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
Origin and evolution of human being.
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
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
4. Theory of biogenesis :
Francisco Redi, Spallanzani and Louis
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
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
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
The Russian scientist Alexander Ivanovich
Oparin(1924) and British scientist J.B.S
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
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
As temperature decrease steam condensed
to water which results in rain and earth
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
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
A) Formation of complex organic compounds:
The hot dilute soup was sterile and oxygen
Simple organic compound come together,
form colloids to form complex compound like
polysaccharides, protiens, nucleosides, fats
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
Oparin called these aggregates as cocervates
while Sidney fox called them Protenoids or
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
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 (or Urey–Miller
experiment) 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 by Stanley Miller and Harold Urey at the University of
Chicago and later the University of California, San Diego and
published the following year.
• 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, 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. Nucleic acids were not formed within the reaction. 18%
of the methane-molecules became bio-molecules. The rest turned
into hydrocarbons like bitumen.