500 million years ago, back in the Cambrian Period,
回到5亿年前的寒武纪时代
a pioneering little mollusk floated up off the ocean floor.
一只开拓性的小软体动物从海底浮了上来
It had developed a way to use its defensive shell
这只小软体动物已经开发出一种使用其防御外壳的方法
for a whole new purpose — buoyancy.
是为了一个全新的目的——浮力
It turned out that by filling its shell with gas,
结果证明 通过在壳里充满气体
this mollusk could literally reach new heights,
这只软体动物的确到达了新的高度
gaining a key advantage over its relatives on the seafloor.
和它在海底的亲戚相比更有优势
Scientists believe that this was the very first cephalopod —
科学家认为这就是最初的头足纲
a group that now includes squids, octopuses, cuttlefish, and nautiluses.
头足纲是现在包括鱿鱼 章鱼 乌贼和鹦鹉螺在内的一个种群
While we might think of the nautilus with its shell as an oddity today,
尽管我们今天可能认为有壳的鹦鹉螺很怪异
the fact is that the ancestors of modern, squishy cephalopods
事实是 现代的黏糊糊的头足纲
like the octopus and the squid all had shells.
比如章鱼和鱿鱼 它们的祖先都是有壳的
And early cephalopods are actually defined by their shells —
而早期的头足纲实际上是由它们的壳来定义的
or more specifically,by how their
或者更明确一些
shells adapted to suit their needs.
是由外壳进化的方式来定义的
Some cephalopods truncated their shell.
一些头足纲截短了他们的壳
Others acquired a different shape.
另一些头足纲演变出不同形状的壳
Some of them internalized their shell like a backbone.
一些头足纲将壳像脊梁一样内化
And, in some cases, they got rid of the thing altogether.
在一些案例中 它们完全摆脱了壳
In ancient times, the shell was cephalopods’ greatest asset.
在远古时期 壳是头足纲最重要的优势
But it also proved to be their biggest weakness.
但壳也被证实是头足纲最大的弱点
Mollusks were some of the first truly complex animals,
软体动物是第一批真正复杂的动物
probably appearing in the late Ediacaran Period.
可能出现于埃迪卡拉纪晚期
Although there is some evidence of hard mineralized shells from this time,
尽管有一些关于坚硬的壳的化石证据来源于这个时期
shells as we know them today
但是我们今天所知道的壳
become much more common after the Cambrian Explosion,
在寒武纪生命大爆发之后变的更加普遍
which,not coincidentally,
并非巧合 捕食者的首个证据
is when the first evidence of predators appears.
也是在这个时候出现的
So, early shells worked like shields,
所以说 早期的壳的作用类似于保护体
protecting the soft body or mantle
保护动物柔软的身体或外套膜
of the animal from predators lurking above.
不被潜伏在上面的捕食者伤害
But by the late Cambrian,
但在寒武纪晚期
one mollusc known as Plectronoceras
一种被称为短棒角石属的软体动物
had acquired a couple of adaptations
已经进行了一些演变
that marked the beginning of a brand new form of transportation
这些演变标志着一种全新的运输方式
and a whole new kind of mollusk.
和一个全新种类的软体动物的开始
For one thing, its shell was divided into seeled-off chambers
一则 软体动物的外壳被称为隔膜的薄壁
by thin walls called septa.
分隔成几个封闭的腔室
As the animal grew, it added new chambers to its shell.
随着动物的成长 壳里会增加新的腔室
This in itself wasn’t new,
这本身并不是新鲜事
but it ended up being instrumental to another adaptation that was.
但它最终成为了另一项崭新的演化的工具
As Plectronoceras added septa to its shell,
由于短棒角石属动物在它的壳里增加了薄膜
it left behind a small, tube-like part of its mantle in each chamber.
它在每个腔室内留下了一个小的 管状的外膜
This little tube of tissue is known as a siphuncle,
这个小的管状组织被称为体管
and as unassuming as it seemed,
尽管看上去很不起眼
it helped Plectronoceras perform a trick the world had never seen before.
体管帮助短棒角石属表演了一个世所未见的把戏
By making the blood that flowed through the siphuncle super salty,
通过让流经体管的血液变得非常咸
Plectronoceras was able to absorb all of the water
短棒角石属可以吸收
from the chambers in its shell.
所有来自壳内腔室的水
As water diffused out of the shell,and into the salty blood,
当水渗透到壳外 进入咸的血液
gas seeped in,
气体渗入
and what was once a suit of armor
之前作为盔甲的物体
became a personal floatation device.
变成了个人的浮力装置
The very first true cephalopods had arrived,
最初的真正的头足纲产生了
and they looked like tiny, adorable, upside-down ice cream cones.
它们看起来像小小的 可爱的 倒置的冰淇淋筒
This development of a gas-filled, chambered shell, also known as a phragmocone,
这个充气的腔室外壳 也被称之为闭锥 它的发展
was a triumphant, history-making adaptation.
是一种成功的 历史性的进化
By the time the Cambrian had segued into the Ordovician,
在从寒武纪转入奥陶纪之时
cephalopods had entered a golden age.
头足类进入了一个黄金时代
There were few predators to threaten them,
几乎没有能威胁它们的捕食者
and a rise in ocean oxygen levels
并且海洋内氧气水平的提升
caused life to flourish, diversify, and occupy new habitats,
使得生命得以繁荣 多样化并占据新的栖息地
providing an abundance of food.
能够被提供丰盛的食物
This is known as the Great Ordovician Biodiversification Event.
这被称为奥陶纪生物大幅射
And that’s when they got very big.
头足纲在这个时期变的非常大
The Ordovician endoceratida cephalopods
奥陶纪头足纲的内角实目
were the biggest animals of their time,
是他们那个时代最大的动物
reaching an impressive 6 meters in length.
达到了壮观的6米的长度
And as the Ordovician progressed,
随着奥陶纪逐渐过去
cephalopods began to leave the shallows to explore the open ocean.
头足纲开始离开浅滩去探索开阔的海洋
So they had to find ways to become more fast and agile.
所以它们必须找到变得更加快速和敏捷的方法
Some species developed shells that coiled,
一些物种进化出了卷曲的壳
forming a more compact and maneuverable form,
形成了一种更加紧凑和灵活的形态
like the modern nautilus.
就像是现代的鹦鹉螺
By the Silurian Period,
到了志留纪
a genus called Sphooceras tried a different tactic:
一种名为Sphooceras的物种尝试了不同的策略
Instead of coiling its shell，
它让壳的末端断掉
it broke off the end of it.
而不是把壳变成卷曲的形状
Sphooceras periodically wrapped part of its soft mantle around the outside of its shell,
Sphooceras周期性地将一部分软外套膜包裹在外壳外面
and then secreted enzymes that helped break off the chambers at the end.
然后分泌用以帮助切断腔室末端的酶
This made the end blunter, shorter, and sturdier.
这使得壳的尾端更钝 更短 更坚固
Which in turn made the shell less vulnerable to breaking
因此壳变得更不容易破碎
and easier to maneuver.
并且更易于操控
Sphooceras might be the very first cephalopod
Sphooceras可能是第一种
that kept its shell inside its mantle for any length of time
让壳长期处于外套膜内的头足纲
— and this was an experiment that was about to be taken to a whole new level.
这是一个即将被提升到全新水平的实验
That’s because a new evolutionary pressure was waiting for cephalopods in the Devonian Period:
这是因为一种新的进化压力在泥盆纪等待着头足纲
fast, jawed fish.
快速的 有颌部的鱼类
While fish with jaws first appeared in the Silurian,
有颌部的鱼在志留纪首次
they proliferated in the Devonian.
出现 在泥盆纪迅速繁殖
And that kicked off an evolutionary arms race
而这种情况在鱼类和头足纲
between fish and cephalopods.
之间开启了一场进化军备竞赛
Up until this point, all cephalopods had been members
截至这个时点 所有的头足纲都成为了
of the slow and steady group known as nautiloids,
缓慢且稳定的 被称为鹦鹉螺的族群成员
from the pioneering little Plectronoceras to the imposing Cameroceras.
该族群从最早的小小的短棒角石纲到壮观的房角石纲
This ancient lineage still survives today in the form of the modern nautilus.
这种远古血统至今仍然以现代鹦鹉螺的形态存活
But in the Devonian, a new branch of the cephalopod family tree appeared: ammonites.
但是在泥盆纪 一个头足纲族谱中新的分支出现了 菊石类
And they coped with the rise of fish
而且他们应对鱼类的崛起的策略
with a live-fast, die-young strategy.
是快速繁殖 快速死亡
Unlike nautiloids, which grew slowly
不同于成长缓慢
and invested a lot of energy into making a few offspring,
而且需要投入大量精力繁殖后代的鹦鹉螺
ammonites grew quickly and had many offspring.
菊石类动物生长很快 并且有许多后代
They ended up being so successful, diverse, and numerous
它们最终是如此成功 多样化且数量众多
that their shells are now used as index fossils
以致于它们的壳现在被作为标准化石
to define Periods in the Mesozoic.
用于定义中生代不同纪元
And ammonites developed a huge variety of shell sizes and shapes,
菊石发展出各种各样的外壳大小和形状
growing shells that looked like hooks or knots or even paper clips.
长出看起来像钩子 结甚至是回形针的贝壳
Then, around the beginning of the Carboniferous, a new lineage appeared
之后 大约在石炭纪的开始期 一个新的血统出现
with an even more radical strategy to deal with the fish problem.
其应对鱼类问题的策略更加激进
They were the first coleoids
这个血统就是最早的鞘形类
Like Sphooceras millions of years before them,
就像是在百万年前出现的Sphooceras一样
coleoids wrapped their soft mantles around their hard shells.
鞘形类用外套膜包裹他们坚硬的外壳
But unlike Sphooceras, they kept it there permanently.
但不同于Sphooceras 鞘形类的外套膜包裹是永久性的
Hematites, for example, was one of the earliest coleoids,
举例来说 赤箭石属 就是早期的鞘形类之一
and it had a cone-shaped shell inside its soft body.
它柔软的身体里有一个锥形的外壳
Then, over millions of years, the shells began to shrink,
然后 经过数百万年 外壳开始收缩
and what remained was built with lighter-weight material.
留下的则由被重量更轻的材质构成
After all, internal shells no longer offered protection,
最终 被包裹在内的外壳不再起到保护作用
so there was no reason to keep lugging around all that extra weight.
所以也没有理由继续搬运多余的重量
So they lost the gas-filled chambers that had kept them afloat,
所以它们丢弃了让它们浮游的充气腔室
and developed new ways to stay buoyant,
而是发展了保持浮力的新方式
and new, faster forms of jet propulsion to get around.
以及新的 更快的喷射推进的游动方式
In time, the internal shell was streamlined down
最终 被包裹在内的外壳
to a long, chitinous structure,
被优化成一个长长的几丁质结构
kind of like a backbone, called a gladius.
有点类似于脊椎 被称为内置鞘
All squid alive today still have some kind of gladius,
今天所有活着的鱿鱼仍然由类似的内置鞘
while octopuses have a pair of similar structures called stylets.
而章鱼有一对被称为口器的类似结构
Armed with these adaptations, coleoids began to take advantage of a new niche:
有了这些演变 鞘形类开始抢占一个新的生态位
the deep sea.
深海
While the old gas-filled phragmocone couldn’t withstand the pressure of the deep ocean,
古老的充气闭锥无法承受深海的压力
the gladius had no such problem.
而内置鞘就不存在这个问题
And their ability to live in the deep turned out to be what saved coleoids from extinction.
结果证明 居住于深海的能力最终使得鞘形类免于灭绝
At the end of the Cretaceous Period,
在白垩纪末期
a fatal blow struck the ammonites and most of the nautiloids:
一次致命的陨石撞击摧毁了菊石类和大部分鹦鹉螺
the Cretaceous-Paleogene extinction event —
白垩纪-古近纪灭绝事件
the same event that killed the non-avian dinosaurs.
也是这次事件灭绝了非鸟类恐龙
Acid rain changed the PH of the oceans,
酸雨改变了海洋的PH值
compromising the integrity of the shells these animals needed to survive.
破坏了这些动物生存所需的外壳的完整性
This hit baby ammonites, which relied on their thin, fragile shells to
这对菊石类的幼仔形成了严重威胁 因为它们依赖薄薄的
passively float near the ocean surface, especially hard.
脆弱的外壳 被动地漂浮接近海面的地方
At the same time, there was likely a massive die-off of ammonites’ main food source, plankton.
在同一时间 菊石的主要食物来源浮游生物可能大量死亡
The nautiloids were probably saved by their slow and steady lifestyles,
鹦鹉螺很可能是被它们缓慢而稳定的生活方式所挽救
and six species in two genera have survived to the present day.
两个属中的六个物种存活至今
But the coleoids were able to take refuge in the deep sea,
但是 鞘形类成功地在深海中避难
and were no longer dependent on their shells.
并且不再依赖于它们的外壳
So with the ammonites gone, when conditions improved,
所以随着菊石类消亡 一旦条件改善
the coleoids rose up and took their place.
鞘形类就成长起来并取而代之
Today, coleoids have colonized every marine ecosystem on the planet,
如今 鞘形类已经在地球上所有海洋生态系统聚居
and they play a vital role in ocean food webs.
并且在海洋食物网中发挥了关键作用
Instead of relying on a suit of protective armor,
它们不再依赖一套保护性盔甲
they now use intelligence, camouflage, and agility
而是利用智慧 伪装和敏捷
to outsmart predators and prey alike.
智胜捕食者和猎物
Their journey from small, passive molluscs
它们从小小的 被动的软体动物
to sleek, voracious predators
到造型优美的的 贪婪的捕食者的进化过程
took hundreds of millions of years of trial and error —
花了数亿年的时间反复试验
from developing shells to survive,
从为了生存进化外壳
to finally learning to thrive without them.
到最终学会在没有外壳的情况下茁壮成长
And the squid still swims around with its gladius intact,
而鱿鱼仍然带着它完好的内置鞘游动
and the octopus with its stylets
而章鱼则是带着它的口器
— reminders of the history they share with the shelled creatures of the past.
提醒人们它们与过去带有外壳的生物共享的历史
Thanks for joining us today!
感谢您今天的加入
And as always, I want to know more of what you want to learn more about!
像往常一样 我想要了解更多您想进一步学习的内容
So leave me a comment below,
在下面给我留言吧
and don’t forget to go to youtube.com/eons and subscribe.
别忘了去youtube.com/eons并订阅
And please. Tell people about how cool our channel is
同时 告诉人们我们的频道有多酷
And if you want to learn more about how life on earth functions
如果你想从我的朋友Dr. Joe Hanson 一位真正的生命生物学家那里
with a real life biologist my friend Dr. Joe Hanson,
了解更多关于生命究竟是如何运作的内容
you can check out It’s Okay To Be Smart, also from PBS Digital Studios.
你可以同样在PBS数字工作室中观看“It’s Okay To Be Smart”