ORIGIN OF LIFE

One of the formost advocates of the special creation theory during chritian times was Suarezi.

According to him The world was made in six natural days. First 2 days------- Media prima 3rd day --------- Ancestors of all living plants 4th & 5th day --------- Ancestors of all existing

animals 6th day ------ man – Adam --- 12th rib Eve The theory was believed until 19th century

1.SPECIAL CREATION THEORY

Greek philosophers of the prechristian era like Xenophanes,Empedocles,Plato.,Aristotle were the followers of this theory

According to this theory,life has originated from non-living organic matter abiogenetically from time to time.

2.Theory of spontaneous generation or abiogenesis

According to Epicuris(342-271.B.C) Worms were produced from manure by the

action of warmth of sun and air. Anaxagoras(510-428 B.C) thought that life

come in tiny seeds(spermia) with the rain water to fruitify the earth.

According to Aristotle(384-322 B.C) living creatures are born from like species no doubt,but they also arise spontaneously.

Till the middle of the 17th century ,the theory of spontaneous generation was widely accepted.but later on Francesco Redi in 17th century and Abbe Spallanzani in 18th century disapproved the idea of spontaneous generation.

Redi (1621-1691)described series of experiment in which he placed meat under clean muslin coverings and demonstrated that flies laid eggs on muslin and larvae appeared only when these eggs were transferred to the meat and allowed to hatch.

He concluded that maggots come only from pre-existing flies and were not spontaneously generated by any other form of material.

3.Theory of Biogenesis

Spallanzani 91729-1799) in his experiment sealed the neck of the flask containing infusion so that even air could not enter inside after boIling for four hours

.some of the flasks were kept open. Microorganisms appeared in those flasks

which were open while the sealed flask didn’t show any life.

According to spallanzani air carried microorganisms which germinate when get moisture and food.they were not formed abiogenetically.

Needham in 1745 described experiments in which sealed vessels having meat extracts were swarmed with microorganisms.

Spallanzani(1765) challenged the views and results of needham.

Louis pasteur rejected the theory of spontaneous generation.

He boiled bouillion in a special flask with a sealed neck ,in which the nutritive medium was sterile for a long time.

No bacterial growth appeared.when the flask was opened the microorganisms entered into the bouillion and started reproducing.

Thus,it was demonstrated that self generation of organisms is impossible.

The theory of eternity states that ,the organisms have been unalterable not only throughout their existence and will continue to exist in the same unchanging state till eternity.

According to Hulton,we find no vestige of a beginning and no prospect of an end.

This theory was supported by Richter,Preyer,Helmholtz,Arrhenius,Hoyle,Bondi etc.

4.Theory of eternity of life.

According to Richter.the protoplasm reached the earth in the form of spores or germ or ether(simple particle with cosmic dust from other planet).

Arrhenius proposed cosmic panspermia theory,according to him the life existed throughout the universe and their spores could freely travel through the space from one palnet to another.

According to Helmholtz the life was brought to the earth with falling meteorites.

4.Cosmozoic theory or theory of panspermia.

This theory has been rejected due to two reasons.

It is not possible that intense cold,extreme dryness and intense radiations of interstellar space had survived life.

The theory does not answer basic question about the origin of life.

Objections

Cuvier “the father of modern paleontology”was the greatest advocate of this theory.

According to them,the earth was periodically visited by catastrophes which destroyed all plants and animals from time to time and created new and special forms of lives

5.Theory of catastrophism

According to this theory the number of different types of fossils correspond to the number of catastrophes which visited the earth.

The number is remarkably high and appears to be impossible.

Drawbacks

In 1920s,a new interest in the origin of life arose from independent speculation of two biologists,A.I Oparin and J.B.S.Haldane.

Both of these scientists got the idea from the new biochemistry that was founded by F.G. Hopkins.

They were of the opinion that evolution is purely chemical --- the gradual transmutation of inorganic compounds into organic ones.

Modern concept about the origin of life or chemical origin of life

Oparin gave the scientific account about the origin of life in his book entitled “The origin of life”,which was published in 1936.

His main postulates are as follows:

Oparin postulates about the origin of life

The earth is presumed to have originated about 5-6 billion years ago.

There are two hypothesis about the origin of life

(i)Planetismal hypothesis believes that the earth is originated as a part broken off from the molten mass of sun.

(ii)Nebular hypothesis:according to this hypothesis the earth is originated by gradual condensation of interstellar dust from which entire solar system is supposed to be formed.

(a) Origin of Earth

In the beginning ,the earth was a spinning ball of hot gases and vapours of elements

Gradually through hundreds of million of years,the gases condensed into a molten mass and different elements got stratified according to their density.

The original temperature of the earth was very high,i.e about 5000-6000c

(b)Presence of elements on earth

Due to this high temperature carbon element existed in the form of

Dicarbon Cynogen(CN) Metal carbides Carbon dioxide etc Oxygen did not exist in free state but oxides

of aluminium,boron,hydrogen were present. Nitrogen existed in combination with metals

in the form of nitrides.

The earth cooled down gradually ,some of the gases liquified and some solidified.

Steam condensed into water and resulted in rain.the water again returned to atmosphere because of superheated earth.this cycle continued for million of years and resulted in the cooling of earth.due to cooling down of earth ,first ocean came into existence.the oceanic water contained methane and ammonia of atmosphere in dissolved form.

The metallic nitrides reacted with superheated steam at high temperature to form ammonia.

Calcium cyanamide was formed by the action of nitrogen on carbides.

(c)Interaction of elements

According to Oparin ,under the influence of electric charges,from these simple compounds more and more complex compounds were formed gradually.

When the temperature of earth surface reduced to 1000c or even lower,a number of saturated as well as unsaturated hydrocarbons like acetylene,ethylene,methane etc were formed.

(d)Origin of organic compounds

The saturated and unsaturated hydrocarbons that were formed combined and recombined in various ways through the process of condensation,polymerization and oxido-reduction to form complex compounds like Acetaldehyde,aldol,ethyl alcohol ,ethyle acetate etc.

During these processes different types of amino acids were also produced.

(e)Formation of complex organic compounds

Ultimately,the amino acids,sugars,glycerol,and fatty acids of the primitive ocean polymerized forming large macromolecules such as proteins,carbohydrates and fats.

Since the complex compounds are the main constituents of the protoplasm of all living cells,their synthesis established the possibility of origin of life in the primitive ocean.

The oceanic water became a rich mixture of organic compound,which had been described by J.B.S Haldane(1920) as ‘the dilute soup”

(f)Formation of proto-proteins,carbohydrates,fats etc

The macromolecules of oceanic soup especially those protein molecules having electric charges,aggregate by mutual exclusion of water ,into small colloid masses,forming insoluble droplets.

These droplets ere surrounded by water film.these were named as microspheres by sydney fox(1964)

(g)Formation of coacervates or primary living molecules

At later stages,the microspheres carriying opposite electric charges aggregated to form larger and denser colloidal systems,named ‘coacervates’ by Oparin.

These coacervate drops were different in chemical composition and reactions which took place in them.they had common properties and still definite individualities and structures.they could grow in size as a result of absorbing substances dissolved in the surrounding waters.

These coacervate drops become more complex and created diversity among themselves.Certain combinations of compounds made them more stable.

Oparin and his co-workers produced several artificial coacervates from gelatin,gum,arabic and other substances as an illustration of this theory of the origin of life.

Another important event in the origin of life was the formation of nucleic acids in the primitive ocean.

The nucleotides were formed in the primitive ocean by combination of nitrogenous bases,pentose sugar and phosphoric acid.these molecules were like naked genes.

Due to aggregation tendency of molecules,macromolecules of nucleo-poteins were formed by the union of nucleic acid and protein molecules.

It is presumed that these macromolecules had certain characteristics of a free living gene.

A number of such free genes might have aggregated to form a single large unit that resembled the present day ‘viruses’ and named as ‘protovirus’.

(h)Origin of primary organisms

Before the gene aggregates could develop into anything like a cell,it was necessary that they produce enzymes.

With the formation of enzymes conditions were now favourable for cells to be formed.

They were very simple cells at first ,without a definite nucleus like bacteria(Prokaryotes).

Simple molecules were abundant in the primitive ocean .they diffused into the cell and by enzymatic action cell protoplasm were formed and growth could took place.

Continued growth of cell created a problem – the cells would have soon become too large for efficient functioning until cell division intervened.

(i)Origin of autotrophim

Being anaerobic heterotrophs,the early bacteria-like prokaryotes gradually consumed all abiotically synthesized organic nutrients of the primitive sea.

At this point there was danger that the life would perish if all the organic nutrients are consumed.

To cope with this situation some of the early prokaryotes acquired enzymes to synthesize simple carbohydrates from inorganic substances of the ocean water.this was the beginning of autotrophic nutrition.

Energy was obtained from anaerobic breakdown.such a method of obtaining energy through chemical recombination with the release of a portion of the energy is known as chaemosythesis.

Chemosynthesis could not become a popular method .later certain autotrophic bacteria synthesize a new ,green substance from the magnesium porphyrin of the ocean water.this substance called bacteriochlorophyll could synthesize food from water and carbon dioxide in the presence of sunlight.

Certain changes in the bacteriochlorophyll resulted into the formation of chlorophyll.

In the photosynthesis oxygen was liberated in the atmosphere.

(ii)Photosynthesis

The entry of oxygen in the atmosphere was a major step.it brought about change in the atmosphere.it reacted with methane and transformed it into CO2 and H2O.

This provided an additional source of CO2 for photosynthesis and changed the atmosphere into a form in which animal life could exist.

Oxygen in the air had another effect of great importance.

It liberated living organisms from the ocean. Intense radiation from the sun was

destructive to life .water is a good absorber of such radiations.thus, life could exist in the oceans ,but not on the land.

some of the oxygen liberated,formed a layer of ozone high in the atmosphere and thus absorb most of the radiations.

Environment

For the first time life on the land was possible as the earth was cooled to a tolerable temperature; the injurious radiations were effectively blanketed;the methane and ammonia in the air were replaced by nitrogen,oxygen and co2.;photosynthesis had become the dominant form of energy capture;life was now free to develop into forms which occur today.

Stanley L.Miller conducted experiment in 1953. He kept a mixture of gases i.e

methane,ammonia,and hydrogen in the flask and a high frequency spark was discharged in a constantly circulating mixture of gases for about a week.during this period a small amount of water was constantly boiled to produce water vapours.at the end of the week a mixture of amino acids like glycine,alanine,alpha-butyric acid,aspartic acid and glutamic acid etc. were obtained.the intermediate products of he reaction were aldehydes and HCN.

Experimental evidences

Bahadur(1954) obtained almost all the possible amino acids by subjecting strong sunlight to a mixture of paraformaldehyde,ammonia and ferric chloride.

Oro and Kimbell(1960-61) obtained adenine by heating a solution of ammonium cyanide at 90c for 24 hours.

Fox and Harade(1961) synthesized uracil from methane, hydrogen and water .

S.w.Fox ,conducted some heating experiments with volcanic eruption and found amino acids undergoing polymerization forming protenoids.

These protenoids with water may form coacervates.thus both Oparin and Fox considered protein coacervates to have given rise to the firstt living organism.

In 1967,J.Willium Schopf detected evidence of 22 amino acids in a rock formation which was about 3000 million years old.these occurrence of amino acids are proof of chemical evolution of life.