听力文本

But then the doubts crept in.
可是后来一些怀疑的声音却掺杂进来。
Only Jarvik had ever analysed the ichthyostega fossil and you just had to take his word for it,
只有Jarvik研究过这块鱼石螈化石，人们只能参照他的说法，
but worse, mutterings began that there was a gaping hole in the story.
更糟的是，有人开始议论这个故事中有一个破绽
Eusthenopteron could not be the immediate ancestor of ichthyostega.
真掌鳍鱼不可能是鱼石螈最近的祖先
The difference between them was too vast.
它们之间的差异太大了
Ichthyostega was a fully formed tetrapod with a ribcage, pelvis attached to the backbone,
鱼石螈是完全成形的四足动物，拥有与脊椎相连的胸廓和骨盆
a spine with interlocking spurs and limbs with fingers and toes.
具备关节突起的脊柱，以及长着爪和趾的四肢

The fish was still a fish, despite its primitive leg bones.
而鱼仍旧是鱼，尽管它有了原始的腿骨
It showed little sign of evolving any of these other tetrapod features.
它几乎没有其它向四足动物进化的特征迹象
To prove one had become the other they needed more evidence.
要想证明一种动物进化成了另一种动物，他们还需要更多的证据
What this really adds up to is that these changes seem to be too gross to have happened in one step.
这里总结起来可以看出，这些变化实在太大了，不可能一步到位
There must be missing animals in here.
这中间一定有缺失的物种
For a start somewhere out there should be other species of Devonian tetrapods
可以预见，某个地方肯定还有其它四足动物
which might shed some light on the theory, but above all they needed to find an intermediate animal,
或许能说明这一理论，但他们首先需要做的就是找到中间物种
one that showed the changes between fish and tetrapod actually happening,
一种能解释鱼如何演变成四足动物的物种
some strange beast that could walk, but was half fish/half tetrapod that was a missing link.
一只能够行走，但却是半鱼半四足类的奇怪动物，它就是进化缺环。

词汇解析

1.creep in 悄悄混进
If you creep in, please don't let it show.
如果你想在外面偷偷混进来，请不要让人知道。

2.take one's word for it 相信某人说的话
But don't take their word for it.
但是，也不要把他们的话当真。

3.in one step 一步完成
We drill the holes, fill them with sodium and seal them all in one step.
钻孔填钠密封，一步就可以搞定。

视频及简介

19世纪，英国生物学家赫胥黎在研究了尼安德特人的头骨以后，认为是一个具有猿脸的头骨，但决不是人与猿之间的中间缺环。这就告诉我们，人与猿之间的亲缘关系，似乎还应该有一环。1891年发现直立猿人（爪哇猿人）的骨胳，证明这个缺环确实是存在的。可是，日本的人类学家认为爪哇猿人尚不是真正的中间缺环。那么，在猿与人之间到底应该是什么样的一环呢？在埃塞俄比亚发现的早期人科动物化石，很可能是过渡物种。这一发现，弥补了原始人类进化中的一个“缺环”。