How life began on the earth
No one knows _______ how the earth began, as it happened so long ago. However, according to a widely accepted ______, the universe began with a “ Big Bang “ that______ matter in all directions. After that, ________ began to form and combing to create stars and other bodies.
For several ______ 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______. The earth became so ______ that it was not clear whether the shape would _____or not. It ______ loudly with fire and rock. They were in time to produce carbon, nitrogen, _________and other gases, which were to make the earth’s ________. What is even more important is that as the earth cooled down, water began to appear on its surface.
Water had also ______ on other planets like Mars but, unlike the earth, it had disappeared later. It was not immediately ______ that water was to be ______ to the development of life. What many scientists believe is that the continued______ of water allowed the earth to ______ harmful gases and acids into the oceans and seas. This produced a ______, which made it possible for life to develop.
Many millions of years later, the first extremely small plants began to appear on the ______ of the water. They ______ and filled the oceans and seas with _______, which _________ the later development of early shellfish and all sorts of fish. Next, green plants began to grow _______. They were followed _______ by land animals. Some were insects. Others, called amphibians, were able to live on land ______ in the water. Later when the plants grew into forests, reptiles _______ for the first time. They produced young generally by________. After that, some huge animals, called dinosaurs, ________. They laid eggs too and______on the earth for more than 140 million years. However, 65 million years ago the age of the dinosaurs ended. Why they suddenly disappeared still remains a ______. This disappearance made ______ the rise of mammals on the earth. These animals were ______ all life forms in the past, because they ______ young baby animals and produced milk to______them.
Finally about 2.6 million years ago some small clever animals, now with hands and feet, appeared and ______ all over the earth. Thus they have, ______, become the most important animals on the planet. But they are not looking after the earth very well. They are putting too much ______ into the______, which ______ heat from escaping from the earth into space. As a result of this, many scientists believe the earth may become too hot to______. So whether life will ______ on the earth for millions of years to come will ______ whether this problem can be solved.
相关推荐：
loading