Dr. N.L. Kachhara
Some scientists believe that life arose spontaneously from available materials present on the early earth. The basic inorganic chemicals from which life was formed are methane (CH4), ammonia (NH3), water (H2O) hydrogen sulphide (H2S), carbon dioxide (CO2), and phosphate (PO43+). As of now, no one has yet synthesized a "proto-cell" using basic components, which has the necessary properties of life. Without such a proof-of-principle, explanations have tended to be short on specifics.
A number of clearly defined stages in explaining the origin of life has been recognized.
Stage 1: The origin of biological monomers
Stage 2: The origin of biological polymers
Stage 3: The evolution; from molecule to cell.
The 'Miller Experiments" (including the original Miller- Urey experiment of 1953) are performed under simulated conditions resembling those thought at the time to have existed shortly after Earth first accreted from the primordial solar nebula. The experiment used a highly reduced mixture of gases (methane, ammonia and hydrogen) in water vapour. The gases passed through a chamber containing two electrodes with a spark passing between them. The experiment showed that some of the basic organic monomers (such as amino acids) that form the polymeric building blocks of modern life could be formed spontaneously.
Simple organic molecules are a long way from a fully functional self-replicating life from; however, in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for chemical evolution ("soup theory"). On the other hand, the spontaneous formation of complex polymers from abiotically generated monomers under these conditions is not at all a straight forward process. Besides the necessary basic organic monomers, also compounds that would have prohibited the formation of polymers were also formed in high concentration during the experiments.
Other sources of complex molecules have been postulated, including sources of extra- terrestrial stellar or interstellar origin. For example, from spectral analysis, organic molecules are known to be present in comets and meteorites. The most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerize and form more complex structures, interacting in consistent ways to form a protocell.
So far no RNA molecules that direct the replication of other RNA molecules have been identified in nature. Some recent experiments provided that an RNA molecule produced by prebiotic chemistry could have carried out RNA replication on the early earth. However, much remains to be done, but scientists hope that some kind of RNA- catalyzed reproduction of RNA will be demonstrated in the not too distant future.
Gunter Wachtershauser provided another possible answer to polymerization conundrum in 1980s, in his iron-sulphur world theory. In this theory, he postulated the evolution of (bio) chemical pathways as fundamentals of the evolution of life. According to latest modification of this theory, the first cellular life forms may have evolved inside so-called black smokers at seafloor spreading zones in the deep sea.
The Bubble Theory
Waves breaking on the shore create delicate foam composed of bubbles. While bubbles comprised of mostly water burst quickly, oily bubbles happen to be much more stable. The phosphor lipid is a good example of an oily compound believed to have been prevalent in the pre biotic seas. They have the tendency to spontaneously form lipid membrane in water. A lipid bilayer bubble can contain water, and was a likely precursor to the modem cell membrane. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of this stuff was released into the medium, the development of the first prokaryotes, eukaryotes, and multi cellular organisms could be achieved.
Clay theory
A hypothesis for the origin of life based on clay was forwarded in 1985. Clay theory postulates complex organic molecules arising gradually on a pre-existing, non-organic replication platform - silicate crystals in solution. Complexity in companion molecules developed as a function of selection pressures on types of clay crystal is then expected to serve the replication of organic molecules independently of their silicate "launch stage".
"Deep - hot biosphere" model
Thomas Gold in the 1990s put forward a theory that life first developed not on the surface of the earth, but several kilometers below the surface. It is now known that microbial life is plentiful up to five kilometers below the earth's surface in the form of archaea, which are generally considered to have originated around the same time or earlier than bacteria, most of which live on the surface including the oceans.
