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Unit 4
第四单元
Astronomy: The Science of the Stars
天文学 星星的科学
Reading How Life Began on the Earth
阅读 地球上生命的起源
No one knows exactly how the earth began, as it happened so long ago.
没有人确切知道地球是怎样开始形成的，因为在很早很早以前它就形成了。
However, according to a widely accepted theory, the universe began with a "Big Bang" that threw matter in all directions.
然而，一种普通为人们所接受的理论是：宇宙起源于一次大爆炸，这次大爆炸将物质投射到四面八方。
After that, atoms began to form and combine to create stars and other bodies.
然后，原子开始形成并结合成恒星和其他天体。
For several billion years after the "Big Bang", the earth was still just a cloud of dust.
大爆炸后好几十亿年过去了，但地球还只是一团尘埃。
What it was to become was uncertain until between 4.5 and 3.8 billion years ago when the dust settled into a solid globe.
随后它会变成什么没人能知道，直到38-45亿年前，这团尘埃才慢慢地形成一个固体的球状物。
The earth became so violent that it was not clear whether the shape would last or not.
地球（开始）变得激烈动荡，不知道这个固体形状是否会继续存在下去。
It exploded loudly with fire and rock.
它（地球）猛烈的爆炸喷出了烈火与岩石。
They were in time to produce carbon, nitrogen, water vapour and other gases, which were to make the earth's atmosphere.
最终产生了碳、氮气、水蒸气和其他多种气体，从而形成了地球的大气层。
What is even more important is that as the earth cooled down, water began to appear on its surface.
更为重要的是，地球冷却了下来，地球的表面就开始出现了水。
Water had also appeared on other planets like Mars but unlike the earth, it had disappeared later.
在火星这些其他行星上都出现了水，但和地球不同的是，这些水后来都消失了。
It was not immediately obvious that water was to be fundamental to the development of life.
水会对生命的发展起关键作用，这一点在当时并不明显。
What many scientists believe is that the continued presence of water allowed the earth to dissolve harmful gases and acids into the oceans and seas.
很多科学家相信，由于地球表面上长期有水存在，它使得地球得以把有害气体和酸性物质溶解在海洋里。
This produced a chain reaction, which made it possible for life to develop.
这就产生了一系列的反应，使得生命就有可能开始发展了。
