Have you ever wondered how Earth came into existence? This unique planet came into existence around 4.6 billion years ago. Earth is the third planet from the Sun and the only planet on which life exists. Like the Sun and the other planets, it was also formed out of dust clouds and gases. Would you believe that before taking its present shape, Earth was a fireball? Earlier, Earth revolved around the Sun in the form of a hot spherical body. Thousands of years later, it gradually started moving away from the Sun but still kept revolving around it. As it moved farther and farther from the Sun, the temperature of Earth kept decreasing. As it started cooling, its outer layer changed into a crust. With the hardening of this crust, cracks developed in it and molten material from inside started coming out. Over a period of millions of years, this molten material gave birth to mountains and valleys. As time passed, the thick layer of hot gases enveloping the Earth started cooling and turned into clouds. These clouds rained on Earth continuously for a long time. Rainwater accumulated in the low-lying areas of the Earth and these areas became the oceans as we know them today. With the passage of time, there were upheavals due to which its surface was raised high or pressed down. This resulted in many volcanoes. Slowly, Earth became calm and the seas and mountains took their definite shapes. Subsequently, around 570 million years ago, micro-organisms started developing on Earth. In the first 345 million years of Earth’s existence, marine (aquatic) animals came into existence. As time passed, the aquatic animals also underwent changes. In the next phase of evolution, the great reptiles came into existence. And finally, around a million years ago, man gradually evolved from lower two-legged species. Today, Earth has the most favourable climate among all the nine planets, which is required for the sustenance of life. Had there been no atmosphere around the Earth, human life would not have existed.