The simplest form of life originated through four main stages:
- Organic molecules were produced before cells existed
- Polymerization of organic molecules formed polymers
- Polymers became enclosed in membranes
- Membrane bound polymers acquired cellular properties
Thereafter, a wide variety of organisms came into existence through biological evolution.
The sources of energy were the ultraviolet rays or electric discharge (lightening) or heat. Either alone or a combination of these energy sources caused reactions that produced complex organic compounds (including amino acids) from a mixture of ammonia (NH3), methane (CH4), water (H2O) and hydrogen (H2).
The amino acids are the building blocks of proteins which are the main components of protoplasm.
Simple organic molecules combined to form large molecules which included peptides (leading to the formation of proteins), sugars, starch and fat molecules.
The large molecules of different kinds combined together to form multi-molecular heaps or complexes. Some simple fat molecules arranged themselves around this molecular complex in a sort of membrane.
When such complexes reached a certain size they separated from the surrounding in the form of coacervate drops of microscopic size, moving in the liquid with a definite boundary. Coacervate like aggregates were probably the precursors of the first living cells.
Some sort of nucleoproteins or nucleic acids may have evolved by random combinations which have provided two more properties to coacervate-like bodies.
- Chemical reactions from the nucleic acids
- Capacity to reproduce through duplication of the nucleic acids
The primitive drop-like forms of life were all heterotrophs, unable to manufacture their own food but derived it from environment.