Life as we know it depends on the availability of certain long molecules (polymers): proteins, RNA, DNA, polysaccharides, lipid bilayers, etc. And the chemistry observed is all organic, i.e. carbon-based.
We have to explain three stages in the emergence of life on Earth:
Stage I. Formation of organic molecules ('monomers') from inorganic matter.
Stage II. Covalent bonding (polymerization) of the monomers into long biomolecules.
Stage III. Emergence of the complete biological cell, ~3.5 billion years ago, capable of survival and propagation.
The first major theory for explaining Stage I was put forward by Alexander Oparin in 1924. He argued that certain organic molecules that were necessary building blocks for the emergence of life required an oxygen-free or 'reducing' ambience for their self-synthesis. He imagined a 'primeval soup' of organic molecules that got created in the oxygen-free atmosphere prevailing at that time, aided by the action of sunlight. And these molecules combined in increasingly complex ways, till they formed the so-called 'coacervate' droplets. Such droplets could 'grow' by fusion with other droplets. And they could 'reproduce' through fission into smaller droplets. This primitive metabolism underwent Darwinian-like chemical evolution in which factors favouring cell integrity survived and evolved. No DNA-type replication was involved in this earliest of 'life forms'.
Similarly, J. B. S. Haldane argued that the prebiotic oceans of the Earth were very different from what they are now, and that a 'hot dilute soup' existed in them in which organic molecules could get formed.
A famous experiment for verifying the Oparin-Haldane hypothesis was carried out by Miller and Urey in 1952. They created electric sparks in a mixture of water, hydrogen, methane, and ammonia. After one week a full 10-15% of the carbon in the mixture was found to have converted to organic compounds, including amino acids (the building blocks of proteins). Recent analysis of their saved vials has revealed that as many as 23 amino acids were formed, whereas they detected only five.
Sidney Fox carried out several experiments that confirmed the formation of long organic molecules from inorganic matter. He allowed amino acids to dry out as if from a warm puddle, mimicking prebiotic conditions. The amino acids were found to form long, even cross-linked, thread-like molecules, now called 'proteinoids'. He also collected volcanic material from a cinder cone in Hawaii, and found that the temperature was over 100oC just four inches beneath the surface. He speculated that this was probably the environment in which life emerged. Biomolecules formed in such conditions could have got washed away to the seas. In an experiment, he placed lumps of lava over amino acids created from a mixture of methane, ammonia and water, sterilized the whole thing, and baked it for a few hours in a glass oven. What he got on the surface was a brown sticky substance. On drenching the lava in a sterilized water tank, a brown liquid leached out. The amino acids had formed proteinoids which, in turn, had combined to form small, cell-like spheres. These 'microspheres' are now known as 'protobionts'. These were not biological cells; they did not contain any functional nucleic acids. But they did form lumps and chains, rather like what cyanobacteria do.
Manfred Eigen and Leslie Orgel demonstrated that a solution of nucleotide monomers can, under suitable conditions in the laboratory, give rise to a nucleic-acid polymer molecule (RNA) which replicates and mutates and competes with its progeny for survival. For achieving this, Eigen used a polymerase enzyme, a protein catalyst extracted from a 'bacteriophage' (the synthesis and replication of the RNA depends on the structural guidance provided by this enzyme).
Orgel did something complementary to the experiment of Eigen. He made RNA grow out of nucleotide monomers by adding a template for the monomers to copy, but did not add a polymerase enzyme.
Thus Eigen made RNA using an enzyme but no template, and Orgel made RNA using a template but no enzyme. Modern-day living cells use both templates and enzymes for making RNA. This work pointed to a possible parasitic development of RNA-based life in an environment created by a pre-existing protein-based life.
There is a strong chance that life appeared on Earth during the so-called 'thermophilic energy regime' (I shall explain this terminology in a later post). This form of life comprised of microorganisms that thrived in hot conditions. Emergence of any form of life means a build-up of order, complexity, and information. For this to have become possible, there had to be conditions far from equilibrium, i.e. an energy gradient (cf. Part 6). One view is that, most probably, sunlight did not play a major role in this, and that the energy source was of geothermal origin. Volcanic heat sources under the sea (hydrothermal vents) provided the upper end of the energy gradient, the lower end being the cold atmosphere above the seas. Under the dominant influence of the driving force provided by this energy gradient, chemical evolution and diversification of molecular structure occurred. As proposed by Kauffman in 1993, such chemical evolution led to the emergence of autocatalytic reactions. His model obviates the need for the prior presence of information-rich DNA molecules for the synthesis of protein molecules, and also provides a non-random mechanism for the origin of life. Closed-loop autocatalytic reactions led to the production of life-like molecules of increasing complexity. Things progressed to a point where the forebears of DNA started appearing, which had the potential for replication. The biological prokaryotic cell emerged in due course.
There is also a viewpoint that, because of the presumed unfeasibility of the time scales involved for a terrestrial origin of life, life might have originated elsewhere in the cosmos, and brought to our Earth by meteors etc.
There is still a lot of debate on what really happened that led to the emergence of life on Earth. In the next two posts I shall describe two major models about the origin(s) of life.