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A procession of segmented creatures moves across the sea floor.
一队节肢动物正在海底爬行。
Up ahead, hundreds have begun shedding their exoskeletons and piling on top of one another in what appears to be... a massive orgy.
前方，上百个节肢动物已经开始蜕去它们的外骨骼，堆在彼此身上，就像是...一场大型狂欢会。
But this is not some alien world. It's Earth about 500 million years ago, when these creatures, called trilobites, prospered.
但这可不是外星世界。而是5亿年前的地球，那时，这些叫三叶虫的动物处于繁盛期。
Prevailing for around 270 million years and encompassing more than 20,000 distinct species, trilobites are some of the most successful lifeforms in Earth's history.
三叶虫在地球上存在了大概2.7亿年，包含2万多个不同的物种，是地球史上最成功的生物之一。
When they sprung into existence, they were among the most diverse and sophisticated organisms Earth had yet seen.
自它们诞生起，它们是地球上迄今为止所见过的最多样、最复杂的生物之一。
And, as the earliest known animals with complex eyes, trilobites had a unique perspective on the ancient world.
同时，作为已知最早的有复眼的动物，三叶虫看古世界有着独特的视角。
For almost all of Earth's history before the rise of the trilobites, life had mostly consisted of microscopic marine organisms.
三叶虫兴起之前，地球历史上绝大部分生命体由海洋微生物构成。
But then, scientists think an increase in oxygen allowed multicellular lifeforms to extract more energy from their food and perform more complex functions.
但之后，科学家认为氧气的增加使多细胞生物从食物中吸取更多的能量，并执行更复杂的功能。
This then enabled the rise of carnivores, which in turn spurred a productive arms race, resulting in what's known as the Cambrian explosion.
这使食肉动物得以兴起，反过来又刺激了一场生产性的军备竞赛，造成了人们所称的寒武纪生命大爆发。
Within about 20 million years, life had branched out to include most of the animal groups we know today.
约在2000万年内，生命进行扩展，已囊括我们如今所知的大多数动物群体。
Trilobites were an important part of this surge. All trilobites had three lengthwise lobes, but building off the same basic components, they varied greatly.
在这次激增中，三叶虫十分重要。所有的三叶虫都有三块纵向叶片，但建立在相同的基本组件上，三叶虫的种类大相径庭。
Species ranged in length from a few millimeters to almost one meter and some were equipped with intriguing ornaments.
不同种的长度区间为几毫米到近一米，有的身上还带有有趣的装饰。
As a result, they filled many distinct niches over the ages. Many trilobites plowed or burrowed into the sea floor while others swam freely.
所以它们在那些年代里填充了许多不同的生态龛。许多三叶虫犁入或钻进海底，其他三叶虫则自由游动。
Certain species had spines, horns, and even protruding tridents.
某些品种有刺、角，甚至突出的三叉戟。
And their social behavior was complex: they came together to search for food, find safety in numbers, migrate and mate.
它们的社交会行为也很复杂：大量聚集在一起觅食、寻找安全的地方、迁移并繁衍。
In fact, their fossilized conga lines represent some of the first evidence of animal group behaviour.
实际上，三叶虫的化石康加线体现了动物存在集体行为的最早证据。
Like modern arthropods, trilobites had compound eyes composed of many tiny lenses.
像现代的节肢动物一样，三叶虫的复眼由许多小型的晶状体组成。
But theirs were made of the mineral calcite, which also constituted their exoskeletons.
但它们的复眼是由矿物方解石制构成的，同时构成三叶虫外骨骼的物质。
These lenses allowed trilobites to form sharp images and quickly sense changes in light.
通过晶状体，三叶虫得以看到清晰的图像，迅速感知到光的变化。
Some had long eye stalks that helped them peep above the muddy sea floor as they burrowed beneath.
有些三叶虫有长长的眼柄，帮助它们在钻进泥泞海底的时候探视海床上方。
Others had large dragonfly-like eyes that they probably used to scour the ocean bottom as they swam upside-down in dim waters.
另一些则有蜻蜓般的大眼睛，这样在昏暗的水域中倒立游动时，它们或许能用眼睛来搜索海底。
And some trilobites had pillar-like eyes that may have offered 360 degree views, along with structures that provided shade from overhead light.
有的三叶虫眼睛则呈柱状，能提供360度的视野，它们还具有遮挡头顶光线的结构。
Yet despite their dominance of the seas, trilobites were vulnerable to environmental changes and predators.
但即便在海洋中占主导地位，三叶虫容易受环境变化影响，且易被捕食者抓获。
About 444 million years ago, Earth cooled and sea levels dropped, radically changing some of their habitats.
约4.44亿年前，地球降温，海平面下降，三叶虫的栖息地遭到急剧变化。
This was the beginning of what would prove to be the trilobite's long demise.
这也被证实是三叶虫长期消亡的起点。
Some 20 million years later, fish with jaws began sweeping the sea. Trilobites developed spiky ornamentation and neat ways of locking their joints shut.
大概2000万年后，海里开始充满长有下颚的鱼类。三叶虫的装饰部位长出尖刺，用妙招让鱼的下颚紧闭。
Many species could curl themselves into balls, entirely sealing their soft parts inside their hard exoskeletons. Then, another extinction event shook the world.
许多种类可以将自己卷成球状，外骨骼完全封住柔软部位。接下来，另一灭绝事件震惊了世界。
Trilobite diversity dwindled, and come about 360 million years ago, only one of the 10 former trilobite orders remained.
三叶虫的多样性逐渐减少，先前三叶虫等级有十个，而在约3.6亿年前，仅剩一个。
Finally, rapid climate change spurred the greatest known mass extinction event in Earth's history.
最终，快速的气候变化刺激了地球史上已知的最大规模灭绝事件。
This catastrophic period finished the trilobites off -- along with approximately 96% of all marine species. But trilobites left a remarkable record behind.
这个灾难性时期导致了三叶虫--以及约96%海洋物种的灭亡。但三叶虫留下了非凡的记录。
Their calcite exoskeletons made for hardy fossils that would remain intact for hundreds of millions of years to come.
它们的方解石外骨骼变成坚硬的化石，在未来数亿年内可完好无损地保存。
We've found trilobite fossils on every single continent -- many in unlikely environments that were once part of the ocean floor.
我们在每片大陆上都发现了三叶虫化石，曾经属于海底但不太可能存在化石的环境中也找到了许多。
Trilobites have no direct descendants, but their evolutionary cousins are alive and well.
三叶虫没有直系后代，但它们进化的表亲生存得很好。
And since arthropods make up over 80% of current animal species,
既然在目前动物物种中，节肢动物的占比高于80%，
we might say that, although Earth may no longer be the planet of the trilobites, their distant relatives still reign supreme.
我们可以说，尽管地球或许不再是三叶虫的星球，但它们的远亲仍享有至高的地位。
