Test: Origin of Life (NCERT) - NEET MCQ

Meat was not spoiled when heated and kept in sealed vessel suggests that simples living organisms could not have originated spontaneously from non-living matter.

The atmosphere of earth before origin of life was strongly reducing (without free oxygen). At high temperature, free atoms combined to form molecules and simple inorganic compounds. Hydrogen, oxygen, carbon and nitrogen could not exist in free state and thus combined variously either among themselves or like CH₄, NH₃, H₂ and H₂O vapour.

The early inorganic molecules interacted and produced simple organic molecules such as simple sugars (e.g., ribose, deoxyribose, glucose, etc.), nitrogenous bases (e.g., purines, pyrmidines), amino acids, glycerol, fatty acids, etc.
CH₄+CO₂+H₂O→ Sugars + Glycerol + Fatty acids
CH₄+HCN+NH₃+H₂O→ Purines + pyrimidines
CH₄+NH₃+CO₂+H₂O→ Amino acids

According to Oparin-Haldane theory, life originated through bio-chemical processes of atoms that combined to form molecules. Molecules in turn reacted to produce inorganic and organic compounds. Organic compounds interacted to produce all types of macromolecules which organised to form the first form of life or cells.

The prebiotic atmosphere of the earth was of a reducing nature, i.e., no free oxygen was available. It was transformed into an oxidising atmosphere due to the emergence of cyanobacteria. They used water to get hydrogen and released oxygen into the atmosphere.

Stanley Miller in 1953 took an air tight apparatus and circulated four gases - CH₄, NH₃, H₂ and water vapour through it. He passed electrical discharges from electrodes at 800^∘C. Then he passed the mixture through a condenser He performed this experiment continuously in this way for a week and analysed the composition of the liquid inside the apparatus. He found a large number of simple organic compounds including some amino acids such as alanine, glycine and aspartic acid. Miller, thus, proved that organic compounds were basis of life.

It is presumed that the first living organisms were chemoheterotrophs that obtained energy by the fermentation of complex organic substances available to them from the sea broth. They were anaerobes.

According to Oparin, the atmosphere of primitive earth was reducing because H atoms were most numerous and most reactive. Large quantities of H₂,N₂ water vapour, CO₂,CH₄ and NH₃ were present but free oxygen was not present in significant amount.

The energy requiring synthetic processes, that occurred on primitive earth obtained energy from the sun, violent electric discharges like lightning, heat produced from volcanic eruptions and ionizing radiations like X-rays, etc.

Organisms have evolved from simpler forms to complex form. Hence the order of the events are genes first, then heterotrophic prokaryotes, then autotrophic prokaryotes, then eukaryotes and then animals.

Since many simpler and lower animals are aquatic and marine, and as the cells and body fluids of all animals contain salts, it is inferred that life originated in ocean (water).

Abiogenesis theory of origin states that life originated from non-living things in a spontaneous manner. This theory was supported by early Greek philosophers. In ancient Egypt, it was believed that the mud of the Nile river could give rise to frogs, snakes, mice and even crocodiles when warmed by the sun. Van Helmont believed that human sweat and wheat grains could given rise to organisms.
So, the correct answer is 'Spontaneous generation'.

Oparin observed that if a mixture of a large protein and a polysaccharide is shaken, coacervates form. The interiors, which are primarily protein and polysaccharides, with some water, become separated from the surrounding aqueous solution.

The primitive atmosphere of earth was reducing atmosphere that lack molecular oxygen but was rich in water vapour, carbon monoxide, carbon dioxide, nitrogen and hydrogen. Absence of oxygen and various high energy sources (UV rays, high temperature and lightening) allowed chemical reaction hydrogen and carbon to produce methane and other hydrocarbons. In absence of oxygen and presence of high temperature methane was present in unspoiled gaseous form and facilitated further chemical reactions to produce more complex protein structures. With origin of photoautotrophs, the organisms that use light as source of energy and carbon dioxide as chief carbon source, earth’s atmosphere II was replaced with atmosphere III. These photoautotrophs use water as electron donor during photophosphorylation that results in production of oxygen thereby adding oxygen to otherwise primitive reducing atmosphere. Photoautotrophs were the source of oxygen. Presence of molecular oxygen oxidized the reduced methane and other hydrocarbons into carbon dioxide.

• Louis Pasteur by careful experimentation demonstrated that life comes only from pre existing life.
• He showed that in pre sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from ‘killed yeast’.
• Spontaneous generation theory was dismissed once and for all.