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!