Introduction How and when did life originate has been a question for man since prehistoric times? Present living organisms comprised of a wide range of plants and animals. All these organisms show a remarkable diversity in their shape, size and functions. Yet, a fundamental similarity exists among them. Such as all living beings are made up of the same elements, they have cells as their structural and functional units and DNA is the hereditary material in all of them. From these similarities, it appears that all the organisms might have evolved from some common ancestor. The study of the condition and the mechanism involved in the creation of primitive life on our earth is the study of the origin of life.
Theories of origin of life The different theories proposed to explain the origin of life on the earth are as follows.
Theory of eternity The theory of eternity suggests the unchangeable characteristics of the universe. The organisms have existed and will remain to exist in the same form, unaltered till eternity. Hutton said that there is neither a beginning nor an end to this world. This theory has no scientific support and since long discarded.
Theory of special creation Until the middle of the nineteenth century, it was believed that life was created by some supernatural power. According to Hindu mythology, the living world was created by God Brahma from his own body. It is believed that Gods, men and devils developed from the head of Brahma, birds from his chest, goats from his mouth and plants from the hair of his body.
As per Christian mythology and as given by a Spanish monk, Father Sudrez, the world was created in six days. God created earth and heaven on the first day and sky on the second day. On third day, earth surface was covered with plants and animals. On the fourth day, God created the sun, moon and stars. The birds and fishes appeared on the fifth day and finally, man and beasts were created on the sixth day of creation. On the seventh day, a woman was created from the 12th rib of man.
Theory of panspermia or Cosmozoic theory According to this theory, life existed somewhere in the universe in the form of spores name cosmozoa. These cosmozoa have reached the earth accidentally from some other planet of the solar system. On getting favourable conditions of life, they developed into organisms. Recently H.F.C. Crick in his book has also written that life on earth could have originated elsewhere in our galaxy and probably reached here accidentally.
Theory of spontaneous generation or Abiogenesis Till the end of the seventeenth century, people believed in abiogenesis or in spontaneous generation of living things from non-living things. This theory was proposed by Anaximander and Aristotle. Van Helmont described the production of mice from wheat grains and soiled clothes kept in a pot for 21 days.
In 1747, Joseph Needham demonstrated the spontaneous origin of minute living organisms in the infusion, which he boiled for a few minutes in a corked flask.
Theory of Biogenesis Italian physician Francisco Redi by conducting a simple experiment demonstrates that abiogenesis cannot exist. He kept cooked fished in three jars:
(Note: Image courtesy www.wikipedia.org)
Maggots appeared in the uncovered jar, but not in covered jars, when observed after a few days. Plenty of flies were seen sitting and laying eggs on the muslin cloth, the covering of the second jar but no maggots appeared in the fish inside. From these observations, Redi concluded that flies arose from the eggs by the parent flies, the maggot could not have come into existence spontaneously. Abbe Spallanzani also confined the views of Redi. Thus with the help of this experiment, Redi proved biogenesis theory, i.e. life arises from pre-existing life. But Redi couldn't explain how the first cell or organism was formed on the earth?
Conclusion All the above theories were ultimately discarded as there were no evidence for them. Then how life was generated on the earth? This mystery was solved by Oparin and Haldane and they proposed the biochemical origin of life. The theory of biochemical origin is the most widely accepted theory to explain the origin of life on earth, as will be seen in detail in the article Biochemical theory of the origin of life.
Hakimuddin Kuwakhedawala is based in India with over 15 years experience as a teacher. Teaching and writing are his passion. Most of the articles of Hakimuddin are related to education, exam, environmental problems, etc.
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Origin of life is a great riddle for mankind and though there are various theories none could explain it clearly and convincingly. The main difficulty is to explain how the first living form, even if it was a single-cell, came into existence. Whether it was eternally there or was created by some accidental incident in nature where some chemicals in a special condition gave rise to the miraculous life form which later could evolve in all possible forms ranging from ants to dinosaurs. This puzzle of the conversion of dead material to life form is the greatest of all the mysteries of nature and once it is found many concepts would change in the human society and belief patterns. It is also strange to note that out of so many animal species only the human being rose to such high levels of consciousness that they are able to study such difficult matters with the help of their advanced brains and logical faculties. Some scientists believe that humans would one day unravel most of the mysteries of nature by this sheer force of consciousness that nature has provided to them. Let us see how scientific progress makes headway in this unexplored and dark direction. Seeing the phenomenal progress of science in the past few centuries this does not appear an impossible task but the only thing is that such deductions take their own time due to higher difficulty level and complexities.