No one knows exactly how the earth began, as it happened so long ago.
However, according to a widely accepted theory, the universe began with
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 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 loudly with fire and rock. They were in time to produce
carbon, nitrogen, water vapour and other gases, which were to make the
earth's atmosphere. What is even more important is that as the earth cooled
down, water began to appear on its surface.
Water had also appeared on other planets like Mars but, unlike the earth,
it had disappeared later. It was immediately obvious that 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, which 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, green 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. After 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 dinosaurs
ended. Why they suddenly disappeared still 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.
Finally about 2.6 million years ago some small clever animals, now with hands and feet,
appeared and spread all over the earth, Thus they have, in their turn, become the most
important animals on the planet. But they are not looking after the earth very well.
They are putting too much carbon dioxide into the atmosphere, which prevent heat from
escaping from the earth into space. As a result of this, many scientists believe the
earth may become too hot to live on. So whether life will continue on the earth for
millions of years to come will depend on whether this problem can be solved.