According to Oparin, after the origin of the Earth, life forms were created by the combination of chemical substances in its water reserves. These were organisms made of single cells. From these, various types of life were born through the process of biological evolution. Literally the biological evolution means unrolling or unfolding to reveal modifications, i.e. gradual origin of complex life from simple life is called biological evolution. In other words, the origin of complex multicellular life by gradual changes from the simpler unicellular life over millions of years is called biological evolution. Decent with modification is the fundamental hypothesis of biological evolution.Living organisms are different from each other, but the structural organization of all of them is the same, that is, each body is made up of cells. The biological functions in all organisms are similar. All organisms begin their life fromsingle cell structure, zygote which reveals a common ancestor of these diverse organisms.