Modern theory of Origin of Life

Biochemical origin of life or Oparin and Haldane theory

The concept of a biochemical origin of life was first proposed by Russian biochemist A.I. Oparin in 1923 and later supported by English scientist J.B.s Haldane in 1928. Thus, this theory is also called Oparin and Haldane theory. They believed the origin of life started along the origin and evolution of earth and atmosphere. As Biochemical origin of life is complex phenomenon it can be studied in three phases;

• Chemogeny (Chemical evolution)
• Biogeny (Biological evolution)
• Cagnogeny(Modification of life)

Chemogeny

Origin of earth and its atmosphere

• The earth was probably formed around 4.6 million years ago.
• The sun and planets including earth were formed by the spherical cloud of gases and cosmic dust.
• Due to gravitation, the sun remained at the centre and other planets towards the periphery.
• At the beginning, the earth was like a fireball (5000-6000°C). Everything in it existed in a gaseous state.
• Gradually earth began to cool which took millions of years and converted into molten core.
• Stratification of elements took place according to their density.
• H₂, He, C and N being lightest elements remained at the outermost strata forming the primitive atmosphere. It is reducing type due to the absence of free oxygen.

Formation of simple molecules

• Numerous numbers of highly reactive element 'H' were found is the primitive atmosphere which reacted with other elements to form methane, ammonia, hydrocyanic acid(HCN), water etc.

2H₂+ O₂→ 2H₂O

N₂+ 3H₂→ 2NH₃

C +2H₂→ CH₄

• All of them existed in the gaseous state due to which temperature of the earth, even water was in the vapour.
• On further cooling of the earth, water vapour condenses to form rain water. Water from the surface of earth returned to atmosphere from heated earth.
• The water cycle continued for millions of years. As a result, water bodies like lake, pond, river, ocean etc came in existence on earth.
• CH₄and NH₃from atmosphere got dissolved in rainwater and ultimately mixed in an ocean.

Formation of simple organic compounds

On the further cooling variety of saturated and unsaturated hydrocarbons were formed which reacted with hot water vapour to form small chain compounds like ketones, alcohols and other organic acids.

Under the influence of UV rays radiation, electricity from the thunderstorm, volcanic eruption etc simple molecules converted into simple organic compounds like sugar, amino acid, purine and pyrimidine.

CH₄ + H₂O→ sugar, fatty acid, glycerol

CH₄ + H₂O + NH₃→ Aminoacid

CH₄ + H₂O + NH₃+ HCN→Purine + Pyrimidine

The hot water ocean containing organic compounds was described by Haldane as "The Hot Dilute Soup" or "The Primordial soup" or "Prebiotic soup". Darwin called it as "Warm little poud".

Formation of complex organic compounds

The simple organic compounds were polymerised to form complex organic compounds like polysaccharides, polypeptides, proteins, lipids etc. There was a large number of complex organic compounds in ocean remained to unspoil due to the absence of life. There was the formation of nucleotides by the combination of sugar, phosphate, purine and pyrimidine. Nucleotides formed nucleic acid which is the basic component of life.

Formation of coacervates

The complex organic compounds of primordial soup in ocean aggregated together through the colloidal system and bounded by water layer was called coacervates. They can grow by absorbing nutrients.

Biogeny

Coacervates may produce the first cell-like structure. The first cell-like structure with the power of division is called Eabiant or Pre-cell. Oparin called it as protobiont.

The coacervates which acquired nucleoproteins gave first cellular structures. Their limiting membranes were formed by fatty acids and showed the great affinity with water. Some of the proteins within coacervates acted as enzymes and began metabolic activities. In this way, first prokaryotic, unicellular, heterotrophs, anaerobic organism evolved.

Cognogeny

• With the gradual increase in the number of heterotrophs as they consumed nutrients of the ocean, there became a declination in organic nutrients. So, they began to search other alternatives for obtaining food i.e. saprophytism, parasitism, chemosynthesis and photosynthesis.
• During photosynthesis, solar energy was trapped by light trapping pigment called chlorophyll. In this way, prokaryotic, anaerobic and photoautotrophic organisms evolved.
• With the increase meant in a number of photoautotrophs O₂ released in great extent in the ocean and came out into the atmosphere. Now the atmosphere became the oxidising type. Then prokaryotes gradually modified to be adapted to the aerobic mode of respiration.
• The true nucleus, mitochondria and other cell organelles were developed. In this way, free eukaryotes were formed from some prokaryotes.
• Around 1500 million years ago, complex organisms were modified from simple ancestral type.