XII BIOLOGY CH-5
Origin and Evolution of life
Origin of life on Earth
Origin of life (Protobiogenesis)-
- Attributes
of living organisms : Responsiveness, growth, metabolism, energy
transformations and reproduction.
- Various theories and hypotheses to
propose origin of life :
Oldest
theory. No scientific proof. Only religious beliefs.All living organisms are
created by a supernatural power.
B)Cosmozoic theory/Theory of Panspermia-
NASA
has reported fossils of bacteria like organisms on a piece of Martian rock
recovered from Antarctica. Life descended to the earth from other planets in
the form of cosmozoa/panspermia.
C)Theory of spontaneous generation (Abiogenesis)
Disproved
by Louis Pasteur. Life originated from non-living material
Explains
only the continuity of life.Living organisms produced from pre-existing living
forms, by process called reproduction.
Chemical Evolution of Life (Self-assembly theory of
origin of life)-
The steps in
the process of chemical evolution :
a)Origin of Earth and Primitive atmosphere
:
Big-Bang
theory of Georges Lemaitre (1931). Formation of reducing atmosphere.
a. The origin of life on earth is closely
related to the origin of earth itself.
b. Evidences suggest that earth originated
about 4.6 billion years ago.
c.
When it was broken from the sun, it was a glowing fire and a rotating cloud of
hot gases, vapours of various elements and pieces of rocks and metals called
Nebulous.
d.
This mass of gas exploded with a Big Bang into several small pieces. The earth,
as a piece was broken from the sun.
e.
As the earth was moving away from the sun, it was getting cooled. This led to
the condensation of gases.
f.
The heavy elements like iron, nickel, etc. sank to the centre and formed the
solid core of earth.
g.
The lightest elements like helium, hydrogen, oxygen, nitrogen, carbon, etc.
occupied the atmosphere of the earth.
h. The geological conditions on the primitive earth about 3600 million years ago were much different from those found today. In fact, the atmosphere of primitive earth was reducing type and there was no free oxygen.
b) Formation of ammonia, water and methane :
Formation
of CH4, NH3, H2O and H2S.
a. The prebiotic environment had very less
resemblance with the present day environment.
b.
The earth was very hot initially and so the atoms could not combine with each
other very easily.
c. The primitive earth contained a large
amount of hydrogen, nitrogen, carbon and oxygen. d. Of these, hydrogen was very
active. e. It combined with nitrogen to form ammonia (NH3), with oxygen to form
water (H2O); and with carbon to form methane (CH4). f. As the temperature was
high, ammonia and methane remained as gases and water as steam. here was no
free oxygen
c) Formation of simple organic molecules :
a.Formation
of monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol,
etc. Formation of water bodies resulting into ‘hot dilute soup’ or ‘primitive
broth’.The next step in molecular evolution was the formation of
micromolecules.
b.
As the earth surface considerably cooled, the highly reactive free radicals –CH
and –CH2 condensed to form a variety of both saturated and unsaturated hydrocarbons.
c.
The simple inorganic molecules of the oceanic water interacted with one another
to form simple organic molecules such as sugar, fatty acids, glycerol, amino
acids, purines and pyrimidines.
d. The energy for these reactions was provided
by the electrical discharges (lightning), ultraviolet rays (solar radiations),
volcanic activities and decay of radioactive elements
•
CH4
+ NH3 + H2O → amino acids.
•
CH4
+ HCN + NH3 + H2O → purines + pyrimidines
•
CH4
+ H2O → sugar + glycerol + fatty acids.
Haldane
described the sea containing molecules of these organic substances in abundance
as ‘the hot dilute soup’ or ‘primitive broth’
d)Formation of complex organic molecules :
a.The
hot dilute soup was sterile and oxygen free. Simple organic substances came
together in increasing numbers.
b.They
colloided, reacted and aggregated forming new complex molecules such as
polysaccharides, fats, proteins, nucleosides and nucleotides.
c.The protein molecules made their existence
by polymerization of amino acids. These proteins showed enzymatic reaction and
were called protoproteins.
d.Due to their enzymatic nature, these
proteins accelerated the rate of other chemical reactions. The formation of
protein molecules is considered as a landmark in the origin of life.
e)Formation of Nucleic acids :
a.Formation of Nucleotides→nucleic acids (RNA,
DNA) →acquired self-replicating ability→ fundamental property of living form.
b.The
next step in chemical evolution was formation of nucleic acid. It was formed by
the aggregation of phosphoric acid, sugar, purines and pyrimidines.
c.
They were linked in various combinations to form different types of
nucleotides. Thousands of nucleotides joined together to form nucleic acid.
d.
It acquired self replication ability, which is a fundamental property of living
forms
f)Formation of Protobionts or Procells
:
First
form of life called protobionts was formed from nucleic acids by coacervation.
Coacervates
and microspheres were non¬living colloidal aggregations of lipids and
proteinoids respectively. They turned into eobionts or protocell.
Showed
growth and division and hence considered as first primitive living system.
The nucleic acid along with inorganic and organic
molecules formed the first form of life and were called protobionts or
precells.
The proteins formed colloidal hydrophilic complexes
surrounded by water molecules. Oparin and Sydney Fox demonstrated the formation
of this aqueous suspensions of polymers.
Oparin called these aggregates as Coacervates, while
Sydney Fox called them Protenoids or Microspheres
g)Formation of first cell :
First cell developed by formation of RNA and
DNA system. First cell was anaerobic, heterotrophic and obtained energy by
chemoheterotrophic processes.
• The
nucleic acids in pre-cells had the capacity to multiply but gradually it
started directing a series of chemical reactions like protein synthesis.
• That was the significant step in the
transformation of pre-cell into a cell.
This chemical evolution gave rise to the biological evolution.
Coacervates-
1)Coacervates are colloidal aggregations of
hydrophobic proteins and lipids (lipoid bubbles). .
2)Coacervates grew in size by taking up material
from surrounding aqueous medium.
3)As they grew, they became thermodynamically
unstable and split into smaller units, comparable to daughter cells of budding
organisms. .
4)Microspheres are protenoids formed from colloidal
hydrophilic complexes surrounded by water molecules.
5)These bodies may have outer double-membrane, like
primitive cell. .
6)Diffusion and osmosis may have occurred across the
membrane. .
7)They were more stable than coacervates.
8)Coacervates and micro spheres were non-living
colloidal aggregations of lipids and proteinoids respectively.
9)They had some basic properties of living cells,
such as growth and division. .
10)These colloidal aggregations turned into first
primitive living system called eobionts or protocell.
Give an account of Urey
and Miller’s experiment.
i. Stanley
Miller and Urey designed an apparatus and created conditions similar to
primitive atmosphere.
ii. The apparatus had a spark chamber with two
tungsten electrodes, flask for boiling water, side tube connected to a vacuum
pump condenser and U-shaped trap.
iii. The electric discharge was created by using
electrodes in a spark chamber containing methane, ammonia, hydrogen and water
vapours in the ratio 1:2:2 without air. This was done to create prebiotic
environment.
iv. The flask containing water was heated to provide water vapour.
