Origin of life begins with the origin of the universe, which has been estimated to be about 20 billion years ago. The Big Bang Theory is the most accepted theory regarding the origin of the planet Earth and the existence of different life forms on it. According to this theory, the universe is a result of a massive explosion that occurred 20 billion years ago. Whether it is a hypothesis or a fact, a new universe was formed. The atmospheric condition after the explosion became more stable. The temperature reduced and gases like hydrogen and helium formed which led to the formation of galaxies of today. Later, after 10 billion years, the earth was formed which was covered by water vapor, methane, carbon dioxide, and ammonia. There was no atmosphere but only gases and moisture. The powerful rays of the sun stimulated and hastened evolution. By making and breaking bonds between gas molecules, the Earth came out with a new face. After millions of years i.e., once the Earth atmosphere was stabled, the first life on earth came into existence (around 4 million years ago). There began the story of the origin of life on earth.
Abiogenesis
It is the generation of life from non-living matter and not precisely known as ‘ Spontaneous generation’. This theory states that complex living organisms are generated from decaying organic matter. Example: Maggots spontaneously appeared in meat.
Francesco Redi, an Italian Physicist, Naturalist, Biologist, and Poet, was the first person to disprove the theory of spontaneous generation by demonstrating that maggots came from the eggs of the flies. Unfortunately, several scientists were still unconvinced by his experiment.
Louis Pasteur, a French Biologist, Microbiologist, and Chemist, performed a similar experiment and successfully convinced the scientific population that Abiogenesis was indeed impossible.
Biogenesis
‘Omne vivum ex vivo’ similar to ‘Omnis cellula e cellula’ which means all cells come from cells. The theory states that living things come from other living things. The term biogenesis was coined by Henry Charlton Bastian.
Chemical Evolution
In the 1920s, Russian scientist Aleksander Oparin and English scientist J. B. S. Haldane both separately proposed what’s now called the Oparin-Haldane hypothesis: That life on Earth could have arisen step-by-step from non-living matter through a process of “gradual chemical evolution.” According to their theory, life evolved in the oceans during a period when the atmosphere was reducing – containing H2, H2O, NH3, CH4, and CO2, but no free O2. Organic compounds were synthesized non-biologically by ultraviolet light energy, which in the absence of an ozone shield would penetrate the upper layers of the ocean. Without free O2 to oxidize them, these organic molecules would be stable and would accumulate in a warm, dilute broth that has been nicknamed “Haldane soup” – as proposed by J.B.S Haldane in 1929.
The first living organism would be little more than a few chemical reactions wrapped up in a film or membrane to keep them from being diluted and destroyed. These organelles would absorb chemicals, grow, divide, and obtain energy by fermenting the available organic molecules around them. Photosynthesis would arise eventually as an alternative energy source when natural foods ran short. The oxygen released by photosynthesis would have the side effect of screening out the ultraviolet radiation with an ozone layer in the upper atmosphere and eventually would turn the atmosphere from reducing to oxidizing. Free oxygen would lead to the evolution of respiration and to modern eukaryotic metabolism.
Miller-Urey Experiment
In 1953, Stanley Miller and Harold Urey did an experiment to test Oparin and Haldane’s ideas. The Miller–Urey experiment was a chemical experiment that simulated the conditions thought at the time (1952) to be present on the early Earth and tested the chemical origin of life under those conditions. It established that the conditions of the early earth were sufficient to produce organic molecules like amino acids as confirmed by chromatographic analysis. In the initial experiment, Miller identified five amino acids present in the solution: Glycine, α-alanine, and β-alanine were positively identified, while aspartic acid and α-aminobutyric acid (AABA) were less certain, due to the spots being faint. 
In a 1996 interview, Stanley Miller recollected his lifelong experiments following his original work and stated: “Just turning on the spark in a basic prebiotic experiment will yield 11 out of 20 amino acids.”
Is the experiment still meaningful?
Scientists now believe that the atmosphere of early earth was quite different from that as proposed by A.I Oparin and J.B.S Haldane i.e. not reducing, and not rich in ammonia and methane. However, a variety of experiments done in the years since have shown that organic building blocks (especially amino acids) can form from inorganic precursors under a fairly wide range of conditions. From these experiments, it seems reasonable to imagine that at least some of life’s building blocks could have formed abiotically on early Earth. However, exactly how (and under what conditions) remains an open question.
Complex Biological molecules
The experiment explained the formation of simple bio-molecules but further evolution depends on the polymerization or condensation of these monomers to polymers.
In the 1950s, Sidney Fox and co-workers at the University of Miami produced synthetic peptides like products of molecular mass between 4000 and 10,000 Da by heating a dry mixture of amino acids at 150℃ and 180℃. He called these ‘Proteinoids’ or Thermal proteins. The sequences of these “thermal proteinoids” are not completely random, but show some internal order. These polymers display a limited catalytic activity, probably resulting from their charged side chains of acidic and basic amino acids. These thermal proteinoids have another interesting property. If a hot proteinoid mixture is washed with water or salt solution, microspheres of a fairly uniform 20,000-Å diameter are formed, as in the photograph to the right.
Microspheres – These are small globules of the proteinoid polymer solution, enclosed by a semipermeable proteinoid film with some of the physical properties of simple cell membranes. Microspheres shrink and swell in salt solutions of different concentrations. They will grow at the expense of dissolved proteinoid material and have been observed to bud like yeast cells to produce “daughter” microspheres. They can be induced to fission by MgCl2 or by a pH change. The enclosing film is a double layer resembling those found in soap films and artificial and natural membranes.
Fox hypothesized that proteinoid material first polymerized on hot, dry volcanic cinder cones, and then was leached into the oceans by rain to form microspheres, which then could have become the early segregated chemical systems that eventually led to protocells. But the weakness of this theory stems from the need for the high concentration of dry amino acids that didn’t fit into the concept of Haldane’s soup.
In Russia, Oparin proposed that Coacervates may be the intermediates between loose molecules and a living system.
Coacervate – is the aggregation of colloidal particles in the liquid phase that persists in the form of tiny droplets. They are capable of exchanging substances with the environment, increasing in size, and selectively concentrating compounds within them.
The spontaneous self-assembly of macromolecules into coacervates and microspheres indicates that the occurrence of similar structures under primitive conditions would give rise to a more organized membrane-bound structure containing molecules, protocells.
Membranes defined the first cell
Oparin has suggested an evolutionary scheme for protocells or “protobionts” along the lines suggested by his coacervate experiments. He proposes that in lakes or ponds with appreciable concentrations of polymerized material, coacervate droplets would be formed naturally by wave action. In general, the composition of these droplets would differ from that of the bulk solution. These “microenvironments” in time could develop into enclosed systems of chemical reactions that absorb high-energy compounds from their surroundings to perform protective reactions or other necessary syntheses.
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Manjushri A 