No one knows exactly ______ the earth began, as it happened so long ago. However, according to a widely ______(accept) theory, the universe began _____ a \"Big Bang\" that threw matter in all directions. After that, atoms began to form and combine to create stars and other bodies.
For several billion years ______ the \"Big Banghe earth was still just a cloud of dust. ______ it was to become was uncertain until _____ 4.5 and 3.8 billion years ago when the dust settled into a solid globe. The earth became so violent ______ it was not clear ______ the shape would last or not. It exploded loudly with fire and rock. They were in time to produce carbon, nitrogen, water vapour and other gases, ______ were to make the earth's atmosphere. ______ is even more important is that ______ the earth cooled down, water began to appear on _____ surface.
Water had also appeared on other planets like Mars ______but, unlike the earth, it had disappeared later. It was not immediately obvious _____ water was to be fundamental to the _____（develop） of life. What many ______(science) believe is that the continued presence of water allowed the earth to dissolve _____（harm） gases and acids into the oceans and seas. This produced______ chain reaction, which made_____ 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 _____（multiply）and filled the oceans and seas with oxygen, _____ encouraged the later development of early shellfish and all sorts of fish. Next, green plants began to grow on land. They _____(follow) 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 _____(appear) for the first time. They produced young generally by _____(lay) eggs. After that, some huge animals, called dinosaurs, developed. They _____(lay) eggs too and existed on the earth for more than 140 million years. However, 65 million years ago the age of the dinosaurs ended. _____ they suddenly disappeared still remains a mystery. This _____(disappear) made possible the rise of mammals on _____ earth. These animals were different _____ all life forms in the past, because they gave birth to young baby animals and _____(produce) milk to feed them.
Finally about 2.6 million years ago some small clever animals, now with hands and feet,
appeared and spread all _____ the earth. Thus they have, in their turn, become the most important animals on the planet. _____ they are not looking after the earth very well. They are putting too much carbon dioxide into the atmosphere, _____ prevents heat from _____(escape) from the earth into space. As _____ result of this, many _____(science) believe the earth may become too hot to live on. So _____ life will continue on the earth for millions of years to come will depend on whether this problem can _____(solve).
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