05 Origin and Evolution of life - part 02 - Chemical Evolution of Life

Chemical Evolution of Life (Self assembly theory of origin of life ) :

According to this theory, life originated on earth by combinations of several chemicals through constant chemical reactions over a long period of time.
This theory is also called self assembly theory of origin of life or biochemical origin of life.
This theory was first formulated by Haeckel but later developed by the Russian scientist Alexander I. Oparin (1924) and British biologistJ. B. S. Haldane (1929).
The process of chemical evolution can be divided into following steps :

1. Origin of Earth and Primitive atmosphere:

The origin of universe was explained by the Big-Bang theory of Georges Lemaitre (1931).
According to this theory the Universe originated about 20 billion years ago by a single huge titanic explosion.
As the universe expanded, the temperature decreased and various galaxies of solid objects were formed.
Milky Way is one such galaxy of which our solar system is one small part.
Earth is one of the planets of solar system and originated about 4.6 billion year ago.
When formed, it was a rotating cloud of hot gases and cosmic dust called Nebula.
The condensation and cooling resulted in stratification with heavier elements like nickel and iron passing to the core and lighter ones like helium, hydrogen, nitrogen, oxygen, carbon,etc. remaining on the surface.
They formed the atmosphere of the earth.
The primitive atmosphere of the earth was quite different from the present one and it was of a reducing type, devoid of free oxygen.

2. Formation of ammonia, water and methane:

Primitive atmosphere was very hot.
As it slowly cooled, the lighter elements started to react with each other.
The early atmosphere was rich in hydrogen, carbon, nitrogen and sulphur
Of which hydrogen being more active, it reacted with other elements to form chemicals on earth like CH4, NH3, H2O and H2S.

3. Formation of simple organic molecules :

As temperature of the earth decreased, steam condensed into water that resulted in heavy rain fall and the earth gradually cooled.
Rain water got accumulated on the land to form rivers, streams, lakes, seas and oceans.
The atmosphere then did not contain ozone layer and thus ultra-violet radiations reached the surface of earth directly.
Under the influence of available energy sources such as ultra-violet rays, radiations, lightening and volcanic activities, the early molecules of hydrocarbons, ammonia, methane and waterunderwent reactions like condensation, polymerisation, oxidation and reduction.
These reactions resulted in formation of simple molecules like monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol, etc.
All these simple organic molecules accumulated at the bottom of water bodies.
Haldane described it as the ``hot dilute soup''or ``primitive broth''.
It did not show any degradation due to absence of free oxygen and enzymes.

4. Formation of complex organic molecules:

The primitive broth was neutral and free from oxygen.
Polymerisation took place and simple organic molecules aggregated to form new complex organic molecules like polysaccharides, fats, proteins, nucleosides and nucleotides.
Polymerisation of amino acids formed protoproteins which later formed proteins.
Formation of protein molecules is considered as landmark in the origin of life.
Proteins (enzymes) accelerated the rate of other chemical reactions.

5. Formation of Nucleic acids :

Number of nucleotides join together to form nucleic acids (RNA, DNA).
Nucleic acids acquired self-replicating abilitywhich is a fundamental property of living form.

6. Formation of Protobionts or Procells :

Nucleic acids along with inorganic and organic molecules formed the first form of life called protobionts.
Protobionts are the prebiotic chemical aggregates having some properties of living system.
Protobionts are formed due to coacervation i.e. aggregation of organic molecules.
Oparin (1924) called them coacervates andSidney Fox called protenoids or microspheres.

Coacervates :

Coacervates are colloidal aggregations of hydrophobic proteins and lipids (lipoid bubbles).
Grew in size by taking up material from surrounding aqueous medium.
Became thermodynamically unstable and split into smaller units, comparable to daughter cells of budding organisms.
Microspheres are protenoids formed from colloidal hydrophilic complexes surrounded by water molecules.
These bodies may have outer double-membrane, like primitive cell.
Diffusion and osmosis may have occurred across the membrane.
They were more stable than coacervates.
Coacervates and microspheres were non-living colloidal aggregations of lipids and proteinoids respectively.
They had some basic properties of living cells, such as growth and division.
These colloidal aggregations turned into first primitive living system called eobionts or protocell.

7. Formation of first cell :

When RNA or DNA system developed within protocells, they look like bacteria or viruses.
They regulated various metabolic activities.
First cell was anaerobic, heterotrophic and obtained energy by chemoheterotrophicprocesses.

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