The history of the Earth has been colourful as today’s Earth is rather different than it was in its beginning. For example, the atmosphere of the Earth did not have the oxygen and carbon dioxide concentrations it has today. Oxygen was added to the atmosphere by photosynthesising plants, and they needed carbon dioxide in the first place to photosynthesise.
Therefore, they could not have been the ones to start off life. Autotrophs create their own energy-yielding molecules e.g. glucose. Heterotrophs don’t, and instead consume autotrophs. Since autotrophs could not have carried out photosynthesis without carbon dioxide, heterotrophs must have been the ones who evolved first. This is known as the heterotroph hypothesis.
Most heterotrophs today, including humans, consume autotrophs, so what did the early heterotrophs consume? They consumed the chemicals themselves, the initial building blocks of organic life. Sources of these molecules could have been lightning, UV radiation and heat.
The initial protocell would have collected organic molecules and natural selection would have resulted in its ability to extract energy from ATP. Then the protocell could divide and reproduce itself.
On early Earth the pre-existing biological materials present today such as seeds, organic matter, oil, etc. were not available. Today’s biosphere is the result of…