Origin of life
When we look at stars on a clear night sky, we are, in a way, looking back in time. Stellar distances are measured in light-years. What we see today is an object whose emitted light started its journey millions of years back and from trillions of kilometers away and reaching our eyes now. However, when we see things in our immediate surroundings, we see them instantly and hence in the present time. Therefore, when we see stars, we are peeping into the past.
The origin of life is considered a unique event in the history of the universe. The universe is vast. The earth itself is almost only a speck. The universe is very old-almost 20 billion years old. Massive clusters of galaxies comprise the
universe. Galaxies contain stars and clouds of gas and dust. Considering the size of the universe, the earth is indeed a speck. The Big Bang theory attempts to explain to us the origin of the universe. It talks of a singular, massive explosion unimaginable in physical terms. The universe expanded, and hence, the temperature came down. Hydrogen and Helium formed sometime later. The gases condensed under gravitation and formed the galaxies of the present-day universe. In the solar system of the milky way galaxy, the earth was supposed to have been created about 4.5 billion years back. There was no atmosphere on early earth. Water vapor, methane, carbon dioxide, and ammonia released from molten mass covered the surface. The UV rays from the sun broke up water into Hydrogen and Oxygen, and the lighter H
escaped. Oxygen is combined with ammonia and methane to form water, CO, and others. The ozone layer was formed. As it cooled, the water vapor fell as rain to fill all the depressions and create oceans. Life appeared 500 million years after the earth's formation, i.e., almost four billion years back.
Did life come from outer space? Some scientists believe that it came from outside. Early Greek thinkers thought units of energy called spores were transferred to different planets, including earth. 'Panspermia' is still a favorite idea for some astronomers. It was also believed that life came out of decaying and rotting matter like straw, mud, etc.
This was the theory of spontaneous generation. Louis Pasteur, by careful experimentation, demonstrated that life comes only from pre-existing life. He showed that in pre-sterilized flasks, life did not come from killed yeast, while in another flask open to the air, new living organisms arose from 'killed yeast. Spontaneous generation theory was dismissed once and for all. However, this did not answer how the first life form came on the earth.
Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g., RNA., protein, etc.). That formation of life was preceded by chemical evolution, i.e... the appearance of diverse organic molecules from inorganic constituents. The conditions on earth were high temperature,
volcanic storms, reducing atmosphere containing CH, NH, etc. In 1953, S.L. Miller, an American scientist, created similar conditions on a laboratory scale. He made electric discharge in a closed flask containing CH, H., NH, and water vapor at 800°C. He observed the formation of amino acids. In similar experiments, others observed—formation of sugars, nitrogen bases, pigment, and fats.
Analysis of meteorite content also revealed similar compounds indicating that similar processes are occurring elsewhere in space. With this limited evidence, the first part of the conjectured story, i. e.. chemical evolution was more or less accepted. of life arose. The first non-cellular forms of life could have originated. We have no idea about how the first self-replicating metabolic capsule of life occurred. The first non-cellular forms of life could have originated3 billion years
back. They would have been giant molecules (RNA, Protein, Polysaccharides, etc.). These capsules reproduced their molecules, perhaps. The first cellular form of life did not possibly originate till about 2000 million years ago. These were probably single-cells. All life forms were in a water environment only. This version of biogenesis. i.e., the majority accepted the first form of life that arose slowly through evolutionary forces from non-living molecules. However, once formed, how the first cellular forms of life could have evolved into the complex biodiversity of today is a fascinating story.
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