Definition : The theory of chemical evolution postulates that “Life occurs as a natural result of the evolution of inorganic matter”.
Introduction : In 1920’s Oparin and Haldane, developed a hypothesis on the chemical origin of life. They proposed that the primitive atmosphere of earth contained primarily reduced substances like methane, ammonia, and water instead of high O2 concentration. It was due to high temperature of about 5000 – 60000C (in primitive earth), elements could not exist in free state and hence primitive atmosphere was “ reducing “ in nature. In such an atmosphere, inorganic materials readily converted into organic forms under high solar radiation and energy.
This probably led to an accumulation of organic molecules. These organic molecules became more and more concentrated, interacted over millions of years to produce life.
Subtitle 1 : Steps in the chemical origin of life as given by Haldane & Oparin
(1) Origin of Organic Compounds :
- Formation of hydrocarbon and their derivatives
- Formation of fatty acids and glycerol
- Formation of carbohydrates
- Formation of amino acids and proteins.
The synthesis of all these organic compounds probably took place in the sea. Holdane described this mixture of oxygenize compounds with sea water as “Hot dilute soup”
(2) Formation of Macromolecules of Complex Organic Molecules :
The organic molecules present assembled randomly in collections capable of chemical interaction with the environment. They collided, reacted and aggregated to form new molecules which were of large size and more complex in nature. Sugar molecules combined to form polysaccharides Fatty acids combined with glycerol to form fats. Amino acids molecules linked together to form a polypeptide.
(3) Formation of nucleic acids and nucleoproteins
Fig : Coacervate
Some of these collections of molecules aggregated in various combinations and later precipitated out of the aqueous medium in the form of large colloidal particles, called Coacervates. These pre-cellular assemblages or coacervates have varied proteins, nucleoproteins and other organic and inorganic molecules in various combinations. These are said to be capable of carrying out primitive living reactions. Later, these were surrounded by a limiting membrane or plasma membrane, and at this stage, some proteins in them started working as enzymes. The coacervate surrounded by a nutrient shell and limiting membranes believed to be First Living Cell.