就是虫子。它们在富含氧气的大气中长得非常大。没错，3.5亿年前，地球上到处都是巨大的虫子。石炭纪的脊椎动物相对较小，但它们发展出了一个重大的进化创新，胎盘动物的卵，这就是你能够储存鸡蛋而不让其变干的原因。胎盘动物的卵不需要在水中孵化，因为它们有坚硬的壳和膜来管理气体交换而不会让胚胎变干。产卵的爬行动物比起还需要回到水中产卵的第一批四足动物来说，对水的依赖性较小。胎盘动物可以在陆地上度过整个生命周期，并且它们变得越来越擅长这一点。它们也变得更大。

The Permian, the last period of the Paleozoic, began 299 million years ago. It was the first age dominated by land vertebrates, including the first big vertebrate land predators like the Finn-backed Dimetrodon.

二叠纪是古生代的最后一个时期，始于2.99亿年前。它是首个被陆地脊椎动物主宰的时代，包括首个大型陆地捕食者——鳍背双臼齿龙。

If you had a dinosaur-themed coloring book or toy set that featured Dimetrodon as a kid, you should know two things. First, Dimetrodon didn't evolve until after the Paleozoic era during the Mesozoic era. Dimetrodon is way older than those guys. Second, Dimetrodon was on the same evolutionary branch as today's mammals, not today's reptiles and birds, so it's more closely related to you than to any dinosaur. It was a member of the group of so-called mammal-like reptiles that came before dinosaurs. Even though they weren't technically reptiles, it can be a helpful way to think of them, not mammals yet, but getting there.

如果你小时候有一个以恐龙为主题的涂色书或玩具套装，并且其中有迪美托龙的形象，你应该了解两件事。首先，迪美托龙在古生代时期之后，也就是中生代时期才出现。迪美托龙比那些恐龙要古老得多。第二，迪美托龙与今天的哺乳动物同属一支进化分支，而不是今天的爬行动物和鸟类，所以迪美托龙与你更近亲，而不是与任何恐龙。它是恐龙出现之前的所谓类哺乳动物爬行动物的成员。尽管它们从技术上来说不是爬行动物，但这种方式有助于我们理解它们，它们还不是哺乳动物，但已经接近了。

Dimetrodon was a carnivore, but there were synopsids that ate plants as well, like the similar looking a daphosaurus, which Dimetrodon probably ate. Land-eating was its own kind of evolutionary innovation, because herbivores couldn't really survive until there were enough plants to sustain the animals that ate them. Plus, herbivorous animals had to evolve digestive systems that could extract nutrients from leaves, which is much harder and less energy efficient than getting all your calories from meat. So plant-eating was another major evolutionary development that happened during the Permian.

迪留龙是一种食肉动物，但也有像外貌相似的海蛇龙一样以植物为食的合弓类动物。迪留龙可能会捕食海蛇龙。陆地动物的出现属于一种独特的进化创新，因为在足够多的植物出现以维持食草动物生存之前，它们无法真正生存下来。此外，食草动物必须进化出能从叶子中提取养分的消化系统，这比从肉类中获取能量更加困难且效率低下。因此，植食是晚古生代发生的另一项重要进化发展。

At the end of the Permian, 251 million years ago, the Paleozoic era ended, and everything else nearly ended along with it. There was a mass extinction event so unimaginably widespread that it's sometimes called the Great Dying. Something like 90% or more of Earth's marine species went extinct. Most of those big synapses died out, marine species were hit even harder. Something so awful happened that life nearly met its match. There were ice ages and smaller extinctions throughout the Paleozoic, but this one was the big one.

在2.51亿年前的二叠纪末期，古生代时代结束了，几乎所有其他的事物也随之结束。发生了一场难以想象的大规模灭绝事件，有时被称为“大灭绝”。大约90%甚至更多的地球海洋物种灭绝了。那些大型神经元也大量死亡，海洋物种更是受到了更严重的打击。如此可怕的事情发生了，使得生命几乎处于边缘。虽然在古生代的过程中存在过冰河时期和较小的灭绝事件，但这次是大规模的灭绝事件。

So what was it? What caused the Great Dying? We don't know for sure. The prime suspect is a plume of lava in present-day Siberia that was deposited 250 million years ago just when the Permian extinction took place. This was a volcanic eruption of sorts, but if you're imagining Vesuvius or Krakatoa, think bigger. A huge plume of heat welled up under Earth's crust and melted it for hundreds of square kilometers. The region was flooded by enough lava to cover two-thirds of the United States.

那么，是什么原因导致了大灭绝呢？我们并不确定。主要嫌疑人是发生在现今西伯利亚地区的一股熔岩羽流，于2.5亿年前刚好发生在二叠纪灭绝事件期间。这是一次类似火山喷发的事件，但如果你想象的是维苏威火山或克拉克火山，那么请认为更加巨大。一股巨大的热量羽流从地壳下涌现并融化了数百平方公里的岩石。该地区被足够的熔岩淹没，足以覆盖美国的三分之二面积。

Now the reason this volcanic plume is such a likely suspect is because it could have done all sorts of life-ending things. It could have caused rapid cooling by blocking out the sun. It also could have set fire to buried coal, releasing carbon dioxide and causing runaway global warming. There's evidence for both kinds of temperature extremes. It could have released chemicals into the atmosphere that led to large-scale acid rain or changed the chemistry of the oceans. We don't know exactly what those eruptions did, but we know they did something, and it probably wasn't pretty.

现在，这个火山喷发的原因是如此有可能是罪魁祸首，是因为它可能引发了各种有可能毁灭生命的灾难。它可能通过遮蔽阳光造成快速降温。它还可能点燃被埋藏的煤炭，释放二氧化碳并引发失控的全球变暖。有证据表明存在这两种温度极端的情况。它还可能释放化学物质进入大气层，导致大规模酸雨或改变海洋的化学结构。我们不知道这些喷发究竟产生了什么效果，但我们知道它们肯定不美观。

Other suspects include methane-producing bacteria, warming the planet, a catastrophe that somehow got rid of all the oxygen in the oceans, an asteroid impact. The Great Dying could also have been caused by the formation of the supercontinent Pangaea. Continents crashing into each other would have destroyed a lot of continental shelf habitat, killing some of the richest parts of the oceans. Having one big continent in one place would also have rearranged ocean currents and altered the climate. But Pangaea formed a little too early to account for such a widespread die-out, and all of the other hypotheses have strengths and weaknesses too. None of them can explain everything.

其他嫌疑人包括产甲烷细菌，因其导致地球变暖；某种灾难导致海洋中的氧气完全消失；小行星撞击。"大死灭"也可能是由于超大陆盘古大陆的形成所引起。大陆相互碰撞会摧毁许多大陆架栖息地，导致海洋中最富有的部分死亡。大陆聚集在一个地方还会改变海洋洋流并改变气候。但是盘古大陆的形成时间稍早，无法解释如此广泛的物种灭绝，而且其他假设也有其优点和缺点。其中任何一个都不能解释一切。

So some scientists have suggested what's known as the Murder on the Orient Express hypothesis, hashtag spoiler, like a neck at the Christie's classic novel, there are multiple culprits. It's like an exam question where the answer might be some of the above, or all of the above. Whatever the cause, nearly everything died out. So we almost didn't make it there, but mass extinctions clear the way for new forms of life to dominate, and in this case, it was the dinosaurs.

因此，一些科学家提出了所谓的“东方快车谋杀案假设”，井号表示剧透，就像阿加莎·克里斯蒂的经典小说中的情节一样，有多个罪犯。就像一道考试题，答案可能是以上某些选项，或者全部都是。无论原因如何，几乎所有的生物都灭绝了。所以我们几乎没有继续发展下去的机会，但大规模的物种灭绝为新的生命形式的主导创造了条件，在这种情况下，就是恐龙。

Life had been brought to its knees by a mass extinction at the end of the Paleozoic Era. The Mesozoic Era, from 251 to 65 million years ago, followed the Great Extinction and would produce some of the weirdest and most fascinating animals of all time, including the dinosaurs. It also led to most of the major land animal groups we know today.

在古生代时期结束时，生命被一次大规模的物种灭绝压到了极限。中生代时期，从2.51亿年前到约6500万年前，紧随大灭绝而来，产生了一些有史以来最奇特和最迷人的动物，包括恐龙。它也导致了我们今天所熟知的大部分陆地动物群体的出现。

Like other eras, the Mesozoic is divided into periods, in this case three of them. The first period, the Triassic, lasted from 251 to 199 million years ago. It was a time of transition, when the dominant vertebrates of the late Paleozoic, the thyroxids pretty much disappeared. A new group of reptiles, the dinosaurs, then rose to become the Earth's new dominant land vertebrates. Throughout the Mesozoic, Earth was warmer than it is now, and it had no polar ice caps. At the beginning of the Triassic, the Earth's land masses were lumped together into the dry, supercontinent Pangaea. Slowly, life started to repopulate the place. Repopulating meant diversifying.

像其他时代一样，中生代被分为若干时期，其中有三个时期。第一个时期是三叠纪，从2.51亿年到1.99亿年前。这是一个过渡时期，晚古生代的主要脊椎动物，甲鳗类几乎消失了。一群新的爬行动物，恐龙，随后崛起成为地球上新的陆地脊椎动物的主导物种。整个中生代，地球的气温比现在更热，并且没有极地冰盖。在三叠纪开始时，地球陆地聚集在一起形成了干旱的超大陆盘古大陆。慢慢地，生命开始重新在这个地方繁衍生息。重新繁衍意味着多样化。

The Thorapsids were declining, but a new group of vertebrates was starting to take over, the Arcosores. Arcosores descended from one of the earliest major groups of land vertebrates, the Diodesids, at the end of the Paleozoic. These major animal groups, the Arcosores and the Diodesids, are defined by holes in their skulls, which attach to muscles and mean that big, heavy bones weigh a little less. It might seem like kind of a strange way to tell these animals apart, but it's actually a very clear marker. In the Diodesids, to have two openings behind their eyes, Arcosores have two extra openings, one in front of the eye, and one in the lower jaw.

海盗类动物减少了，但一种新的脊椎动物群开始接管，那就是鳄形类动物。鳄形类动物起源于古生代末期的早期陆生脊椎动物中的一个重要群体，即二字兽类。这些重要的动物群，鳄形类动物和二字兽类，其颅骨上有孔洞，这些孔洞与肌肉相连，使得大而重的骨骼减轻了一些重量。这种方式可能看起来有点奇怪来区分这些动物，但实际上这是一个非常明确的标记。在二字兽类中，在它们的眼睛后面有两个开口，而鳄形类动物有两个额外的开口，一个在眼睛前面，一个在下颌上。

Dinosaurs are Arcosores, but so is another group that almost took over instead of the dinosaurs, the Pseudo-Suchians, which evolved very similar body plants to dinosaurs that came later, including standing on two legs. They came in a lot of different shapes and sizes, but if you want to know what a Pseudo-Suchian looked like, picture something a bit like a crocodile, but about 12 times more terrifying, because it's able to stand up straight on its long legs and run really fast over land. Pseudo-Suchians were nearly wiped out in another mass extinction at the end of the Triassic. Only one lineage survived, one that took to living in swamps and gave rise to modern crocodiles and alligators. Although, thankfully, the modern versions don't run around on two legs. The only other Arcosores around today are birds.

恐龙是鳄形类动物，但还有另一群动物几乎取代了恐龙，它们是伪鳄类动物，演化出与后来的恐龙非常相似的身体结构，包括站立在两条腿上。它们有各种各样的形状和大小，但如果你想知道伪鳄类动物是什么样子，可以想象一下鳄鱼的样子，但是要可怕十二倍左右，因为它能够直立在长腿上并且在陆地上快速奔跑。在三叠纪末期的另一次大规模灭绝事件中，伪鳄类动物几乎灭绝。只有一条血脉幸存下来，它们适应了沼泽生活，并导致现代鳄鱼的出现。不过，值得庆幸的是，现代版本没有用两条腿四处奔跑。现今唯一剩下的鳄形类动物是鸟类。

So the Arcosores took over from the Theropids, which had mostly died out at the end of the Paleozoic. Some Theropids hung on through the Triassic, and some didn't die out at all, luckily for us. The Theropids are descendants of another major branch of land vertebrates, the synapsids, who are also classified by their skullholes. Synapsids have only one skull opening, not four, and they're the ancestors of mammals. The earliest mammals appeared in the middle of the Triassic, at practically the same time as the dinosaurs. Yes, we mammals started out as dinosaur buddies, and we still hang out with dinosaurs now, just in bird form.

所以，阿尔科索雷斯（Arcosores）从六叶龙（Theropids）手中取代了掌握主导地位，因为六叶龙在古生代末期大部分已经绝迹了。一些六叶龙熬过了三叠纪，而另一些幸运地没有完全消失，对我们来说这是幸运的。六叶龙是另一个重要的陆地脊椎动物类群的后代，这个类群是颅骨孔所分类的。六叶龙只有一个颅骨开口，而不是四个，并且它们是哺乳动物的祖先。最早的哺乳动物出现在三叠纪中期，几乎与恐龙同时存在。是的，我们哺乳动物最初是恐龙的伙伴，现在只是以鸟类的形式与恐龙继续相处着。

Speaking of dinosaurs, there probably weren't many Triassic dinos in the coloring books you had when you were six, because they hadn't developed much yet. But they had some advantages that would eventually make them the ruling reptiles. Like I mentioned earlier, pseudosuchians and dinosaurs had very similar body plans. But the dinosaurs had a slight physiological edge. Their breathing was more efficient. And while both groups evolved legs that were positioned straight under them, instead of sprawling out to the sides, dinosaurs were the stronger movers. The earliest dinosaurs that we can be confident about go back 230 million years. But there are some very dino-like animals from 10 million years before that. An animal called niacosaurus may or may not be a true dinosaur, depending on who you ask. But it's definitely close. And it comes from 243 million years ago in present-day Tanzania. As is often the case with evolution, it's hard to draw the line between true dinosaurs and their immediate ancestors. But somewhere between niacosaurus and later dinosaurs like eoraptor, they had officially evolved ready to take over the world.

说到恐龙，当你六岁的时候，你的彩色书里可能没有太多三叠纪恐龙，因为它们还没有发展起来。但它们有一些优势，最终使它们成为统治爬行动物。正如我之前提到的，伪鳄类和恐龙有非常相似的体型。但恐龙在生理上稍微占优势。它们的呼吸更加高效。虽然两组动物都进化出了直立的腿，而不是呈扇形伸出来，但恐龙的行动能力更强。我们可以确定的最早的恐龙可以追溯到2.3亿年前。但在此之前的1000万年，还有一些非常类似恐龙的动物。一个叫做尼亚科龙的动物可能或可能不是真正的恐龙，这要看你问谁。但它肯定很接近恐龙。它出现在现在的坦桑尼亚，距今2.43亿年。就像进化常常发生的情况一样，很难划定真正的恐龙和它们的直接祖先之间的界限。但在尼亚科龙和晚期恐龙（如始祖鸟）之间，它们已经官方地进化成为准备统治世界的生物。

A couple of other animal groups turned up during the Triassic. One was the ichthyosaurs, the first reptile group to become fully aquatic again after evolving a land-based lifestyle. They're also one of only two groups to evolve a fish-shaped body from a four-footed animal body, the other group being the whales. Finally, toward the end of the Triassic, a group of archosaurs that were closely related to dinosaurs, but weren't dinosaurs themselves, evolved the power of flight, the pterosaurs.

在三叠纪时期，还出现了另外几个动物群体。其中之一是鱼龙，它们是第一批从陆地生活方式进化成为完全水生爬行动物的爬行动物群体。它们也是唯一两个从四足动物体形进化为鱼形体的动物群体，另一个是鲸鱼。最后，在三叠纪末期，一群与恐龙密切相关但并非恐龙本身的主龙类动物进化出了飞行能力，它们是翼龙。

After the Triassic came a period that you might have heard of called the Jurassic Period, which lasted from 199 to 146 million years ago. Although I do feel like I need to point out that some of the dinosaurs from the movie franchise are partly or totally made up, and others aren't from the Jurassic at all.

在三叠纪之后，有一个你可能听说过的时期，叫做侏罗纪时期，该时期从1.99亿到1.46亿年前持续存在。尽管我需要指出的是，电影系列中的一些恐龙是部分或完全虚构的，而其他一些则根本不属于侏罗纪时期。

During the Jurassic, Pangaea was beginning to separate into two continents, Loresia and Gondwana, shallow seas covered parts of the land. This is when dinosaurs diversified into their more familiar forms. The Jurassic was a great time to be a sauropod, for example, a huge, long-necked plant eater that walked on four legs. The Plodocus, Brachyosaurus, and Apatosaurus all lived during the Jurassic. Then there were the Theropods, the meat eaters that walked upright. Allosaurus was one major predator, but even bigger and meaner theropods were yet to come. The Stegosaurus also evolved during the Jurassic. It was a big plant eater with plates all along its back, and a spiked tail weapon called a Thagomizer, because if you're gonna pick a name for a giant, spiky tail weapon, you might as well make it awesome.

在侏罗纪时期，盘古大陆开始分裂成洛雷西亚和冈瓦纳两个大陆，浅海覆盖了部分陆地。这时期恐龙开始多样化为更为熟悉的形态。对于巨大的蜥脚类恐龙来说，侏罗纪是一个伟大的时代。蜥脚类恐龙是长颈、四足行走的大型植食动物。普洛多克斯、短鼻龙和甲龙都生活在侏罗纪时期。此外，还有以直立行走为特点的食肉类恐龙，如异特龙是其中一种重要的掠食者，但更大更凶猛的食肉类恐龙尚未出现。侏罗纪还见证了剑龙的进化。剑龙是一种大型植食性恐龙，背部长满了板状突起，尾巴上有一个被称为“索命尖器”的尖刺武器，因为如果你要给一个巨大、多刺的尾巴武器取个名字，那就应该让它令人惊叹。

Meanwhile, the Pleasiosaurus, a group of reptiles not closely related to dinosaurs, joined the ichthyosaurs in the ocean. There was one other major group of dinosaurs that appeared during the Jurassic, birds. We know that birds are descended from dinosaurs because of the similarities in their skeletons, and the fact that many dinosaurs had feathers. Because people who study evolution like to include all of a group's descendants in that group, birds technically are dinosaurs. So if you've ever fed a chicken nugget that's shaped like a dinosaur to a child, that's a weird experience. That's a whole strange thing. Archaeopteryx, which is usually considered the earliest bird, dates back to the Jurassic, and so do lots of other early birds. They, along with the pterosaurs, were the two kinds of flying archosaurs during the Jurassic, and during the next period, the Cretaceous.

同时，晚龙和鱼龙这两类与恐龙没有密切关联的爬行动物加入了海洋。还有一个在侏罗纪时期出现的重要恐龙群体，就是鸟类。我们知道鸟类是从恐龙演化而来，因为它们的骨骼结构非常相似，而且很多恐龙长着羽毛。由于进化研究的人们喜欢将一组动物的所有后代都归入该组，所以鸟类从技术上来说就是恐龙。所以，如果你曾经给孩子喂过形状像恐龙的鸡块，那真是一种奇特的体验，是一件很奇怪的事情。考古鸟（即始祖鸟）通常被认为是最早的鸟类，可以追溯到侏罗纪，而且还有很多其他早期鸟类也是侏罗纪时期的。它们与翼龙是侏罗纪以及下一个时期白垩纪时的两类飞行主龙形动物。

The Cretaceous, which means chalk-bearing, lasted from 146 to 65 million years ago, and was even warmer than the earlier Mesozoic. The continents continued to drift apart, heading for where they are now. As the sea floor spread, it released carbon trapped in the Earth's crust, and caused some serious global warming. Ichthyosaurs disappeared sometime during the Cretaceous. But a new type of marine reptile appeared, the Mosasores. Aquatic lizards related to the monitor lizards we have today, but not closely related to dinosaurs.

白垩纪（Cretaceous）指的是白垩岩的时期，此时距今约1.46亿至6500万年前，温度甚至比较早期的中生代更高。大陆继续分离，向着现在的位置漂移。随着海底扩张，地壳中被困的碳释放出来，导致了严重的全球变暖。白垩纪时期，海生爬行动物鱼龙逐渐消失，但一种新的海洋爬行动物出现了，称为蜥蜴鱼龙（Mosasores）。它们是我们今天所看到的蜥蜴的水生亲戚，但并不与恐龙有密切的关联。

Another new arrival? Flowering plants, which were excellent at getting animals to spread their pollen. That's why, at the same time as flowers, we see pollinators like bees appearing on the fossil record. Whether flowers or pollinators came first is a kind of evolutionary chicken and egg question. Probably neither one of them came first, exactly. The flowers and pollinators influenced each other's evolution, and became more interdependent as time went on.

又一位新来的？开花植物非常擅长引诱动物传播花粉。这就是为什么在化石记录中我们同时看到了出现蜜蜂等传粉者。花朵还是传粉者先出现的问题有点像进化中的鸡生蛋问题。很可能两者都不是先出现的，而是彼此影响并相互促进着进化，随着时间的推移变得更为相互依赖。

Mammals, which you'll remember had been around since the Triassic, evolved into the major lineages alive today. Placental mammals, like us, marsupials, like the opossum, and monotremes like the platypus. The Cretaceous also meant even more dinosaurs, like the frilled seratopsians and the duck build hadrosores. And, of course, Tyrannosaurus rex. I don't know about you, but I think T-Rex is pretty cool. I'm also very glad that predators that size aren't around today to snack on us.

哺乳动物，你可能还记得它们自三叠纪以来一直存在，进化成了如今存活的主要类群。胎盘哺乳动物，比如我们人类，有袋动物，比如负鼠，还有卵生哺乳动物，比如鸭嘴兽。白垩纪还出现了更多的恐龙，比如带领群众前行的蜥脸角龙和鸭嘴龙等。当然，还有霸王龙。不知道你，但我觉得霸王龙很酷。我也很庆幸现如今没有那么大型的捕食动物来咬我们了。

And why aren't they around anymore? Well, like the Paleozoic, the Mesozoic ended in a mass extinction. But there were a few differences between the two dieouts. So one thing, the extinction at the end of the Mesozoic wasn't as bad. Maybe about 50% of the Earth's species went extinct, which, yes, is a lot, but not nearly as many as during the extinction at the end of the Paleozoic, when almost all life died out.

为什么它们不再存在了呢？嗯，就像古生代一样，中生代也在一次大规模灭绝事件中结束。但是这两次灭绝之间存在一些区别。首先，中生代末期的灭绝没有那么严重。大约有地球物种数量的50%绝灭，是很多，但远不及古生代末期那次几乎所有生命灭绝的灭绝事件所造成的数量。

And while we don't know exactly what caused the earlier Paleozoic extinction, we have a major clue about the event at the end of the Mesozoic. It's a crater in the Yucatan region of Mexico. Most scientists agree that a meteor impact at this site must have been what wiped out all of the dinosaurs except for birds. There might have been other factors at play, but the meteor didn't help. So most of the diversity dinosaurs had to offer is gone for good. Yes, birds are very cool, but they represent only one lineage of dinosaurs. Those big four-footed plant eaters and the walking around armored vehicles, they're not around anymore. But once they were gone, mammals had a chance to take over, which is what happened during the Cenozoic, the era that we are still in.

虽然我们不知道早期古生代灭绝事件的具体原因，但是对于中生代末期的事件，我们有一个重要的线索。它就是墨西哥尤卡坦半岛地区的一座撞击坑。大多数科学家都同意，在这个地点发生的陨石撞击事件可能是导致除鸟类外所有恐龙灭绝的原因。可能还有其他因素参与其中，但撞击事件并没有起到积极作用。因此，大部分恐龙所具有的多样性就此消失。是的，鸟类非常酷，但它们只代表恐龙的一个支系。那些四足植物食草动物和装甲车辆一样行走的恐龙已经不复存在了。不过，一旦它们消失了，哺乳动物有机会接管地位，这正是新生代所发生的事情，而我们目前仍处于这个时代。

So dinosaurs are pretty cool, but finally it's our time to shine. It took another 65 million years or so, but who's counting? The Cenozoic is divided into three periods. The names of the periods have been switched up a bit recently, but these days, the era is divided into the Paleogene, Neogene, and Quaternary Period.

恐龙非常酷，但最终我们终于站在了舞台上。虽然花了大约6500万年，但谁会去计算呢？新生代被分为三个时期。时期的名称最近有点变化，但在现今，这个时代被划分为古近纪、新近纪和第四纪时期。

And each period is subdivided into even smaller units of time, called epoch.

每个时期又被细分为更小的时间单位，称为时代。

So first, the Paleogene Period. It covers the time from the extinction of the dinosaurs 65 million years ago to about 23 million years ago, and it's divided into the Paleocene, Eocene, and Oligocene Epox. The Paleocene, from about 65 to 56 million years ago, came right after the extinction of the non-evian dinosaurs, and even the ones that didn't die out, the birds, took a big hit on the diversity front. The dinosaurs left behind huge ecological shoes to fill. There were lots of feeding strategies and body plans that suddenly weren't being used. The place an organism fills in its environment is its niche, and usually two animals can't use the same one at the same time. After the dinosaurs gone, the mammals started exploiting those niches. So that's why mammals expanded a lot in diversity during the Cenozoic, even though they'd existed since the early Mesozoic.

首先是古近纪时期。它涵盖了从6500万年前恐龙灭绝到约2300万年前的时间，被分为古新世、始新世和渐新世。古新世大约从6500万年前到5600万年前，正好发生在非鸟类恐龙灭绝之后，即使那些没有灭绝的鸟类也在多样性方面受到了重大打击。恐龙们留下了许多巨大的生态空缺。许多饲食策略和体型规划突然不再被使用。生物在环境中的角色称为其生态位，通常两种动物不能同时占据相同的生态位。在恐龙灭绝后，哺乳动物开始利用这些生态位。这就是为什么哺乳动物在新生代期间扩展了很多多样性的原因，尽管它们早在中生代早期就已经存在。

By the Eocene Epoch, from 56 to 33.9 million years ago, mammals had diversified into some pretty neat forms, including orders that still exist, like rodents and primates, but also some that don't, like the enormous and bizarre titanathers and euintathers. One little antelope-like mammal had even wandered into the sea to become the ancestor of whales.

到了始新世这个时代，距今5600万至3390万年前，哺乳动物已经形成了一些相当神奇的形态，包括一些仍然存在的目，如啮齿目和灵长目，还有一些已经灭绝的目，如庞大而奇特的泰坦兽和伊兽。还有一种类似羚羊的小型哺乳动物甚至进入海洋，演变成为鲸类的祖先。

The oligocene, from 33.9 to 23 million years ago, saw the introduction of carnivores, but not what we usually mean by carnivores. There were already animals that ate meat. I'm talking carnivora, the taxonomic order of mammals that includes cats and dog. There were also lots of different kinds of rhinos all over the place. The paleogene was warm. Despite the mass extinction, the climate carried on more or less the way it had in the Mesozoic. Bommie with no polar ice caps. But all of that was about to change, because the continents were shifting.

始新世从3390万年前到2300万年前，见证了食肉动物的出现，但这里的“食肉动物”并不是我们通常所指的。在那个时候已经有吃肉的动物存在了，我说的是食肉目，即包括猫和狗在内的这一哺乳动物分类目。还有许多不同种类的犀牛散布在世界各地。始新世时期气候温暖。尽管发生了大规模灭绝，但气候在更或多或少与中生代时期相似的情况下继续存在着，没有极地冰盖。但所有这一切即将发生改变，因为大陆正在漂移。

Antarctica drifted over the South Pole and was surrounded by a cold current, which led to global changes in the circulation of the oceans. Antarctica started to ice over.

南极洲漂移至南极地区，并被一股寒冷的洋流所包围，这导致了全球海洋环流的变化。南极洲开始结冰。

The later two periods of the Cenozoic, starting with the neogene, were characterized by the rise to prominence of one extraordinary life form. It changed the course of evolution for every species that encountered it. I'm talking, of course, about grass. Grass is so common that most of us probably don't think about it. It just belongs on the ground. Always has. But grass is a relative newcomer to the evolutionary scene. The first grass is showed up just before the end of the Mesozoic. But the C4 grasses, so-called because of the way they process carbon, only showed up between 25 and 35 million years ago. Those are the important ones, and the major changes they influenced mostly happened within the last 10 million years. Grass is so important because it's hard to eat. It's tough, low in nutrients, and has little bits of silica incorporated into its tissues, specifically to discourage herbivores. Technically called phytopliths, they're basically sand. And chewing on sand is less than amazing for your teeth. Rather than not eating it, a lot of mammals just got really good at chewing and digesting grass. They evolved teeth with high crowns more resistant to being ground down. They evolved complex stomachs, like the four chambered arrangement in cows, to extract as much nutrition as possible. And they evolved long legs adapted to running around to the new open grassland habitats. Horses and antelope were the big winners in the Neo gene. But it wasn't just them. Grasses have become so widespread that all kinds of creatures depend on them for food, including us. The leaves with the sandy bits in them, but most humans depend on grain like corn, wheat, and rice. All grasses. We also feed grass and grain to our livestock. That means, for the rest of the cenozoic up to the present, the evolution of mammals was tightly bound to the spread of grassy habitat. Paleogene herbivores had been mostly browsers, animals that eat leaves from trees and shrubs.

新生代的后两个时期，从新近纪开始，以一种非凡的生物形式崛起为主导。它改变了与之相遇的每个物种的进化历程。当然，我说的是草。草是如此普遍，以至于大多数人可能都不会去思考它。它天然就存在地面上，一直都有。但是，草在进化历史上是相对较新的。第一代草出现在中生代末期之前。但是C4草，因其处理碳的方式而得名，直到2500万到3500万年前才出现。这些是重要的草类，它们影响的主要变化主要发生在过去1000万年内。草之所以重要是因为它难以食用。它坚硬、营养含量低，组织中还有一些硅的微粒，专门用来阻止食草动物。严格来说，它们被称为植物矿物体，基本上就是沙子。含磨砂的食物对牙齿来说并不理想。很多哺乳动物并没有放弃食用草，而是演化出了非常擅长咀嚼和消化草的能力。它们进化出了更耐磨的高冠齿缘。它们演化出了复杂的胃，如牛的四室胃，以尽可能提取更多的营养。它们还演化出了适应于奔跑于新开放的草原栖息地的长腿。马和羚羊是新近纪的大赢家。但不仅仅是它们。草类已经变得如此普遍，以至于各种生物都依赖它们作为食物，包括我们人类。草叶中含有沙粒，但我们大部分人都依赖粮食，如玉米、小麦和大米。这些也都是草类。我们也把草和谷物喂给家畜。这意味着，在剩下的新生代直到现在，哺乳动物的进化与草地栖息地的扩张紧密相连。早新生代的食草动物大多是浏览动物，它们吃树木和灌木的叶子。

In the Neo gene, they were outnumbered by grazers. The Neo gene is divided into two epochs, the Miocene and the Pliocene. In the Miocene epoch, beginning 23 million years ago, the continents were already close to where they are today. Ocean circulation became more modern too, which meant things were cooling down. The giant shark, Sea Megalodon, patrolled the ocean. Grasers like horses and camels were all over the place. And toward the end of the epoch, in Eastern Africa, a group of apes was learning how to walk on two legs.

在新后这个时代，他们被食草动物所占优势。新后时代分为中新世和上新世两个时期。在中新世时期，从2,300万年前开始，大陆已经接近于今天的位置。海洋循环也变得更加现代化，意味着气候趋于凉爽。巨型鲨鱼海大白鲨在海洋中巡逻。马和骆驼等食草动物遍布各地。而在该时期末期，东非地区的一个猿类群体正在学习如何两腿行走。

At some point before or during the Pliocene epoch, from 5.3 to 2.6 million years ago, North and South America crashed into each other. During the Pliocene, animals crossed the new land bridge and switched up their places on the continent. Apossums colonized North America, and as anyone who's driven around here knows, they stuck around. Camels and bears moved into South America, and they're still there too. And in the off-our region of Africa, there lived the early human relative Australopithecus Afarensis. Australopithecus' upright body plan was adapted to a shifting climate, and by the end of the Neo gene, that climate was shifting quite a bit. Antarctica had already started to freeze into the southern polar ice cap, and in the Pliocene, the Arctic began to get chilly too. This was the first time Earth had ice caps for a long time, possibly since the early Paleozoic.

在距今5.3到2.6百万年之前或之间的上新世某个时期，北美洲和南美洲相互碰撞。在上新世期间，动物们跨越这座新的陆地桥，改变了它们在大陆上的位置。负鼠开始在北美洲定居，正如任何在这里开车的人都知道的那样，它们一直都在这里。驼和熊搬进了南美洲，它们至今仍然存在。而在非洲的离我们较远的地区，生活着早期人类的近亲南方双足猿人。南方双足猿人的直立体型适应了不断变化的气候，而到了新基因纪的末期，气候变化变得相当大。南极洲已经开始冻结成南极冰盖，而在上新世，北极也开始变得寒冷起来。这是地球自早古生代以来很久以来首次出现冰盖。

So by the time the quaternary period came around, starting 2.6 million years ago, things were a little different from the mild times that came before. The quaternary period is divided into the Pliocene and Holocene epochs. You might recognize the last one as the epoch were in now. The Pliocene epoch, from 2.6 million to 12,000 years ago, is sometimes called the Ice Age. But it was more like a series of ice ages, with ice sheets advancing over the Earth than receding in dozens of cycles. The reason the ice sheets advance in retreat in cycles has to do with minor, predictable variations in Earth's orbit. When the ice sheets get more sun, they melt more than they freeze, and vice versa. Atmospheric carbon dioxide also tracks closely with global temperature during these cycles. When CO2 drops, the temperature plunges too. We coexisted with lots of cold weather organisms in the Pliocene, like woolly mammoths, sabertooth cats, and the actually real not just from Game of Thrones dire wolf. Many of these are extinct, even though we aren't. The post-mortem on the Pliocene megafauna seems to be some combination of climate-various variations that caused the glacial cycles and the arrival of hungry humans with pointy sticks.

所以到了四期从260万年前开始的时候，情况与以前的温和时代有所不同。四期被分为上新世和全新世。你可能会认出来，我们现在正处于全新世。上新世，从260万年前到12000年前，有时被称为冰河时期。但实际上它更像是一系列的冰河时期，冰盖在地球上先前退缩了很多周期。冰盖先进后退的周期与地球轨道的细微且可以预测的变化有关。当冰盖受到更多阳光照射时，它们会融化得比结冰多，反之亦然。大气中的二氧化碳与全球气温在这些周期内也密切相关。当二氧化碳减少时，温度也会下降。在上新世，我们与许多适应寒冷气候的生物共存，如毛象、剑齿虎和真实存在而不仅仅是《冰与火之歌》中的幻想生物狼。尽管我们还存活着，但其中许多物种已经灭绝。上新世巨型动物的死因似乎是多种原因的结合，包括引起冰河周期的气候变化以及饥饿的人类带着尖锐的棍子的到来。

So-called archaic humans, also sometimes called homo-hidal burgensis, date to around 400,000 years ago. Then anatomically modern humans showed up, a little less than 200,000 years ago. Homo sapiens, that's us, also coexisted with other branches of our human family tree, like the Neanderthals. We're the only ones left, but for a while there were a handful of different species of humans running around at the same time. These early humans hadn't yet developed the complex cultures and traditions that make us truly ourselves, but they mostly just needed time. There's evidence for art as old as 40,000 years. Our big brains probably evolved as an adaptation to the unpredictable climate. With the glaciers coming and going, we needed flexibility to survive. Also, tools and fire helped. We are technically still in the Pliocene Ice Age in what's called an interglacial period, even though we considered the Pliocene epoch to be over. The ice is supposed to return eventually, just not yet. With recent epoch, the Holocene is a tiny slice of time covering only the most recent warm interglacial cycle. That's just shy of 12,000 years ago. It's not the formal definition of the Holocene or anything, but that period of time also happens to correspond with humans learning to farm and keep animals.

所谓的古代人类，有时也被称为希德尔堡人，大约出现在40万年前。然后，大约不到20万年前，解剖结构上现代人类出现了。智人也与我们人类家族树上的其他分支，如尼安德特人，共同存在。我们是唯一剩下的人种，但曾经有几种不同的人类物种同时存在。这些早期人类还没有发展出构成我们真正自己的复杂文化和传统，但他们主要只需要时间。有关40,000年前的艺术的证据存在。我们的大脑很可能是对不可预测气候的适应进化而来的。由于冰川的来去，我们需要灵活性来生存。同时，工具和火的使用也起到了帮助作用。在所谓的间冰期中，我们技术上仍处于上新世的冰河时期，尽管我们认为上新世时期已经结束了。冰川很可能最终会回归，只是现在还没有。在最近的地质纪Holocene中，它只是覆盖最近一个温暖间冰期循环的短暂时间。这发生在大约12,000年前。虽然不是Holocene的正式定义，但那段时间也巧合地与人类学会耕种和饲养动物相一致。

We started this mini-series with life emerging nearly 4 billion years ago. 200,000 years of human history isn't much compared to that, and our actual recorded history compared to prehistory is on a geological scale very short. We're basically a blip, but even though we're a relatively young species, we've already had a lot of influence on Earth's geology through things like nuclear tests and our use of plastic. A group of scientists has argued that this is enough to define the start of a new epoch within the last century, the Anthropocene, or human epoch. The powers that be in geology haven't adopted this term yet, but it's often used informally. So welcome to the Anthropocene, the latest slice of time in geologic history.

我们以近40亿年前生命出现为开端开始了这个小系列。相比于那个时间，人类的历史只有200,000年，我们真正记录下来的历史相较于史前时期来说，在地质时间上是相当短暂的。我们基本上只是一个瞬间，但即使我们是一个相对年轻的物种，我们已经通过核试验和塑料的使用等方式对地球的地质产生了很大的影响。一群科学家认为这足以定义过去一个世纪的一个新纪元的开始，即人类纪，或者说人类纪。地质学中的权威尚未正式采用这个术语，但它经常非正式地被使用。所以欢迎来到人类纪，地质历史上最新的时间片段。

Well, thanks for revisiting our mini-series on the history of life with us. It took a long time for it to get to where it is today, but the diversity of life on our planet is amazing. And there's so much more to the history of life on Earth than we were able to cover in this series.

嗯，感谢您再次与我们共同回顾有关生命历史的迷你系列。生命的多样性让我们的星球变得令人惊叹，但地球生命历史上的内容远不止我们在这个系列中所能涵盖的部分。

So if you love learning about this stuff as much as we do, you can check out our sister show Eons, which is all about the most bizarrely fascinating things that have happened on this planet over the last 4.6 billion years. From Sabertooth Salmon to dinosaurs with hollow butts, just head over to youtube.com slash Eons and subscribe.

如果你和我们一样热爱了解这些东西，你可以去看看我们的姊妹节目Eons，它讲述的是地球过去46亿年发生的最奇异且引人入胜的事情。从剑齿鲑到拥有空心臀部的恐龙，只需前往youtube.com slash Eons并订阅即可。