The earth is a special planet in the solar system. The explosion of the earth produced carbon, nitrogen, vapor and other gases,       (make) the earth’s atmosphere. What is even more important is _  as the earth cooled, water which is fundamental to the    （develop） of life appeared and stayed on the surface. The continued presence of water allowed the earth to dissolve       (harm) gases into the oceans and this produced a chain reaction which made it possible for life to develop.
Many millions of years later, the _  （arrive） of small plants encouraged the development of early shellfish       all sorts of fish. Then, with the first green plants appearing on land, land animals appeared and dinosaurs developed and existed on the earth for millions of years. Then Dinosaurs’ sudden disappearance made the rise of mammals possible on the earth. They were different from all life forms      ever existed in the past because they produced young from within their bodies. Finally, some small clever animals with hands and legs ______(appear) and spread all over the earth. Thus they, in their turn, have become       most important animals. However, they are putting too much carbon dioxide into the atmosphere. So for millions of years to come,       life will continue on the earth will depend on whether the present problem can be solved.