Millions of animals and plants exist now on the surface of the earth. Naturalists through many centuries tried their best to gather the knowledge about those varieties of living organisms. Naturally there have many queries about the beginning of Life and diversities of animals and plants have been developed in the mind of the naturalist. How the life arose on this earth and where and when did the first appearance? Did all the organisms appear at a time or at different times and how they appear? There were so many such types of questions in the mind of the naturalist. All the questions are not yet been solved by some of the problems about the origin of life and their diversities have been solved some extent. There have many more theories in the relation of the beginning of Life. We now hear by going to discuss about some important theories on this regard.
Theory of special creation :
This is the theory which states that live on earth is a special creation by divine or spiritual power; since then, the life is continuing to exist in the condition unchanged. This concept has been that teaching of the authorities of the religious persons for many centuries. Though the theory is not based on the facts but it may have some historical importance.
Spontaneous generation theory :
People of many ancient centuries believed that various forms of organisms were developed directly from the mud, manure and other nonliving substances. This theory believed that the living organisms were developed instinctively from nonliving substances. It was thought that fishes, toads and frogs arose from mud; maggots were developed from decaying flesh and microorganisms were formed suddenly from air or water. But it was Louis Pasteur (1822-1895), a great French microbiologist demonstrated that living organisms cannot arise from nonliving substances and finally the purity of spontaneous generation for origin of life was disapproved.
The catastrophism theory :A famous palaeontologist, Baron George Cuvier (1769-1832) strongly advocated this theory. According to this theory, there had been several creations of the life on the earth and at the end of creation there was a great catastropy, that and wiped out the living organisms and a new set of life was created again and again. The existence of fossils on the earth can explain by this theory but it failed to explain that how the close relationship have been developed between the fossils of the recent layer with the existing life. As a result this theory of origin of life was completely rejected by most of the geologists.
The Cosmozoic theory :
This theory explains that prehistoric life may have originated in some other planet and from that planet those primitive life reached in the earth accidentally. But how those living methods come to the earth and could survive the extreme climatic conditions of interplanetary space? This theory failed to explain it. However the space scientists are still trying to search for the existence of life in some other planets.
Chemosynthesis theory of origin of life :
The origin of life is actually concerned with the conditions and mechanisms that are occupied in the creation of most ancient living structures. From the beginning of human civilisation, man has speculated on the origin of life. In the ancient teachings on the basis of religious faith it was spoken that the life is created by some supernatural power. After that it was believed for many centuries about the spontaneous beginning of Life from nonliving substances. But the modern thought about the origin of life is based on the evolution of plain chemical substances that lead to the formation of organic materials such as protein or protein like compounds. In the theory of chemosynthesis it has been tried well explain about the beginning of Life as a modern concept.