Brief Account On Evolution:
About 2000 million years ago, the first cellular forms of life appeared on the earth. Some of these cells had the ability to release oxygen. Slowly, these single-celled organisms became multicellular life forms.
About 500 million years ago, invertebrates were formed. Jawless fish evolved around 350 million years ago. Sea weeds and few plants existed around 320 million years ago. The first organisms that invaded land were plants.
About 350 million years ago, the animals lobe fins (fishes) evolved into first amphibians who lived both land and water. These were the ancestors of modern day frogs and salamanders.
The amphibians evolved into reptiles which lay thick-shelled eggs that do not dry up in sun. The turtles, tortoises and crocodiles are the descendants of these reptiles. In the next 200 million years, reptiles of different shapes and sizes dominated on earth.
Some land reptiles went back into water to evolve into fish like reptiles about 200 million years ago. The land reptiles were the dinosaurs. About 65 million years ago, the dinosaurs suddenly disappeared from the face of earth.
The first mammals were like shrews. Their fossils are small sized. Mammals were viviparous and protected their unborn young inside the mother’s body. When the reptiles come down, mammals took over the earth.