IT IS a curious thing that so long as man was viewed as the centre of the universe because of his unique relationship to God, the earth which is his home automatically achieved its special status by association, and very little thought was given to its peculiar fitness in performing this function. It was only after man had been dethroned and the geocentric concept of the universe had been abandoned, that man suddenly began to realize what a unique body the earth really is.
The uniqueness of the earth as a setting for life is indeed quite extraordinary and the fact is very widely recognized among scientists who nevertheless view it as a purely accidental circumstance. The kind of uniqueness here in view involves a number of factors: (1) its size, (2) its rate of revolution, (3) its mean distance from the sun, (4) the variations in its distance as it circles the sun, (5) the constitution of its surface, and (6) its satellite, the moon.
(1) The size of the earth determines the constitution of its atmosphere, and the constitution of its atmosphere determines the nature of the living forms upon it. (6)
If it were much larger, it would have retained a large percentage of gases inimical to life. If it were much smaller, its gravitational forces would have been insufficient to retain virtually any atmosphere at all. The smaller planets with smaller gravitational fields lose a large proportion of their lighter elements during the cooling process. The larger planets retain most of their original atmosphere. Actual measurements show that although the weight of Jupiter is only 317 times that of the earth, so great is the amount of atmospheric strata around it that its volume appears to be 1300 times greater than that of the earth. The planet Mercury, on the other hand, has a mass approximately one twenty-third that of the earth and is known to have no appreciable atmosphere surrounding it, its gravitational field being too weak to retain nitrogen, oxygen, and water vapour.
The earth has, as a result, just sufficient mass that it is able to hold around itself a blanket of gases which both supports life and shields it from lethal rays of the sun. Its size is such that poisonous gases which formed as the earth cooled were not held in the atmosphere but escaped into space. The carbon dioxide, which was held, ultimately supported luxuriant vegetation, which in turn purified it for animal life by setting oxygen free through photosynthesis. Gases, like all other things, have mass, some being heavier than others. It so happens that the gases unsuitable for life were light enough and the earth's gravitational pull small enough that they were lost into space and thereby eliminated.
(2) The rate of revolution of the earth is just right for the continuous renewal of the atmosphere for animal life. Nothing gets too cold or too hot over most of its area, and plants have just sufficient times of light and of darkness to perform their function of regenerating the air (since the unique stability of carbon dioxide depends upon alternating light and darkness)
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(3) The distance from the sun determines the mean temperature of the atmosphere and the earth. The pliable materials of which living tissue is composed are made up of chains of molecules which retain their physical characteristics within a comparatively narrow range of temperature variation. It appears that apart from the very exceptional properties of carbon in forming these long chainlike molecules, such structures as ourselves and all other pliant forms would not be possible at all. It is only in a very restricted range of temperature that these carbon compounds are stable. If the temperature becomes too cold, these chains become inflexible, and if the temperature becomes too high, they lose their bonds and disintegrate. The range of temperature within which living flesh can continue without artificial protection is quite small relative to the ranges of temperature which may exist on a body in space.
(4) The seasonal variations which take place throughout the year, due to the 23° axial tilt of the earth, are very important for the continuance of human life. Were it not for these changes, micro-organisms which cause diseases and which are favoured by certain environmental conditions, would multiply so extensively that the
human race might very well suffer extinction because of them. Man is not the only animal to suffer on this account. Consider what would happen to the mosquito population if the conditions ideal for their multiplication were to persist throughout the year all over the globe. Surgeon-General C. A. Gordon pointed out that not only does the persistence of a particular temperature and humidity have to be taken into account here, but even the length of day. The length of day, of course, is governed by the rate of revolution of the earth about its axis. In his paper, Gordon gave a chart showing the distribution throughout one year of some of the major diseases caused by these micro-organisms. Were the conditions favouring any one of the disease micro-organisms maintained throughout the year, the consequences would probably be disastrous for man.
(5) The surface of the earth is part water, part dry land, in a ratio of approximately 3 to 1. The uniqueness of water has been pointed out by countless authorities so that the existence of water in a fluid state is itself fundamental to the continuance of life. On this point Harold Blum makes the following observations:
Water makes up perhaps 80 to 90% of all living organisms, and may be regarded as their principal environmental component, since even forms living in air maintain an aqueous internal environment in one way or another. Most of the water on the earth is in the liquid state, but it is also of importance as an environmental factor when in the vapor state and even as a solid.Blum then elaborates upon these properties. His elaboration leaves one filled with wonder at the power and wisdom of God in creating such a medium. But this medium requires a quite specific environment for its continued usefulness. That is to say, it is useful in a unique way -- in a unique environment. Blum sums this up by saying:
Water seems admirably suited for the major role it plays in maintaining a relatively constant temperature for the earth's surface, a matter of paramount importance to living organisms, which can serve only within a very restricted range of temperature. It owes this aspect of its fitness to several properties.
So fitness partakes of the nature of uniqueness, the uniqueness of the earth as an abode of life is a matter that strikes one more forcibly the more he tries to break out of the circle. Not only is the earth as it is, but it has reached that state through an evolutionary process, each step of which has been dependent upon the one preceding it.The stage upon which living systems bowed their debut was set by all the preceding events in the history of the earth ‹ or, for that matter, of the Universe. These events placed important restrictions upon the nature of life and its evolution.To be continued . . .
Life, it seems, did not arise and evolve as a system free to vary in any direction whatever; but as a system upon which great restrictions were placed, some of them even before the earth came into existence.
Harold Blum concludes his chapter "Fitness of the Environment" with these words, "This aspect of fitness is not, then, universal, but exists only in relation to the planet Earth, or to planets that are very nearly like the Earth.''
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