Origin and evolution of life

Origin of life : Theories of the origin of life, early earth, modern self assembly theories, Oparin-Haldane theory of chemical evolution of life.

Organic evolution : Darwin’s theory, modern synthesis theory of evolution, geological time scale.

Origin and evolution of human being.

introduction It is presumed that the universe came into

existance with a single titanic explosion called big bang about 15000 million year ago.

The fragments of the fire ball expanded and cooled to give rise to many celestial bodies.

This results in the formation of solar system consisting of sun and planets.

The Earth originated about 4.6 billion years ago.

Earlier life was thought to be created by some super natural power.

Origin of life Origin of life on Earth is also called

protobiogenesis.

It is clear the that life came into existence from non living substances of primordial earth about 3 billion year ago.

Origin of life can be explained by following theories :

1. Theory of special creation : Spanish monk Father Sudrez. According to this theory, all the living beings

on earth were created by god or by some super natural power.

2. Cosmozoic theory (panspermia theory) :

Richter in 1865

Life on earth came from different planet in the form of spores or microorganisms.

This is called cosmozoa or panspermia which were preserved inside meteorites.

These meteorites struck into barren earth to release cosmozoa and helpin development of various creature.

3. Theory of spontaneous generation or theory of abiogenesis :

It is also called as autobiogenesis.

Greek philosophers in 600 BC.

Life is originated from non living things spontaneously, without any interruption.

Louis Pasteur disproved this theory and gave the scientific explanation that life originated only from pre existing life or biogenesis.

4. Theory of biogenesis :

Francisco Redi, Spallanzani and Louis Pasteur.

This theory could not explain first life on earth but able to explain continuty of life.

Living organisms are always produced from pre-existing living forms by reproduction and not from non-living or lifeless matter.

Modern self-assembly theories

Modern self assembly theory holds that protobiogenesis occurred due to inherent property of chemical molecules of self organization or self assembly.

This theory include conversion of micro molecule into macro molecules to cell like microsystem.

First demonstration was provided by Schmitt (1956) who has synthesized collagen from simple micro molucules.

Reed (1967) demonstrated a inter macromolecular assembly by assembling two protein molecules which resulted into enzymatically active complexes.

The accepted model of protobiogenesis suggested that information flowed from amino acid in geochemcal matrix to protein in first organism.

First life came into existence by combination of chemical compound by constant chemical reaction over long period of time.

This is known as chemical evolution of life or self assembly theory of origin of life or biochemical origin of life or molecular evolution.

This theory was formulated by Haeckel but developed by Oparin and Haldane.

Oparin haldane theory of chemical evolution

The Russian scientist Alexander Ivanovich Oparin(1924) and British scientist J.B.S Haldane(1929).

Process of chemical evolution can be divided into three step.

A) Origin of earth and primitive atmosphere:

When earth was broken from sun, it was a glowing fire and a rotating cloud of hot gasses, vapours of various elements, pieces of rocks and metals called nebulous.

As the earth was moving away from sun, the temperature slowly decreased.

This led to condensatiion of gases.

The heavy elements (iron, nickel) sank to the centre and form solid core of earth.

Lightest elements (He, H, O, N, C) occupied atmosphere of the earth.

B) Formation of ammonia, water and methane:

The earth was very hot initially and so the atoms could not combine with each other easily.

It contain H, O, N and C.

Hydrogen was very reactive it combine with nitrogen to form ammonia, with oxygen to form water, with carbon to form methane.

As temperature is high ammonia and methane remained as gas and water as steam.

As temperature decrease steam condensed to water which results in rain and earth become cold.

Water gradually accumulated and this led to the formation of rivers, sea lakes etc.

Thus the first chemicals formed on the earth were water, ammonia, methane etc.

C) Synthesis of simple organic compound:

Next step is formation of micromolecules.

When temperature of earth gradually cooled down then the highly reactive free radicals CH and CH2 condensed to form variety of saturated and unsaturated hydrocarbon.

Hydrocarbon, ammonia, water show condensation, polymerization, reduction and oxidation

This results into formation of various biologically active molecules like monosaccarides, amino acids, purines, pyrimidines, fatty acids and glycerol etc.

J.B.S Haldane described the sea containing molecules of organic substances in abundunce as “the hot dilute soup or primitive broth”.

A) Formation of complex organic compounds:

The hot dilute soup was sterile and oxygen free.

Simple organic compound come together, form colloids to form complex compound like polysaccharides, protiens, nucleosides, fats and nucleotides.

The protein molecules formed by polymerization of amino acids (protoprotein).

The formation of protein molecules is considered as a land mark in the origin of life.

B) formation of nucleic acid:

C) Formation of protobionts or precells :

The nucleic acid along with inorganic and organic molecules formed the first form of life precells or protobionts.

Protiens formed colloidal hydrophilic complexes surrounded by water molecules.

Oparin and Sidney Fox demonstrated the formation of this aqueous suspension of polymers.

Oparin called these aggregates as cocervates while Sidney fox called them Protenoids or Microspheres.

D) Formation of first cell:

This was significant step in transformation of precells into cell.

E) Biological evolution:

The first cell or primitive cells were marine and hetrotrophic in nature.

Growth and multiplication of these cells caused depletion of food and increase in Co2 due to fermentation.

This led to development of chromophores.

This event help to transform heterotrops to autotrophs.

Due to release of oxygen as a byproduct of photosynthesis, the primitive reducing atmosphere was slowly and gradualy converted into oxidizing atmosphere.

• The Miller–Urey experiment[1] (or Urey–Miller experiment)[2]

was an experiment that simulated the conditions thought at the time to be present on the early Earth, and tested for the occurrence of chemical origins of life. Specifically, the experiment tested Alexander Oparin's and J. B. S. Haldane's hypothesis that conditions on the primitive Earth favored chemical reactions that synthesized more complex organic compounds from simpler organic precursors. Considered to be the classic experiment concerning the experimental abiogenesis, it was conducted in 1953[3] by Stanley Miller and Harold Urey at the University of Chicago and later the University of California, San Diego and published the following year.[4][5][6]

• The experiment used water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2). The chemicals were all sealed inside a sterile array of glass flasks and flasks connected in a loop, with one flask half-full of liquid water and another flask containing a pair of electrodes. The liquid water was heated to induce evaporation, sparks were fired between the electrodes to simulate lightning through the atmosphere and water vapor, and then the atmosphere was cooled again so that the water could condense and trickle back into the first flask in a continuous cycle.

• Within a day, the mixture had turned pink in colour,[9] and at the end of two weeks of continuous operation, Miller and Urey observed that as much as 10–15% of the carbon within the system was now in the form of organic compounds. Two percent of the carbon had formed amino acids that are used to make proteins in living cells, with glycine as the most abundant. Sugars were also formed.[10] Nucleic acids were not formed within the reaction. 18% of the methane-molecules became bio-molecules. The rest turned into hydrocarbons like bitumen.