The Evolution of the Earth
Scientists are still debating about how our Earth was formed. Most scientists
are of the view that Earth began to form around four and a half billion years ago.
The Earth has reached its present form through several phases. It began as a ball of
swirling dust and clouds, and passed through a molten stage. At that time, the Earth
was very hot and was constantly bombarded by massive rocks and other materials
from the space. In this way, the size of the Earth grew. The Earth was so hot that it
was molten (in hot liquid form). If you boil a thick soup containing many substances,
you may observe that the heavier particles tend to go to the bottom and lighter
particles come to the top. These lighter particles cool at the top and form a layer
of crust (like the cream of milk). Similarly, while heavier substances formed the
part of the molten core, lighter substances rose to the surface and cooled. Slowly,
an upper crust of lighter and cooler materials formed covering the molten interior.
As the Earth's interior continued to cool, it contracted and the outer crust
wrinkled, forming ridges (mountains) and basins (low areas which became oceans
later).
The atmosphere of the Earth consisted of different kinds of gases including
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Explanation:
The Earth was formed at the same time as the rest of the solar system by the coming together of materials that were not incorporated into the sun. The Earth was very hot (perhaps molten) at first, but quickly cooled off. Oceans appeared very early and have been on the surface continuously ever since. Life in the form of single-celled algae was present when the Earth was 3.5 billion years old. Animals with hard skeletons and shells appeared and formed fossils 540 million years ago. The period since then is usually broken up into the Paleozoic Era (“old life”) from 540 to 225 million years ago, the Mesozoic Era (“middle life”) from 225 million to 65 million years ago, and the Cenozoic Era (“modern life”) from 65 million years ago to the present. Dinosaurs flourished in the Mesozoic, whereas human beings have been present for only a few million years.