Big-Bang Theory :
Proposed by Abbe Lemaitre.
According to it, the universe originated about 20 billion years ago due to a thermonuclear explosion.
This thermonuclear explosion is called Big-bang.
Flat-disc like structure is called SOLAR-NEBULA formed.
About 4.5 billion years ago, the origin of our solar system took.
The very hot central part of this solar Nebula became still hotter & converted into the sun. Now, due to condensation of atoms & dust particles moving around the sun, formation of the other planets took place
[Mercury, venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune].
The solid part of our planet earth was called Lithosphere & the gaseous part was known as atmosphere.
When the earth's surface cooled down and its temperature decreased to 100°C, water formed on it.
Ancient Theories for origin of life :
1. Theory of special creation –
The greatest supporter of this theory was father Suarez According to Bible life and everything was created by god in 6 days.
on first day : Earth and heaven
on second day : Sky and water
on third day : Land and plants
on forth day : Sun, moon and stars
on fifth day : Fishes and birds
on sixth day : Land animals and first man Adam and from his 12th Rib first woman Eve.
According to hindu mythology the world was created by God Brahma. (The first man was Manu and the first woman was Shraddha)
According to it life has not changed ever since its origin.
Special creation theory lacks scientific evidences so is not accepted.
2. Theory of Spontaneous Generation (Abiogenesis or Autogenesis) –
This hypothesis was supported by ancient Greek philosophers like Thales, Anaximander, Xenophanes, Plato. Empedocles, Aristotle.
According to this theory life was originated from nonliving things spontaneously.
They believed that the mud of the Nile river could give rise to frogs, snakes, crocodiles.
Abiogenesis was strongly supported by Von Helmont. He claimed formation of mice in 21 days. If a sweat soaked dirty shirt is kept in wheat barn.
3. Cosmozoic Theory –
Proposed by Richter.
Protoplasm reached on earth in the form of spores or other simple particles from some unknown part of the universe with cosmic dust and they gave rise to various forms of life.
4. Cosmic panspermia theory –
Proposed by Arrhenius.
According to this theory organisms existed throughout the universe and their spores could freely travel through space from one star to the other.
5. Theory of Eternity of Life –
Helmholz believed that life is immortal.
6. Theory of Biogenesis –
Harvey (1651) and
Huxley (1870)
{Omnis vivum ex ovo or vivo.}
New organisms can be originated on earth only by pre-existing life.
This theory rejected the theory of Spontaneous generation but cannot explain origin of life.
To prove Biogenesis and to disprove abiogenesis experiments were performed by –
Francesco Redi (Italian 1668) –
He took cooked meat in three jars, one was uncovered, the second was covered with parchment and the third was air tight.
He observed that maggots developed only in the uncovered jar while maggots could not develop in the meat in closed jars.
This proved that larvae were formed from eggs laid by the flies in open jars. Since the meat in closed jars could not be visited by flies so no larvae could develop. Therefore life originated from pre existing life.
Lazzaro Spallanzani (Italian 1767) –
He boiled vegetables and meat to prepare a sterlized nutritive soup and he kept some of it in air sealed flasks and some in loosely corked flasks.
He observed that the soup in sealed flask remained sterile while micro organisms appeared in the soup in loosely corked flasks.
Thus even micro organisms were formed from pre existing ones in the air rather then spontaneously.
Louis Pasteur (French 1862) –
Pasteur is popular for Germ Theory of Diseases or Germ theory and he disproved abiogenesis.
He prepared sterlized syrup of sugar and yeast by boiling them in flasks.
He took two flasks one of broken neck and another of curved neck (swan neck flask). No life appeared in swan neck flask because germ laden dust particles in the air were trapped by the curved neck which serves
as filter while in broken neck flask colonies of microorganism were developed.
Modern theory of origin of life :
(Oparin-Haldane theory of origin of life)
Naturalistic theory Or Theory of Chemical Evolution –
This theory was proposed by Russian Scientist A.I. Oparin and J.B.S Haldane (England born Indian scientist)
Oparin's theory was published in his book 'ORIGIN OF LIFE'.
Important Points
– According to this theory life originated by the composition of chemicals. (Chemical evolution).
– Oparin's theory is based on Artificial Synthesis. So also called as artificial synthetic theory.
– Ist life originated in the water of oceans. So water is essential for origin of life.There is no life on moon due to absence of water.
– At the time of origin of life free O2 was absent, so first life was anaerobic.
– In the primitive atmosphere free oxygen was present but complete oxygen consumed in composition so primitive atmosphere of earth was reducing.
– Oxygen was reproduced by photosynthesis and atmosphere converted in oxydising.
Chemical Evolution :
1. The atomic stage –
The earth was originated about 4.5 billion years ago. Early earth had free atoms of all elements which are essential for the formation of protoplasm.
The lightest atoms like carbon, hydrogen, nitrogen and oxygen formed the primitive atmosphere.
2. Molecular stage (Origin of molecules and simple Inorganic compounds) –
Free atoms combined to form molecules and simple inorganic compounds.Due to presence of high temperature, active hydrogen atoms combined with all oxygen atoms to form water and leaving no free oxygen.
Thus the primitive atmosphere was reducing (without free oxygen) unlike present oxidising atmosphere (with free oxygen).
Hydrogen atoms also combined with nitrogen to from NH3.
(The first molecular compounds formed were probably water and Ammonia).
These Lighter elements also formed CO2 , CO, N2 , H2 etc.
3. Origin of early organic compounds –
The nitrogen and carbon of the atmosphere combined with metallic atoms forming nitrides and carbides.Water vapour and metallic carbides reacted to form to first organic compound Methane (CH4). Later on hydrogen cyanide (HCN) was formed .
Water which formed on earth due to high temperature evaporated so clouds were formed.
Water vapour changed into rain drops and by the collection of water on earth primitive oceans were formed.
4. Origin of simple organic compounds –
Water of primitive oceans contained large amount methane, ammonia, hydrogen, cyanides, carbides, nitrides.These early compounds interacted and formed simple organic compounds like, aldehyde, Ketones. Alcohols Pentose and hexose sugar, Amino Acids, Glycerol, Fatty Acids, Purines, Pyrimidines etc.Energy was obtained from U.V. Rays of sunlight, cosmic rays and heat of volcanic eruptions
5. Origin of complex organic compounds –
The small simple organic molecules combined to form large complex organic molecules, e.g –
– Amino acids Joined to form polypeptides and proteins, which were non-enzymatic.
– Simple sugar units combined to form polysaccharides.
– Fatty acids and glycerols united to form fats and lipids.
– Sugar, nitrogenous bases, phosphates combined into nucleotides which polymerized into nucleic acid,which unable to replicate.
These macromolecules forms main component of protoplasm hence the possibility of origin of life in primitive oceans could be established.After long time the water of primitive oceans became rich mixture of organic compounds as a result of chemical evolution.Haldane called this saturated water of oceans as prebiotic soup or hot dilute soup.
The major requirement for promoting polymerization is the availability of continuous source of energy and removal of water from the surface of reactants so that they can concentrate and prevent depolymerisation.Experimental evidence for formation of simple organic compounds –
By Stanley Miller who was a student of Harold Urey.
In this experiment Miller took the mixture of methane, ammonia and hydrogen (ratio 2 : 1 : 2) in a large flask and passed steam over it by boiling water and connecting it with a glass tube. Electric spark discharged in the mixture by using two tungsten electrodes as source of energy.After 18 days this fluid was collected and analysed. This dark red fluid was found to contain.
– Simple amino acids – glycine, alanine, aspartic acid.
– Simple organic acids – formic, acetic, oxalic, lactic, succinic acids.
– Pentose, hexose, aldehyde, ketone etc.
