BIG BANG THEORY

Origin of life is inseparably linked with the formation of our solar system, particularly with the origin of earth. According to Big Bang Theory, some 15 billion years ago, the universe originated by a big bang (thermonuclear explosion) of a dense entity . This big explosion threw out numerous pieces of cosmic material (nebulae) which slowly condensed and became organized slowly into groups of stars called galaxies. These galaxies appear to be moving away from one another at a speed nearly equal to the speed of light.

FORMATION OF SOLAR SYSTEM

Evidences suggest that our solar system was formed about 4.5 billion (4500 million) years ago from a huge spinning cloud of cosmic dust and gas called solar nebula. As this cloud condensed, the rotational movement threw a ring of cloud which kept on rotating around the central mass. The whole matter condensed under the force of its own gravity to become a flattened spinning disc of atoms and dust particles . The central mass condensed further and formed the sun. As the sun condensed, it became hotter and brighter due to conversion of gravitational energy into heat. Atoms and dust particles orbiting around the central mass, i.e., sun then aggregated into clumps (about the size of asteroids). These clumps finally swept up more particles and grew into different sized planets. The latter kept revolving round the sun in different elliptical orbits.

THEORY OF SPECIAL CREATION

It stated that life was created by the wishes of a divine being or some supernatural power, the Creator or God out of nothing at a particular time in the remote past. According to Christian belief, God created the universe, plants, animals and human beings in six natural days. Adam and Eve were the first man and woman created by God. According to Hindu mythology, Lord Brahma created the world with his wish. Manu and Shradha were the first man and woman respectively.

The theory of special creation is purely a religious concept. It is not based on scientific facts.

THEORY OF SPONTANEOUS GENERATION

It stated that life originated spontaneously from non-living materials-abiogenesis. For example, fly larvae were believed to develop from rotten meat; frogs, snakes and crocodiles could form from the moist soil (mud) of the river. However, this idea was laid to rest by Louis Pasteur (1862) who experimentally proved that microorganisms, such as protists and bacteria, arise from the pre-existing microorganisms of their kind. Since then, it is being widely accepted that life originates from pre-existing life-biogenesis.

INTERPLANETARY OR COSMOZOIC THEORY 

According to this theory, life could have originated once or several times in various parts of the galaxy in universe and it reached the barren earth from some other heavenly body in the form of resistant spores or seeds. The latter grew and then evolved into various existing forms. This theory is called theory of panspermia.

Because of hazards like low temperatures, very high influx of cosmic and UV radiation, lack of atmosphere, extreme dryness etc. that exist in the interplanetary space and are conductive to life, this theory of origin of life. 

CHEMICAL ORIGIN THEORY

It is now well accepted that life originated from inanimate matter by a series of chemical reactions on the primitive earth. This concept was put forward by Alexander. I. Oparin (1923) and J.B.S. Haldane (1929) independently. It was summarized by Oparin in his book origin of life' as 'abiogenesis first, but biogenesis ever since'. This theory is now known as primary abiogenesis.

J.B.S. Haldane, a British scientist suggested in 1929 that life must have arisen on the primitive earth from a collection of chemicals through a progressive series of chemical reactions in which atoms combined into inorganic molecules, inorganic molecules into simple organic compounds, simple organic compounds into complex organic compounds (polymers), and polymers into aggregates, which finally got organized into the living matter. This primary abiogenesis is thought to have occurred about 3.6 billion years ago. Haldane speculated that the conditions on the primitive earth were far from the conditions we see today. The atmosphere was highly reducing and oxygen in molecular form was not available. The earliest molecules on road to life might have been the formed gases and inorganic compounds, namely, hydrogen, nitrogen , ammonia, methane, carbon dioxide and water vapour. The energy for the formation of molecules might have come from reflected solar radiations, lightning or volcanic eruptions . The primitive inorganic molecules later interacted with one another to form simple organic compounds, namely, sugars, fatty acids, amino acids and organic bases (purines and pyrimidines). Energy for such interactions was probably provided by solar radiations and electrical discharges. The reactions that led to the formation of organic compounds partly occurred in the primitive atmosphere but mainly in the oceans. Absence of life and reducing atmosphere preserved these compounds long enough.

MILLER AND UREY’S  GLASS EXPERIMENT

Major support to this theory was provided by Stanley L. Miller and Harold C. Urey in 1953. They designed a glass apparatus and simulated the conditions that thought to have existed on primitive earth. They circulated a mixture of methane (CH4), ammonia (NH3) and hydrogen (H2) in the ratio of 2:2:1, and water vapour (H2O) in this apparatus. These gases were believed to prevail in the primitive earth. The mixture was maintained at a temperature just below 100°C. They provided energy for interaction of gases in the form of electric sparks in the gas flask. At the end of a week, 15% of the carbon (from methane) had been converted to simple organic compounds such as amino acids, simple sugars, purines and pyrimidines.