Extraterrestrial Civilizations - Isaac Asimov [93]
We might conclude, then, that the Earth, in accomplishing the very ticklish task of capturing the Moon (so difficult a task that astronomers can’t figure out just how it might have happened) made it possible for land life to exist.
When we calculated how many myriads of habitable planets there were, we left out of account how few of them might have succeeded in capturing a large satellite that just happened to be there, and how few might therefore have developed land life and in that way have the kind of intelligence and technology that we are looking for.
And yet this argument in favor of Earth’s being unique in possessing land life, and therefore intelligence and technology, is also not compelling. We don’t need a captured Moon to explain the coming of land life. During the billions of years that life existed in the sea and not in the land, the Moon’s tides, however high, probably could not have brought about a transfer of life to the land.
During most of Earth’s existence after all, the Earth’s atmosphere did not contain more than a very small percentage of free oxygen, if any at all. This meant that there was no ozone layer in its upper reaches, and ultraviolet radiation from the Sun could reach the Earth’s surface in large quantites.
The energetic ultraviolet radiation is inimical to life since it tends to break up the complicated molecules on which life depends. This would not affect life in the ocean, however, which could drift just far enough under the ocean surface to receive enough of the ultraviolet energy without receiving too much.
On land, however, it is not that easy to escape the deadly radiation of the Sun, so the land remained dead, sterilized by sunlight.
Even at the beginning of the Cambrian, 600 million years ago, the Earth’s atmosphere was not quite 5 percent oxygen. The oxygen content was now increasing rapidly, however, and an ozone layer was forming and growing denser. The ultraviolet was increasingly blocked by the forming ozone layer and by 400 million years ago, it no longer reached the Earth’s surface in deadly quantity. Now for the first time, fragile living tissue pushed up on the shore by the tides was not killed at once. Slowly, the land was colonized.
This is a much more persuasive argument for the delay in establishing land life than are those involving the capture of the Moon.
It would seem, then, that we must abandon the thought of the Moon’s playing a crucial role in the development of civilizations. Whether a habitable planet has a large satellite, a small satellite, a captured satellite, several satellites, or no satellites should not affect, as nearly as we can judge from the evidence on hand,* the development of land life, and intelligence will move on undisturbed.
Then where is everybody?
INTELLIGENCE
Granted, then, that there are as many habitable planets as we have estimated and that all of them are filled with land life, can we really be sure that an intelligent species will inevitably arise on each of them?
Are we perhaps wrong to apply the principle of mediocrity to this phase of the calculations? Can it be that the development of intelligence on Earth is an unbelievably lucky chance, and that while the Galaxy and the Universe swarm with life, even with land life, intelligence and, hence, civilizations might be be altogether absent—except here?
Are the requirements for an intelligent species all but impossible to meet? What are they?
In the first place, an intelligent species must be rather large, for it must develop a large brain; though it must not be too large, in the sense that its body must not outrun its brain.
Thus, the human being is more intelligent than its larger relative,