As we have seen, free oxygen in a planetary atmosphere is essential for a planet to be considered habitable. It is highly probable that virtually all the free oxygen in the Earth's atmosphere has come from the decomposition of water by green plants during photosynthesis. Thus the question becomes: what is the likelihood that this or a similar process would develop on other planets having all of the essential astronomical conditions ?
We do not actually know how life started on the Earth, although this topic has been much analyzed in recent years. If we assume that the origin of life has been a natural evolutionary process, however, there is no reason to suppose that life would not always originate whenever the conditions were correct. In support of this view is the evidence that microscopic forms of life appeared on the Earth rather soon after the Earth's formation. The oldest rocks, on the basis of analyses for radioactive decay products, are estimated to be about 4.5 billion years old, while the earliest detectable life forms appear in rocks that are about 2.5 billion years old, or even older in the opinion of some investigators, including T. C. Hoering of the Carnegie Institution's Geophysical Laboratory (Simons, 1962). Since the appearance of detectable life forms must have been preceded by a long period of evolution of chemical precursors and life forms that could leave no permanent evidence of their presence on the Earth, it seems that life originated on the Earth very soon after the environmental conditions became suitable.
Since living matter as we know it is composed of some of the most abundant elements in the universe and in the Earth's crust, there is no reason to suppose that life elsewhere on habitable planets, although undoubtedly differing from Earth life in fine structure, would be greatly different in general chemical composition. Wherever they are found, living organisms must always depend on the same basic chemical processes and physical laws with which we are familiar on the Earth's surface. If water is the only major source of hydrogen available, then living materia], wherever it exists, will depend on the development of some process for extracting hydrogen from water and incidentally releasing oxygen to the atmosphere.
Although we are concerned here with the occurrence of life on planets suitable for human beings, many life forms can survive, and probably could originate, under conditions not tolerable to human beings. Thus life-bearing planets are undoubtedly much more numerous in the Galaxy than are planets habitable by man.
It will be assumed here that life will always appear on planets having the correct combination of astronomical conditions and that free oxygen in the atmosphere will always accompany the appearance of life.
Whether or not chlorophyll (magnesium-porphyrin protein), for instance, would be found on other planets as the main agent of photosynthesis is an interesting question. Apparently chlorophyll, in various forms designated as chlorophyll-a, -b, -c, -d, and bacteriochlorophyll, has been the undisputed champion among photosynthetic compounds on the Earth for 1J to 2\ billion years, although other compounds apparently capable of carrying out much the same role, specifically the carot-enoids (Fruton and Simmonds, 1959) are employed in conjunction with chlorophyll in various plant species. It is quite possible that life, wherever it appears, may depend on very similar organic compounds for the following reasons. Water (HaO), ammonia (NH3), and methane (CH4) should be among the most abundant compounds in the primitive atmospheres of terrestrial planets in the early stages of their development. A great variety of more complicated organic compounds including the amino acids, other organic acids, pyrroles, purines, and pyrimidines, are formed when mixtures of methane, ammonia, water, and hydrogen are acted upon by electric discharges or by various kinds of radiation (Miller, 1955; Palm and Calvin, 1959; Berger, 1961). These organic compounds would be available as building blocks in living processes (once these had begun); consequently, the most useful products formed in the process would tend to be much the same on every planet. Since chlorophyll and its variants are so very useful and vital on the Earth, perhaps very similar compounds would inevitably evolve elsewhere in the universe through mutation and selection.
For present purposes, then, it will be assumed that PL is equal to 1.0.
We are here depending on indigenous life to provide an oxygen-rich atmosphere. It does not necessarily follow that any of the indigenous plant life would be edible or palatable from the standpoint of man or animals that have evolved on the Earth. Human settlers on planets beyond the solar system should be prepared to take their own seeds, soil bacteria, et cetera, to start crops of the food plants familiar on Earth. Incidentally, it is quite probable that life forms that evolved on the Earth will encounter no natural pathogens or parasites on an alien planet, since parasites and hosts typically must evolve together.
The probability that indigenous intelligent life will be present on a given planet seems to be quite low. This is the conclusion of Beadle (1960) based on the enormous number of possible paths that evolution could take. He indicates that the probability of an organism evolving with a nervous system like man's is extremely small. On the Earth, it has been only within the past 100,000 years or so (out of perhaps 2 to 3 billion years during which living things have existed) that the presence of an intelligent species would have been apparent to a visitor from some other planet. It may be that, given enough time, an intelligent species will eventually appear on a given habitable planet. However, its time of appearance is probably not highly predictable. If, through some unlucky accident, the ancestral family group of the human race had become extinct so that there were now no human beings on Earth, how long might it be before some other intelligent species would evolve from the existing animal species of the Earth? The answer is by no means obvious.
On the other hand, once an intelligent species has evolved on a planet, there is a good probability that it will quickly spread to other unoccupied habitable planets in its region of space, as the human race may well do within the span of not too many generations.
The probability of finding a habitable planet already in the possession of an intelligent species is one of the interesting question marks of the future. For present purposes, however, this probability will be regarded as too remote to be considered in making our calculation.
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