No one knows exactly     1     the earth began. However, according to a widely accepted theory, the universe began     2     a “Big Bang”. Then the dust     3    (settle) into a solid globe. As the earth cooled down, water began to appear on its surface. Many     4    (science) believe that it was the presence of water that allowed the earth to dissolve     5    (harm) gases and acids into oceans and seas, which made it possible for life to develop.

Millions of years later, the first small plants appeared on the surface of the water. Their development     6    (follow) by early shellfish, all sorts of fish, green plants and land animals. Reptiles     7    (produce) young generally by laying eggs while mammals gave birth to young baby animals. Dinosaurs existed on the earth for over 140 million years before they suddenly disappeared. The reason     8     they disappeared still remains a mystery.

Finally, humans appeared and spread all over the earth. But they are putting too much carbon dioxide into the atmosphere, which prevents heat from     9    (escape) from the earth into space. As a result of this, people believe the earth may become too hot to live on. So     10     life will continue on the earth will depend on how this problem can be solved.