The organic molecules formed in primitive ocean under primitive reducing conditions of earth served as the only source of carbon for first life form of earth. The first life forms, bacteria like prokaryotic cell, originated from protocell in Archaeozoic era and were anaerobic chemoheterotrophs that used the organic component as a source of energy and carbon. The organisms that use organic/inorganic compounds as their carbon source are called as heterotrophs while those that can synthesize their two organic compounds are known as autotrophs. Since the first life forms fed on available organic compounds, they were heterotrophs. Due to the limited supply of organic compounds, organisms evolved photosystem to capture sunlight to be used as a source of energy thereby leading to the origin of photoautotrophs. The organisms that use light as a source of energy and carbon dioxide as a chief carbon source are referred to as photoautotrophic organisms. phototrophs appeared much later in evolution.