If life comes only from life, then every living creature which exists now is a direct descendant of the first bit of living protoplasm which appeared on earth (unless life arose from inanimate nature repeatedly). The origin of the first life of necessity is a highly speculative issue. Indeed, our inability to observe spontaneous generation in nature or to bring it about artificially in laboratory experiments shows that life must have arisen under some conditions which no longer obtain at present and about which we can make only the vaguest guesses.
Attempts have even been made to avoid the issue by supposing that life was introduced on our planet from other heavenly bodies. Spores or other germs of life may have arrived on earth in a meteor or with cosmic dust. But we are not certain that life exists or ever existed outside the earth, and, if it did, could stand the transport through the interplanetary space. In any case, we must face the problem of the origin of life in the universe.
All life that now exists, including the simplest viruses, has as its physical abode highly complex organic compounds, nucleoproteins. Not only nucleoproteins but even their constituent amino acids and nucleic acids are synthesized exclusively in living organisms and never spontaneously from inorganic substances (some of the amino acids can, however, be made synthetically in laboratories). Spontaneous formation of these energy-rich compounds is highly improbable on the basis of physicochemical considerations. Nevertheless, several scientists, Oparin in Russia, Dauvillier and Desguin in France, Bernal in England, Urey, Miller, and Blum in America, have tried to visualize conditions under which chemical substances now formed only in living organisms could have arisen without intervention of life. (USA Today, 1994).
In the early stages of the history of the earth the prevalent high temperatures permitted the existence of water only in the form of superheated steam.....................
