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为什么人有第三层眼皮? – 译学馆
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为什么人有第三层眼皮?

Why do humans have a third eyelid? - Dorsa Amir

TED-Ed 值得分享的课程
大脑是我的一切 沃森 身体不过是一个附件罢了——阿瑟·柯南道尔《福尔摩斯探案集》
You know that little pink thing nestled in the corner of your eye?
你知道位于眼角那粉红色的小东西是什么吗?
It’s actually the remnant of a third eyelid.
它叫做“半月皱襞”
Known as the “plica semilunaris,”
实际上是剩余的第三层眼睑
it’s much more prominent in birds and a few mammals,
这种结构在鸟类和一些哺乳类动物中更为显著
and functions like a windshield wiper to
它们的作用就像是挡风玻璃一样
keep dust and debris out of their eyes.
防止灰尘和碎屑进入眼睛
But in humans, it doesn’t work.
但是在人类中 它不能发挥作用
It’s vestigial, meaning
这是因为它退化了
it no longer serves its original purpose.
也就是说无法发挥原有的功能
There are several other vestigial structures
人体中的一些其他结构
like the plica semilunaris in the human body.
也像半月皱襞一样退化了
Most of these became vestigial long before homo sapiens existed,
它们大多数早在智人存在前就已经退化了
quietly riding along from one of our ancestor species to the next.
悄悄地从我们的一个祖先物种传给下一代
But why have they stuck around for so long?
但为何它们存在了如此之久?
To answer this question,
为了回答这个问题
it helps to understand natural selection.
我们要先了解自然选择
Natural selection simply means that traits
简单来说 自然选择意味着
which help an organism survive and reproduce in a given environment
那些在特定环境中有利于物种生存和繁殖的性状
are more likely to make it to the next generation.
更容易遗传给下一代
As the environment changes,
随着环境的改变
traits that were once useful can become harmful.
曾经有用的性状变得有害了
Those traits are often selected against,
那些性状通常就会被淘汰
meaning they gradually disappear from the population.
也就是说它们逐渐地从群体中消失了
But if a trait isn’t actively harmful, it might not get selected against,
但如果一个性状不是非常有害的话
and stick around even though it isn’t useful.
即便它没有什么作用 可能也不会被淘汰
Take the tailbone.
拿尾椎骨来说
Evolutionary biologists think that as the climate got drier
进化生物学家认为 随着天气的干燥
and grasslands popped up,
以及草原的出现
our tail-bearing ancestors left the trees
我们的有尾祖先便从树上下来
and started walking on land.
并开始在陆地上行走
The tails that had helped them in the trees
这种在树上可以帮助他们的性状
began to disrupt their ability to walk on land.
开始妨碍他们在陆地上行走
So individuals with mutations that reduced the length of their tails
所以那些产生了使尾巴变短的突变的个体
became more successful at life on land,
在陆地上就变得更加成功
surviving long enough to
因此可以生存足够长的时间
pass their short tails on to the next generation.
以便把短尾性状传递给下一代
The change was likely gradual over millions of years until,
数百万年中 这种改变都是潜移默化的
about 20 million years ago,
直到大概两千万年以前
our ancestors’ external tails disappeared altogether.
我们祖先外部的尾巴完全消失了
Today, we know human embryos have tails
如今 我们知道人类胚胎仍有尾巴
that dissolve as the embryo develops.
只不过随着胎儿发育 尾巴逐渐消失了
But the stubby tailbone sticks around,
但是短小而粗壮的尾椎骨保留了下来
probably because it doesn’t cause any harm—
或许是因为它并不会造成任何伤害
in fact, it serves a more minor function
实际上 它的作用很小
as the anchor point for certain other muscles.
只不过是一些特殊肌肉的锚定点
Up to 85% of people have a vestigial muscle
多达85%的人群拥有一块逐渐退化的肌肉
called the “palmaris longus.”
它叫做掌长肌
To see if you do,
要查看你是否有这块肌肉
put your hand down on a flat surface
将手放在平坦的表面上
and touch your pinkie to your thumb.
并用拇指触摸小指
If you see a little band pop up in the middle of your wrist,
如果你看到手臂中间隆起一条筋
that’s the tendon that attaches to this now-defunct muscle.
那就是附在现已失去用处的肌肉上的肌腱
In this case, the fact that not everyone has it
在这种情况下 并非每个人都具有这种肌肉
has helped us trace its function.
这可以帮助我们弄清它的功能
Vestigial traits can persist
当无诱使其消失的因素时
when there’s no incentive to lose them—
退化性状可以存留
but since there’s also no incentive to keep them,
但是由于保留它们的诱因也不存在
random mutations will sometimes still eliminate them
有时随机突变还是会
from part of the population.
把它们从种群中清除
Looking at our primate relatives,
看看和我们血统相近的灵长类动物
we can see that the palmaris longus is sometimes absent
我们会发现 那些待在陆地上更久的灵长类动物
in those that spend more time on the land,
有时是缺失掌长肌的
but always present in those that spend more time in trees.
但是那些经常呆在树上的灵长类都有掌长肌
So we think it used to help us swing from branch to branch,
因此我们认为它用来帮助我们在树枝间摆荡
and became unnecessary when we moved down to land.
而当我们移动到陆地上它们就变得多余了
The appendix, meanwhile, may once have been part of the intestinal system
同样地 阑尾曾经可能也是肠道系统的一部分
our ancestors used for digesting plant materials.
我们的祖先用它来消化植物
As their diets changed, those parts of the intestinal system began to shrink.
随着饮食结构的改变 那部分肠道系统开始退化
Unlike other vestigial structures, though,
而和其他退化结构不同的是
the appendix isn’t always harmless—
阑尾并不总是无害的
it can become dangerously inflamed.
危险的是它可能会发炎
For most of human history,
历史上大多数情况下
a burst appendix could be a death sentence.
急性阑尾炎是致人死地的疾病
So why did it stick around?
那为什么阑尾还被保留下来了呢?
It’s possible that it was very slowly on its way out,
有可能是因为它处于缓慢退化的过程中
or that mutations simply hadn’t arisen to make it smaller.
或者根本不存在使它缩小的突变
Or maybe it has other benefits—
又或者它有着其他的益处
for example, it might still be a reservoir of bacteria that helps us break down food.
例如 它可能容纳着帮我们降解食物的菌类
But the fact is, we’re not really sure why the appendix persists.
但事实是 我们并不清楚阑尾存在的原因
Evolution is an imperfect process.
进化是一个不完美的过程
Human beings are the result of millions of years of trial, error, and random chance—
人类是数百万年不断地试验 犯错和随机事件的结晶
and we’re full of evolutionary relics to remind us of that.
进化遗留物时刻提醒着我们这一点
No organs evolve faster and into more varied shapes
没有哪个器官比生殖系统的器官
than those evolved in procreation.
进化得更快 形状更多样化
Watch this video to learn more about
观看这段视频来了解更多
the evolution of animal genitalia
关于动物生殖器的进化
Or consider that humans are one of only five
考虑到人类是地球上
mammals on earth that mainstream
仅有的五种主流哺乳动物之一
Can you guess what the other four are?
你能猜出其他四种是什么吗?
Watch this video to find out.
观看这个视频来寻找答案

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