Extraterrestrial Civilizations - Isaac Asimov [75]
Our calculations concerning extraterrestrial intelligence must therefore come to a halt right here, unless we can say something reasonable about the chance that a habitable planet actually has life on it.
In order to do that, we must again turn to something that is known, and that is the one habitable planet that we know to have life on it—Earth itself. In other words, before we can say anything sensible about life on habitable planets in general, we must be able to say something sensible about how life came to exist on the Earth.
Early speculations about the existence of life on Earth invariably assumed it to have been created through some nonnatural agency, usually through the action of some god or demigod. The best-known story in our Western tradition is that humanity was created in the same series of divine acts that created the Universe generally.
In six days of creation the job was done. God created light on the first day; the land and sea on the second; plant life on the third; the heavenly bodies on the fourth; animal life of the sea and air on the fifth; and animal life on land on the sixth. As the last creative act on the sixth day, humanity was brought into being.
Life, created on three different days, was considered as having come into being in separate species (“after his kind” it says in the King James Bible). Presumably, these were the species that continued to exist into contemporary times. As some believed, no species were added to the first creation and none subtracted.
As to the date of this Divine creation, the Bible is not specific, for the habit of dating with compulsive precision is a rather late development in historical writing. Deductions based on various statements in the Bible, however, place the date of creation only a few thousand years in the past. The precise date usually found in the headings of the King James Bible is 4004 B.C., this date having been worked out by the Irish theologian James Ussher (1581–1656).
Although the creation of the world (or of different worlds) was assumed to be a once-for-all act, it was common in early times to assume that this was not necessarily true for life.
Actually, this is a reasonable attitude. After all, while there was no visual evidence of any creation of worlds in the course of human history, there did seem to be visual evidence for the creation of living things without the intervention of earlier living things.
Field mice may make their nests in holes burrowed into stores of wheat, and these nests may be lined with scraps of scavenged wool. The farmer, coming across nests from which the mother mouse has had to flee, and finding only tiny, naked, blind infant mice, may come to the most natural conclusion in the world: he has interrupted a process in which mice were being formed from musty wheat and rotting wool.
Let meat decay and small wormlike maggots will appear in it. Frogs can seem to arise out of river mud.
If the notion were true for various species of vermin, it might be true for all species of organisms, though perhaps less common for the larger and more complex species such as horses, eagles, lions, and human beings.
In fact, if one were sufficiently daring, one might suppose that the tale in Genesis was a fable; that this sort of “spontaneous generation” of living things from nonliving antecedents might account for the original beginning of life. Little by little each species might have formed, first the simple ones and later the more complex ones, with human beings, naturally enough, last of all.
And in that case, if we were to apply this to habitable planets generally, we would see that they, too, would naturally form life.