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We are tetrapods, to whit one, two, three, four.
我们是四足动物，瞧着，1，2，3，4
Every mammal, every dog, cat is, is a tetrapod, four legs.
所有哺乳动物、每只狗、每只猫都是四足动物，都有4条腿
A horse is a tetrapod evidently enough, so is a bird.
马很明显是四足动物，鸟类也是
All the reptiles, all the frogs, the salamanders and even the snakes which don't have legs at all,
所有爬行动物、所有青蛙、蝾螈、甚至是没有腿的蛇
whales which don't seem to have legs, they're all tetrapod animals.
还有看似没有腿的鲸鱼，它们都是四足动物
Scientists became bewitched by the fact that under the skin all tetrapods are basically the same.
所有四足动物皮下结构基本相似的现象使科学家们很感兴趣
They all had spines kept firm by special interlocking spurs.
它们都具备由特殊关节突起牢固连接的脊柱
It is as true of us as it was of the dinosaurs.
我们和恐龙都是如此
All tetrapods had a pelvis attached to the backbone to support their weight.
所有四足动物都有与脊椎相连的骨盆以支撑它们的体重
They all had a ribcage to protect their heart and lungs, and they all breathed air through nostrils.
它们都有用以保护心脏和肺的胸廓，它们都用鼻孔呼吸空气
Their limbs invariably consisted of one single bone at the top.
它们的四肢总是由上面的一根骨头
A pair of bones underneath leading to feet or hands
和下面连着足或爪的一对骨头组成
which scientists noted never seemed to have more than five fingers or toes.
而且科学家们发现似乎从来没有超过5根手指或脚趾
It was true of dinosaurs, human beings and even whales, for under their flippers they have five fingers.
这一现象存在于恐龙、人类、甚至是鲸鱼中，因为它们的鳍肢内也有五趾
This similarity convinced scientists all tetrapods must come from just one type of creature, a single common ancestor.
这种相似性使科学家们确信，所有四足动物都起源于一类动物，一个共同的祖先。
