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逆向进化

How the Smallest Animal Got So Simple

A new episode of our podcast,
我们的播客目前推出了新一集:
Eons: Mysteries of Deep Time, is out now!
永恒:深时间之谜
In this episode, we grapple with one of the
在此集中 我们设法理解
biggest open questions in paleontology:
古生物学中最大的开放性难题之一:
what was the first dinosaur?
最早出现的恐龙是什么?
Check it out now wherever you get your podcasts!
无论你从哪点进本播客的 现在来看看吧!
Okay, I think that a lot of us tend to think about,
好的 我认为我们之中有许多人往往会
and talk about evolution in the wrong way
以错误的方式思考或谈论关于进化的内容
– and hear me out because I know I’ve done this for a long time too.
听我说完 因为我也曾有很长一段时间这样做
Like, we think of the aquatic creature
就像 我们构思出水生生物
climbing out of the water onto land for the first time,
第一次从水里爬上陆地
with the benefit of bigger, stronger forelimbs.
得益于其更大 更强健的前肢
We think of that terrible misleading illustration,
我们构思出的那糟糕的误导性插图
the so-called March of Progress,
号称“前进的步伐”
which gets so many things wrong,
有很多地方是错误的
not the least of which is the idea that
最主要的错误是以下观点:
humans are somehow the pinnacle – the ultimate culmination –
人类某种程度上是进化过程中的顶峰
of the evolutionary process.
或者说最终的高潮
We think that evolution only goes in one direction –
我们认为进化的方向只有一个
toward getting bigger, more advanced, and more complex.
变得更大 更先进 更复杂
But I’m here to tell you that’s not the case.
但这里我要告诉你的是 事实并非如此
Things turn out to be much more nuanced than that.
事实证明 进化方向与此大不相同
And the proof is what’s perhaps the most bizarre animal in the world.
证据是世界上大概最奇特的动物
Yes, this is an animal.
没错 这是一个动物
It’s among the smallest and simplest animals ever found.
它是目前已发现的最小和最简单的动物之一
It has no mouth and no nervous system,
它没有嘴巴和神经系统
and at least one species can’t even breathe oxygen.
且至少有一个种类甚至不能呼吸氧气
How did this thing come to be?
它是如何变成这样的呢?
Well, it used to be … something else.
唔 它过去曾是…其它东西
Something a lot more complex.
是更加复杂得多的东西
Until it evolved …
直到它进化…
but in a way that’s the opposite of the way we’re used to thinking.
但是哦 其进化方式与我们习惯认为的相反
The name of this strange little animal is myxozoan,
这个奇特的小动物名为粘原虫
which means “slime animal,”
意思是粘液动物 哈哈
I mean honestly I’ve been called worse…
说实话 我被叫过更难听的(外号)…
and there are more than 2000 known species of them today.
粘原虫目前已有超过2000个已知的物种
And they all make their living by being parasites –
它们都以寄生的形式生存
they rely on infecting hosts in order to survive.
依赖于感染宿主而存活
The vast majority of them have life cycles that involve alternating
它们中绝大多数都有着 这样的生命周期
between infecting invertebrates – mostly annelid worms –
在生命周期中 它们交替感染无脊椎动物(多为环节动物蠕虫)
and vertebrates – mostly fish.
以及有脊椎动物(多为鱼类)
And while their life cycles are kind of complex,
尽管它们的生命周期有点复杂
that’s really the only thing about them that is,
但那真是它们唯一的复杂之处了 真的
Like I said, they’re incredibly small, both in terms of their physical size and in the size of their genome.
我说过 它们的身体和基因组尺寸都惊人的小
For most of their lifecycle, they’re smaller than the width of a human hair.
在生命周期的大部分时间里 它们都比人类头发的宽度小
And one species has a genome that’s a contender
一个种类粘原虫只有一个很小的基因组
for the smallest animal genome ever known.
都可以竞争目前所知基因组最小的动物了
And they’re missing some of the most basic features that we associate with animals,
它们缺少一些和动物相关联的最基础特征
like a mouth, a gut, a nervous system,
比如嘴巴 内脏 神经系统
and true gametes – egg or sperm cells.
及真正的配子 如卵细胞或精细胞
This level of simplicity is why myxozoans were grouped together with
这种水平的简单使得粘原虫在1838年
single-celled protozoa for more than a century
被发现后 已有超过一个世纪之久
after they were discovered in 1838.
被归类于单细胞原生动物
But, over time, we began to realize
但久而久之 我们开始
that this classification just couldn’t be right.
意识到这种分类应该是不正确的
For one thing, myxozoans aren’t single-celled –
首先 粘原虫不是单细胞的
at least, not all the time.
至少并不总是
At certain points in their complex lifecycle,
在它们复杂的生命周期的特定时间点
they’re multicellular – although just barely,
它们是多细胞的 最多只由几个
consisting of only a few cells, at most.
细胞组成 尽管这也很罕见
And as researchers dove deeper and deeper into myxozoan biology,
科学家们对粘原虫的生活习性越来越深入地钻研
they came to a bizarre, but inescapable, conclusion:
最终得出了一个奇怪但却无法避免的结论:
Myxozoans started out much larger and more complex – specifically,
粘原虫一开始是更大 更复杂的动物 具体来说
they used to look something like modern jellyfish.
它们过去看起来像现代的水母
But then, over time, they shed many of their biological components to
但久而久之 它们脱落了大部分生物组件
become much smaller, simpler animals.
变成小得多 简单得多的动物
Experts were able to figure this out,
专家们能把这个弄明白
because these animals still hold
是因为这些动物仍保留着
some clues about their evolutionary past.
一些它们过去进化的线索
For example, they still have a few genes that are found only in the animals
例如 它们仍具有一些仅在刺细胞动物
known as Cnidarians, which include jellyfish, sea anemones, and corals.
包括水母 海葵 珊瑚中才能找到的基因
And an analysis of their DNA has found that myxozoans are closely related to
它们的DNA分析发现 粘原虫与刺细胞动物的
the subgroup of cnidarians called Medusozoans – aka the jellyfish.
子群密切相关 该子群叫水母纲 又名水母
And beyond the genetic evidence, they’ve also kept at least one physical feature
除基因证据外 粘原虫从过去类似水母演变到现在
from their ancient jellyfish-like past:
还至少保留了一个外在特征
they have nematocysts, or stinging structures.
具有刺细胞 或者说具有刺状结构
Nematocysts are only found in cnidarians,
刺细胞只存在于水母中
and they use them to do things like capture prey or defend against predators.
水母利用这些细胞捕获猎物或抵御捕食者等
And myxozoans still have them,
而粘原虫仍含有刺细胞
along with the genes and proteins needed to build and use them.
也含有构造和使用这些细胞所需的基因和蛋白
But they seem to have adapted them
但它们似乎为了寄生的生活方式
for their parasitic lifestyle.
而把刺细胞改造了
I think it’s funny that we call it a lifestyle-
我认为称之为生活方式挺有趣的
because it’s kind of a charitable way of putting it don’t you think.
因为 你不觉得这样说有点仁慈吗?
like
就像
keto diet is a lifestyle, stand up paddling is a lifestyle,
酮饮食是一种生活方式 站着划船也是一种
being a parasite is like….
而成为一个寄生虫就像…
I would classify that differently, but…
我会将其归为另一类 但…
They’ve lost the toxins that the nematocysts would normally inject,
粘原虫已失去了刺细胞平常注入的毒素
so they’re no longer used as stingers.
因此其刺细胞不再被当成刺使用
Instead, they’re used as anchors to help the animal attach to its host.
反而被当成了该动物连接到宿主身上的锚
It’s one of the few physical features that they kept and adapted,
这就是粘原虫保留并改造 而不是干脆一起
rather than just losing altogether.
丢失的为数不多的外在特征
This process of simplification is sometimes known as regressive evolution,
这个简化的过程有时被认为是退化演变
and it’s actually pretty common among parasites.
而且在寄生虫中真的非常常见
I mean, after all when species can exploit some of the features of their hosts,
我指的是 毕竟这些物种在利用宿主的某些特征时
many of their own features just become …
它们自身的许多特征就变成
redundant like – nothing more than evolutionary baggage.
像进化的包袱一样的累赘
These unnecessary features can then be lost over time,
随时间推移 这些非必需的特征就被丢弃了
which means the animal can expend less energy and resources on them.
这意味着该动物能对自身花费更少的能量和资源
For example, parasites often become less mobile,
例如 寄生虫经常变得不太灵活
because their hosts can move around and get them from place to place.
是因为它们的宿主可以到处移动 带着它们四处游走
And they have reduced sense organs because,
它们还简化了感觉器官 又是因为
again, the host is keeping track of their surroundings so they don’t have to.
宿主会时刻留意周围环境 所以它们就不用这么做了
But, we’ve never seen any other group of animals go through such
但是 我们从未见过任何其它动物经历了
an extreme process of regressive evolution as the myxozoans have.
像粘原虫这样极端的退化演变
I mean, at least one species of myxozoan
我是说 至少有一种粘原虫
has even lost its mitochondrial DNA, and with it,
甚至已丢失了它的线粒体DNA 同时
the ability to carry out aerobic respiration –
失去了进行有氧呼吸的能力
that’s the process of generating energy in the presence of oxygen –
有氧呼吸即在有氧环境下产生能量的过程
because the genes that make respiration possible are in the mitochondria!
因为能使呼吸作用进行的基因存在于线粒体内!
This makes it the only known animal that doesn’t,
这使其成为了唯一已知不使用
and physically can’t, use oxygen!
也不能使用氧气的动物
So how do we know that these parasites are actually animals,
那么 即使缺失了这么多动物界的特征
even though they’re missing so many hallmarks of the animal kingdom?
我们咋还知道这些寄生虫实际就是动物呢?
Well, they trace their ancestry to the animal kingdom,
嗯 因为追查到它们的祖先属于
which makes them still animals.
动物界 那它们就也还是动物
It’s kind of like how birds are still considered dinosaurs,
这有点像 鸟是怎么被认为仍是恐龙的
even though they don’t look much like what people picture
即使它们看起来并不很像人们听到恐龙
when they hear the word dinosaur.
这个词时 脑中浮现的形象
It all comes down to a group’s evolutionary history.
这归根结底是一个族群的进化历史
So when did myxozoans begin their weird evolutionary journey?
那粘原虫的古怪进化之旅是何时开始的呢?
Well, the fossil record of myxozoans is basically non-existent,
嗯 粘原虫的化石记录基本上是不存在的
because they’re tiny and … squishy.
因为它们又小又…湿软
But by comparing the genes of myxozoans with the genes of other animals,
但通过比较粘原虫与其它动物的基因
researchers could roughly calculate how long it would have taken the differences – aka
科学家们可以粗略计算它们在彼此分化后
mutations – to take place, after they diverged from each other.
已经产生差异 或者说发生突变多久了
And this analysis, called the molecular clock,
而这个分析 叫做分子钟
places the origins of the group at around 650 million years ago,
分辨出这个族群起源于大约6.5亿年前
when they last shared a common ancestor with the Medusozoans.
当时它们与水母纲拥有着最后的相同祖先
These ancient roots make them the oldest known
这古老的起源使粘原虫成为了已知的
parasitic animals.
最古老寄生动物
And while they were probably more anatomically complex than they are now,
虽然古老粘原虫的生理结构可能比现在的更复杂
their lifecycles were simpler back then.
但它们的生命周期在当时却是更简单的
Today, most myxozoans alternate between using invertebrates and fish as hosts,
在今天 大多数粘原虫交替采用无脊椎动物和鱼作为宿主
but 600 million years ago, fish didn’t exist yet.
但6亿年前 鱼还不存在呢
So scientists think that their original hosts were
因此 科学家们认为它们最初的宿主是
aquatic invertebrates, like bryozoans and annelid worms.
水生无脊椎动物 如苔藓虫和环节动物蠕虫
Then, when fish arrived on the scene over a hundred million years later,
然后 当鱼在1亿多年后出现时
myxozoans began infecting them too,
粘原虫也开始感染它们了 然后
and they wound up with the two-host, invertebrate-to-fish lifecycle
最终形成了双宿主 即无脊椎动物-鱼的生活方式
that most of them still have today…
也就是如今大部分粘原虫的生活方式
which probably happened independently multiple times,
该生活方式可能在鱼类每次进化出新族群时
as new groups of fish evolved.
都独立发生了多次
As their hosts diverged and radiated, so did the myxozoans,
当宿主出现分化并分散到不同地方时 粘原虫
in a co-evolutionary dance that continues to this day.
也开始了持续至今日的分化-分散联合进化
And this is probably the key to the incredible diversity of living myxozoans.
这可能是活粘原虫具有惊人多样性的关键
The 2000-or-so species that have been described
截止目前被描述的大约2000个粘原虫物种
so far represent around 20% of all known cnidarians.
占已知刺细胞动物的大约20%
The story of the myxozoans makes us fundamentally reconsider what
粘原虫的故事通过展示 在给予足够的时间
it means to be an ‘animal’, by showing just how unrecognizable one can become,
和正确的选择压力后 动物会如何变得难以辨认
given enough time and the right selective pressures.
使我们从根本上重新思考什么意味着“动物”
And it shows us that evolving doesn’t have to mean becoming bigger,
并且它告诉我们 进化并非意味着必须变得更大
or more complex.
或更复杂
Sometimes, less really is more.
有时候 少亦即是多
Okay Before you go,
好了 在你退出前
big news for history buffs like myself-
面向我这样的历史爱好者的大新闻:
PBS Digital Studios has a new history-themed channel
PBS数字频道有一个叫PBS起源的
called PBS Origins, and we think you’re gonna love it.
新的历史主题频道 我们认为你们会喜欢
The channel will be home to multiple shows
多个节目会在该频道发布
including a new series called Historian’s Take.
包括一个新的系列 叫历史学家之见
This show explores history through the lens of pop culture,
这个节目通过流行文化的视角探索历史
examining what that culture can tell us about its time period,
考察这种文化在其所在时期的样子
and what it means for our future.
以及其对我们的未来意味着什么
PBS Origins will have even more new stuff on the way,
PBS起源以后甚至会有更多的新鲜玩意
so hit the link in the description and subscribe to the channel so you don’t miss anything!
因此点击简介中的链接 订阅该频道 就不会错过任何消息
And thanks to this month’s Eontologists, who are always with us:
还要感谢本月总与我们同在的“永恒学家”Kallie
Kallie told me that was supposed to be a parasite pun but…
她告诉我(粘原虫这个名字)应该是寄生虫的双关 但…
感谢本月的“永恒学家”:(见屏幕)
By becoming an Eonite at patreon.com/eons,
成为patreon.com/eons的“永恒粉”
you can get fun perks like submitting a joke for us to read,
你可以得到有趣的特权 如提交供我们阅读的笑话
like this one from Jacob
就像下面来自Jacob的这则:
How do blind megafauna read their books?
眼盲的巨型动物要怎么看书呢?
They get them printed in La Breaille
它们将书打印在La Breaille上
And as always thanks for joining me in the Adam Lowe studio.
像往常一样 感谢你与我一起在亚当·劳工作室度过
Subscribe at youtube.com/eons for more fabulous fossils.
在youtube.com/eons订阅本频道 观看更多难以置信的化石

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视频概述

粘原虫,最小的动物,最古老的寄生虫,进化过程中越变越简单,它是如何做到的?本视频将带你理解不一样的进化。

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视频来源

https://www.youtube.com/watch?v=GKh8g44UnAc

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