ORIGIN OF LIFE

 In 1920's,Haldane and Oparin independently proposed the modern hypothesis on the origin of life.That is now the prevailing view held by most scientists.they proposed that biological evolution was proceded by a long interval of chemical  evolution,during which simple natural compounds(water,ammonia,methene,hydrogen etc.)under the influence of natural energy sources (lightening,uv radiation,heat etc.)reacted to form simple organic molecules, these inturn interacted to produce ever more complex organic molecules,eventually to lead to a self replicating cell.

Stanley Miller(1953)conducted an experiment to test the hypothesis of chemical evolution of life,using a mixture of simple gases like H2,H2o,CH4,NH2 which are the principal chemical elements of life (C,H,N,O:carbon,hydrogen,nitrogen and oxygen).In a closed system without free oxygen and using high electric sparks as an energy source to stimulate lightening,the resulting products consisted of a mixture of aminoacids,the building blocks of proteins which aree the structural expressions of genes and other simple organic compounds.Different experiments revealed that variety of simple organic compounds can be obtained from various gas mixtures and energy sources.It is really very difficult to assess the axact conditions and these products might have assembled in closed circuits /areas in aquatic conditions,to produce the first group of organisms with vital activities and this can be the possible step for initiation of life.The second speculation on the next steps in chemical evolution can be the dissolutions of chemical substances  in an ocean (called as dilute soup) reacted with eachother to form larger molecules and later still large molecules.

These reactions might have occured on mineral surfaces,where the molecules were preferentially absorbed and concentrated.Eventually these molecules became large enough to be hydrophobic and separated from the water into discrete "droplets"that grew in size, as they absorbed other organic molecules.These membrane bound systems(proto cells) were organic molecules.These membrane bound systems(protocells) were isolated from the surrounding water and the complex mixture of molecules within them continued to react.Eventually the droplets grew too large and later split into two.Initially the separated halves may not have been identical but eventually the system might have designed a means of divisions/replication to produce identical halves or copies of itself the first cells.With advancement of science some and all of these steps,or variations of them or alternatives of them,might eventually be the guideline for demonstration of origin of life in the laboratory.This might take many many years to constitute and produce a live cell,but this is also not certain in a life time schedule.

The first major evolutionary advance after life appeared on earth was the ability of organisms to manufacture their own food from simple inorganic compunds such as carbondioxide and water.The first organisms undoubtedly were not capable of doing this and had to depend on pre-existing organic compounds in the ocean for their food.This limited supply or no supply,togather with the remains of dead cells that might be harvested,could not sustain the growing population of organisms.At some point,one or more organisms developed the ability to use chemical energy to manufacture the organic compounds that they needed for life processes.A number of primitive one celled organisms continued  to use this process is called chemosynthesis.

slowly a more advanced method evolved that used the readily available incoming light energy from sun.The first step is photosynthesis.on later with time ,did this is the process involve into the oxygen releasing photosynthesis and it is the primary basis of all plant life and provides food to all the food chains now available in all types of ecosystem.The organism which prepare their own food is called autotroph and those who depend on others for food are called heterotrophs.These autotrophs utilize the mineral resources and energy and produce energy rich compounds and oxygen.This oxygen enriches the atmosphere.

One of the most important event occured in the earth surface is release of molecular oxygen by autotroph which enriched the surface environment.All animals and plants need this surface molecular oxygen to breathe and these organisms could not have evolved until our atmosphere acquired oxygen in sufficient quantities.Earlier many elements that have multiple oxidation states existed in their reduced forms.With the enrichment of oxygen,these multivalency elements began to oxidise,changing the chemistry of the oceans and land surfaces.The enrichment of molecular oxygen played a key role for the sustainance of all life forms,which existed to day.