CBSE Class 6 Answered

Our Earth is about 4.5 billion years old. Earth was hot earlier and the atmosphere mainly consisted of carbon dioxide and other gases like ammonia and methane. Many theories have been forwarded by the scientists regarding the evolution of life forms on the Earth. One main theory is that living things developed from molecules which could replicate. The life on Earth may have begun due to the following reasons:

• Organisms were probably produced by the conditions on earth at that time
• They came from somewhere else, such as another planet in our Solar System

Over the years these molecules joined other molecules and became complex and dependent on each other. The earliest samples of life forms formed in fossils are prokaryotes (cellular organisms without a membrane enclosed nucleaus), just like bacteria. Samples of Blue green algae have also been found in rock structures. Thereafter life began to evolve on the planet Earth.