Our Earth was born around 4.6 billion years ago. Like the sun and other planets, it was also formed out of the clouds of dust and gases. However, before turning onto present shape it was a fireball surrounded by an atmosphere of burning gases. At that time it revolved round the sun in the form of a hot spherical body. Hundreds of years latter, it gradually started moving away from the sun while still revolving round it. As it moved further & further from the sun, its temperature kept on decreasing. It started cooling off its temperature kept on decreasing. It started cooling off its outer layer changed into a crust. With the hardening of this crust, cracks developed in it and molten material gave birth to mountains and valleys.
As time passed by, the thick layer of hot gasses enveloping the Earth for a long time. There was upheavals due to which its surface was raised high or pressed down. This produced many volcanoes slowly it became calm and the seas and mountain took there definite shapes.
Subsequently, around 570 million years ago, micro organisms started developing on the earth. In the first 345 million years, marine ( aquatic ) animals came into existence. As more time passed, the aquatic animals also underwent changes.
Today, earth has all favourable conditions required for all the existence of life. It has an atmosphere absolutely essential for the living beings.
