If the assumption about the primitive atmosphere of earth is correct, then one is led to conclude that the development of life is quite natural and not at all unique to earth. That this ideally suited atmosphere existed, that just the right molecules were formed, that they, by chance, organized into a magnificent cooperative enterprise to produce self-duplicating macromole-cules, that these joined together instead of competing with each other, and that they evolved the mechanics of heredity in order to possess the genetic systems to perpetuate—all this staggers the imagination and taxes credibility. It has been compared to the oft-cited example of the monkey randomly pecking at a typewriter. Given enough time, measured in the billions of years, he will eventually type all possible arrangements, and so produce—Hamlet! The key here is—"given enough time." Geologists, calculating from the extent of radioactive decay in ancient rock formations, estimate the age of the earth, in its present form, at 4.5 billion years, and the emergence of life at 2.5 billion years. In other words, the incredible became not only credible but real in the space of two billion years.
Of course, man has not yet succeeded in synthesizing living material (defined as a self-replicating molecule). But, as Van-nevar Bush avers, "there is little doubt that he soon will. Some very simple short-chain nucleic acid, synthesized from inert matter and placed in a chemical soup, will suddenly assemble accurate images of itself and the job will be done."
The assumption is that if man can do it in the laboratory, Nature has done it by chance. Given the immensely long time of two billion years, the overwhelming odds against such random occurrence are severely diminished and natural bio­genesis, or spontaneous generation, may have taken place.
There are other theories advanced about the origin of life which ought to be mentioned in passing. One of these is the "pan-spermia hypothesis" of Svante Arrhenius, according to which life originated on earth through the migration of spores to earth from some other planet. But this only defers the question of the origin of life to some other site. Another, equally fantastic notion advanced by J. B. S. Haldane in 1954, is based on the "steady-state" theory of the universe.
Since the world, according to this theory, had no beginning, then life may be co-eternal with the universe, i.e., life always existed and also had no beginning.12 There are a number of other such theories, all of them (with the exception of the one just mentioned) assuming that life developed naturally from pre-living material.
The Historical Antecedents
Current speculation on extraterrestrial intelligent life is not exactly new. Both the astronomical ideas necessary for such life, and the conjecture itself about rational and sentient beings elsewhere, were known to antiquity. About 2500 years ago Anaximander proposed the idea of an infinite number of worlds, some in the process of being born and some dying. Two hundred years later another Greek, Democritus, inventor of the Theory of Atoms, elaborated the same idea in the con­text of his theory of the infinity of both space and time. A generation after Aristotle, Aristarchus already ventured a helio­centric conception of the universe.
In the sixteenth and seventeenth centuries, with the develop­ment of the new cosmography and the opening up of the limited, walled-in universe, speculation was rife about the ex­istence of extraterrestrial races of intelligent beings superior to man. Kepler, Galileo, and Descartes entertained such notions and discussed them quite openly. Giordano Bruno, in 1586, concluded that there must be an infinite number of morally imperfect beings, like man, on an infinite number of worlds. Lovejoy, the great historian of ideas, has shown that this in­terest was not the result of the new scientific conceptions initiated by Copernicus and Galileo, but rather of the philo­sophic development of certain ideas implicit in Plato.13
Thus, some three to four centuries before technology pro­pelled us beyond the gravitational pull of the earth, scholars were already discussing the possibilities of races of intelligent beings on some planet in this or some other solar system.
A new outburst of such speculation took place in the middle of the last century. In his Plurality of Worlds, William Whewell expressed his opposition to the idea that other planets
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