Chemical Evolution or Molecular Evolution of Origin of Life

It has been estimated that some 5000 million years ago the Earth was formed from the Sun . The Earth , as a piece , was broken from the Sun and was a glowing fire containing a molten mass of gases and vapours of various elements . The temperature was very high and it was about 5000-6000 degree C . As the Earth was moving away from the Sun , it was getting cool . This led to the condensation of gases . The heavy elements like iron , nickel etc. occupied the core of the Earth and the lightest elements like helium , hydrogen , oxygen , nitrogen and carbon occupied the atmosphere of the Earth . Life originated only from these elements . The chemical changes leading to the formation of life is called Chemical Evolution or Molecular Evolution of Origin of Life . Oparin(1924) and Haldane(1929) developed the theory of Chemical Evolution and according to the theory life originated as follows -

Formation of Water , Methane and Ammonia 

The primitive Earth contained a large amount of hydrogen , nitrogen , carbon and oxygen . Of these , hydrogen was very active . It combined with nitrogen to form ammonia , with oxygen to form water and with carbon to form methane .As the temperature of the Earth was high , ammonia and methane remained as gases and water as superheated steam .As years passed , the temperature of the earth came down . Steam condensed into water and resulted as rain . Since the Earth was very hot , the rain drops were evaporated immediately on reaching the surface of the Earth . Water gradually accumulated and this led to the formation of rivers , streams , lakes ,seas and oceans . Compounds like ammonia , methane etc. were dissolved in rain water and were accumulated in the sea . Mineral rocks also dissolved leading to the accumulation of minerals and salts in sea water . Thus the first chemicals formed on the Earth were water , ammonia , methane and others .

                                                                   Formation of Micromolecules

The next step in Molecular Evolution was the formation of micromolecules like amino acids , fatty acids , monosaccharides , purine , pyrimidines , adenosine monophosphate(AMP) , adenosine dipphosphate(ADP) in form of colloids .

         Formation of Macromolecules 

The next step was the formation of macromolecules . . The micromolecules combined in various ways to form large molecules called macromolecules . They consisted of proteins , lipids , polysaccharides , nucleic acids and nucleoproteins . The combination of smaller molecules into macromolecules is called polymerization .Moody(1970) , Fox (1974) and Dose(1972) suggested that proteins were the first macromolecules to be originated during molecular  evolution . The first-formed proteins were called protoproteins .

Formation of Nucleic Acids 

The next step was the formation of nucleic acids . They were formed from phosphoric acids , sugar , purines and pyrimidines . In the formation of nucleic acids , first of all , nucleotides were formed . The nucleotides include purine-sugar-phosphate and pyrimidine-sugar-phosphate . Thousands of nucleotides were linked together in different combinations to produce nucleic acids .

Formation of Nucleoproteins 

The nucleic acids and proteins combined together to form larger molecules called nucleoproteins .

Coacervation 

When large molecules were formed they underwent aggregation and precipitation in the sea .The aggregation and precipitation led to the formation of organised structures called coacervates . The formation of coacervates was called coacervation . Fox named the colloidal droplets (coacervates ) as microspheres . They exhibited certain properties of cells . They were spherical , microscopic and uniform in diameter . They had double layered boundaries around them and the capacity for binary fission . budding and fragmentation .

Pre-cells or Protocells 

The earliest first living units are called protocells or precells or protobionts . The protocells were spherical in shape and a double layered membrane was present around them . They exhibited reproduction by budding and mutation . The protocells were heterotrophs ; they obtained the energy by the fermentation of organic substances which were dissolved in the sea . Thus the protocells were anaerobic heterotrophs .

There are two major objections against Chemical Evolution theory . The first is that , Oparin in his theory argued that a 'primeval soup' of organic molecules could be created in an oxygenless atmosphere through the action of sunlight . But some researchers have suggested that such primitive atmosphere did not exist for any appreciable length of time . Second objection is about the 'primeval soup' . Such soup should contain relatively large quantities of nitrogenous organic compounds , such as amino acids , bases etc. It is expected that such nitrogen rich soup would have given significant quantities of nitrogenous cokes trapped in early Precambrian sediments . No such nitrogen rich material has yet been found in early Precambrian rocks on this planet . In fact , the opposite is true ; nitrogen content of early Precambrian organic matter is relatively much less .

                 But there is good evidence in the rocks that simpler living beings were present and their early evolution did take place throughout the early Precambrian time . Life probably originated on Earth at least 3,800 million years ago as evident from different Precambrian metasediments and stromatolites . Obviously , Chemical Evolution had occurred before 3,800 million years and unfortunately , no Precambrian rocks found are old enough to test these ideas . So , where else we can look to explain existence of early life? Scientists studied meteorites and interstellar molecules for useful clues .

How can we define '' LIFE '' ?

All living things on this planet are qualified with some properties , which we forward as characteristic of life . Apparently these properties are used to differentiate living beings from non-livings . There have been many suggestions , suppositions and theories about how life began on Earth . But , in the discussion of origin of life, a major problem is how to define life . Some say it is ''unique'' some a ''miracle'' . But it is obvious that , anyone who has the power of thinking is puzzled and bewildered by ''life'' .

Then '' what is life '' ? To give a complete definition of it is a real hard task . There are countless definitions but none is absolutely fool proof because there always seem to be one or more exception to any particular rule . In the primary classes of Biology , the simple definition of a living object is that it moves , eats , grows , excretes , reproduces etc. But these features do not suffice because there are always significant exceptions to the list .

Aristotle believed that throughout the study of evolution of life from a simplest atom to a complex living organism , it is impossible to say where non-life ends and life begins .Many modern scientists do agree with him and opined that matter simply differs according to its degree of organization . In recent advancement in Biology , Biochemistry and Biophysics , several definitions of living system have emerged with rigid physical and chemical terms , such as --

1. A living organism can be defined as a metabolizing system that reproduces itself , mutates and reproduces mutation .
2. Living things are likely to be Carbon-based , dependent on a supply of liquid water and on either absence of free Oxygen or the presence of suitable Oxygen-mediating enzyme system .
3. Life is the structure-replication of enzymes , ensured by exactly-produced nucleic acid molecules .
4. A living organism is a molecular system , capable of reproducing itself from generation to generation .

These definitions are definitely detailed molecular-biological attributes but heir rigidity is their weakness . They are actually describing only a few of the important functions of the system , not the system itself .Cairns-Smith(1993) argued that there should be no absolute qualities of life , because it has emerged gradually during the early evolutionary processes . The purposes of life are to survive , to compete , to reproduce and to adapt , but all these inbuilt qualities of life are imposed on life by the process of evolution . So life is a product of evolution .

Life on Earth is a miracle . But far from being a miracle , life may be a necessary and inevitable consequence of the pathway through which the Earth originated and evolved . Indeed , right from the beginning , the mass , the chemical composition , and the physical and astronomical parameters of or planet is ideal for origin and evolution of life . On many other Earth-like planets in the Universe , life must have originated and evolved . Probably , life also originated on Mars , but due to its loss of atmosphere to outer space , life did not go far . All other planets and moons in our solar system are physically and chemically unsuitable for development of life . So , we may consider the opposite as miracle ; the absence of life on a suitable planet like Earth could be a miracle!