The entire story of the origin of life is based on the assumption as no one has seen it happening. But, there are some evidences which helped to trace the possible events of the past.
The origin of life is closely related or associated with the origin of the universe, i.e., the entire solar system including our planet, the earth.
The earth is presumed to have originated about 4.5 billion years ago. At that time, the earth had a reduced atmosphere and not oxidizing like that today. The earth was too hot to provide an environment for any living organism. The earth gradually cooled down and gases condensed with the passage of time. Torrential rains for thousands of years together resulted in the formation of water bodies. The atmospheric compounds and inorganic salts and minerals gradually dissolved in the large water bodies to form oceans. The oceans were like a ‘broth’ and gave rise to a variety of compounds and finally to self duplicating molecules. These self duplicating molecules ultimately enclosed in membranes derived from lipids and proteins forming the first ‘Cell like‘ structures.
The early atmosphere of the earth was devoid of oxygen. So, the first organism formed must be anaerobes. They must have been dependent on the organic molecules present in the ‘broth’. Such organisms are called chemoheterotrophs. Then sulfate-reducing bacteria called anaerobic chemoautotrophs came into existence. These organisms were able to obtain energy by the degradation of chemical compounds for the synthesis of food molecules. Next to occur were anaerobicphotoautotrophic bacteria. There is a recent finding that the aerobic photoautotrophs, i.e., cyanobacterium-like, oxygen-producing photosynthesizers originated from 3300 to 3500 million years ago. Such oxygen-producing photosynthesizers started liberating oxygen into the environment and changed the reducing atmosphere into an oxidizing life giving atmosphere for organisms. Therefore, oxygen became a means of releasing energy by respiration and autotrophs became a source of food for heterotrophs. In this way, photosynthesis and respiration became established as the main balancing mechanism of nature at an early stage. Hence from simple organisms more complex forms or organisms evolved with the passage of time. The first mammals originated on earth dates back to 220 million years.