Many people think that the environment of the planet earth never changes. This is a wrong idea. The rivers, the mountains, the seas we see may remain unchanged during one's lifetime. But our earth has existed for long time and during the past millions of years the features of the earth's surface has changed beyond recognition. Once upon a time there were no mamals like cows, dogs, tigers, whales, men etc. on the earth's surface. Before that there was a time when not even a blade of grass grew on the surface of the earth. The continents and oceans looked quite different. Even now they are changing. The earth is 4.6 billion years old and only 0.22 billion years ago all the continents were close together. It is called Pangaia. At that time South India was touching Africa and the entire north India including the Himalayas was under the Tethys sea. The northern part known as Laurasia started separating from the southern part known as Gondoanaland. India separated from Africa and Antarctica only 100 million year ago. In these days Pangia was dominated by huge raptiles like the dinosasur. Trees and plants were quite different from what they are now. In those days even there were no monkeys or apes.