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Many millions of years later, the first extremely small plants began to appear on the surface of the water.
好几百万年以后，水的表面开始出现了极小的植物。
They multiplied and filled the oceans and seas with oxygen, which encouraged the later development of early shellfish and all sorts of fish.
它们繁殖起来就使得早期的海洋充满了氧气，这为早期的贝类及其他各种鱼类的发育进一步创造了条件。
Next, green plants began to grow on land.
接着，绿色是植物开始出现在陆地上。
They were followed in time by land animals.
随后，陆地上出现了动物。
Some were insects.
其中有一些是昆虫。
Others, called amphibians, were able to live on land as well as in the water.
另外一些叫两栖动物，既能生活在陆地上水里，也能生活在水里。
Later when the plants grew into forests, reptiles appeared for the first time.
后来，当这些植物成长为森林，爬行动物首次亮相。
They produced young generally by laying eggs.
它们一般是通过孵蛋而繁衍后代的。
After that, some huge animals, called dinosaurs, developed.
再后来一些叫做恐龙的巨型动物出现了。
They laid eggs too and existed on the earth for more than 140 million years.
它们也生蛋，在地球上生存了一亿四千多年。
However, 65 million years ago the age of the dinosaurs ended.
然而，六千五百万年前，恐龙时代结束了。
Why they suddenly disappeared still remains a mystery.
它们为什么会突然灭绝至今仍是个谜。
This disappearance made possible the rise of mammals on the earth.
恐龙的灭绝使地球上哺乳动物的产生有了可能。
These animals were different from all life forms in the past, because they gave birth to young baby animals and produced milk to feed them.
哺乳动物不同于以往所有的生物形式，因为它们能从体内生产出幼仔并给幼仔哺乳。
Finally about 2.6 million years ago some small clever animals, now with hands and feet, appeared and spread all over the earth.
最后，大约260万年前，一些小巧聪明、长着手脚的动物出现了，它们散布在地球的各个地方。
Thus they have, in their turn, become the most important animals on the planet.
于是，他们接着成为了这个行星上最重要的动物。
But they are not looking after the earth very well.
然而，他们对地球却并不怎么在意。
They are putting too much carbon dioxide into the atmosphere, which prevents heat from escaping from the earth into space.
它们把过多的二氧化碳释放到大气层中，这使得地球上的热不能释放到太空中去。
As a result of this, many scientists believe the earth may become too hot to live on.
因此很多可科学家相信地球可能会因此变得太热而不适合生物的生存。
So whether life will continue on the earth for millions of years to come will depend on whether this problem can be solved.
所以，在未来的数百万年中，生命能否在地球上延续取决于这个问题能否得到解决。
