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1、How life began on the earthNo one knowsexactlyhow the earthbegan,as it happenedso long ago.However, according to a widely accepted,thetheoryuniverse began with a “BigBang” that threw matter in all directions. After that,atomsbegan to form and combine to create stars and other bodies.For several bill
2、ion years after the “Big Bang”, the earth was still just a cloud of dust. 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. The earth became so violent that it was not clear whether the shape would last or not. It exploded loudl
3、y with fire and rock. They were in time to produce carbon, nitrogen, water vapor and other gases, which were to make the earthssphere.atmo What is even more importants that as the earthcooleddown,waterbeganto appearon its surface.Water had also appeared on other planets like Mars but unlike the eart
4、h, it had disappearedlater. It was not immediatelyobviousthat water was to be fundamental to the development of life. 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. This produced a chain reaction, w
5、hich made it possible for life to develop.Many millions of years later,the first extremely small plants began to appear on the surface of the water. They multiplied and filled the oceans and seas with oxygen, which encouraged the later development of early shellfish and all sorts of fish. Next, gree
6、n plants began to grow on land. They were followed in time by land animals. Some were insects. Others, called amphibians, were able to live on land as well as in the water.Later when the plants grew into forests, reptiles appeared for the first time. They produced young generally by laying eggs. Aft
7、er that, some huge animals, called dinosaurs, developed. They laid eggs too and existed on the earth for more than 140 million years. However, 65 million years ago the age of the dinosaursended.Why they suddenlydisappearedstill remains a mystery. This disappearance made possible the rise of mammals on the earth. 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.