The Oparin-Haldane hypothesis of biochemical evolution is at the heart of modern scientific ideas about the origin of life.
According to the theory of biochemical evolution, the formation of life on Earth proceeded in three stages:
- abiogenic synthesis of organic substances;
- the formation of biopolymers;
- the formation of membrane structures and the emergence of self-reproduction.
Abiogenic synthesis of organic substances
According to Oparin’s theory, the emergence of life on Earth was possible only in the conditions of an ancient atmosphere and the absence of living organisms.
At the first stages of its existence, our Earth was a red-hot ball.
As it cooled, a primary atmosphere gradually formed, consisting of ammonia, methane, carbon dioxide, hydrogen cyanide and water vapor. There was no oxygen or ozone in the atmosphere of the ancient Earth.
With a further decrease in temperature, a primary ocean was formed. Under the influence of various types of energy (electrical discharges, nuclear reactions, solar radiation, volcanic eruptions), simple organic compounds were formed: formaldehyde, alcohols, formic acid, amino acids, etc.
Oxidation of the resulting substances did not occur, since there was no free oxygen. The synthesized substances gradually accumulated in the ancient ocean for tens of millions of years. Their accumulation eventually led to the formation of a homogeneous mass – “primary broth”. According to Oparin, it was in the “primary soup” that life arose.
This stage of biochemical evolution was confirmed experimentally by biochemists S. Miller, J. Oro and other scientists. In experimental installations simulating the conditions of the primitive Earth, they obtained aldehydes, amino acids, simple sugars, purine and pyrimidine bases, and nucleotides.
Biopolymers were synthesized from simple organic substances under certain conditions. Amino acids were combined into polypeptides, simple sugars were converted into polysaccharides, and nucleotides into nucleic acids. Carboxylic acids, combining with alcohols, could form lipids, which covered the surface of reservoirs with a fatty film.
The resulting proteins formed colloidal complexes attracting water molecules to themselves. This is how coacervates appeared – clumps of organic matter, isolated from the rest of the water mass. Organic compounds constantly entered the coacervates, resulting in the synthesis of more complex substances. They could merge and grow in size.
The formation of biopolymers and coacervates in the conditions of the ancient Earth was confirmed experimentally by the works of L. Orgel and S. Akabori. They obtained the simplest proteins and nucleotide chains.
Formation of membrane structures and the emergence of self-reproduction
A biological membrane could form from lipid films on the surface of coacervates.
The combination of coacervates with nucleic acids led to the formation of primitive self-reproducing living organisms – probionts. These primary organisms were anaerobes and heterotrophs and fed on the substances of the “primary broth.”
Thus, about 3.5 billion years ago, according to this hypothesis, the origin of life on Earth was completed.