Extraterrestrial Civilizations - Isaac Asimov [88]
It is no wonder that even after life in the sea grew energetic and complicated it took hundreds of millions of years to conquer the land.
But the conquest took place. The pressures of competition forced organisms of various sorts to spend more and more time upon the land, until such time as they could live on land more or less permanently.
About 370 million years ago, the first plants invaded the land. The land that had been lying sterile and dead for 4¼ billion years began to turn a faint green about its edges.
Animals followed the plants over the next few tens of millions of years. Insects and spiders appeared as the first true land animals about 325 million years ago. Snails and worms appeared on land. The first vertebrates to be entirely land animals, primitive reptiles, appeared 275 million years ago.
A rich land life appeared when the Earth was about 4.3 billion years old and had passed through 36 percent of its lifetime. By the principle of mediocrity, then, we can say that 64 percent of the habitable planets have a rich land life.
That gives us our eleventh figure:
11—The number of planets in our Galaxy bearing a rich land life = 416,000,000.
INTELLIGENCE
Even a land species is not necessarily intelligent. To this day, cattle and other grazing animals are not particularly bright.
Nevertheless, one can see a steady progression of intelligence and a steady elaboration of the brain. Mammals, which first appeared about 180 million years ago, were on the whole an advance in intelligence over the reptiles.
The order of primates, the earliest records of which date back 75 million years, moved toward specialization in eyes and brains. About 35 million years ago, the primates split into the less brainy and smaller monkeys and lemurs on one side, and the more brainy and larger apes on the other.
Some 8 million years ago, a particularly brainy species developed that was the first hominid. About 600,000 years ago, Homo sapiens had developed, and about 5,000 years ago, human beings invented writing, so that written history began and civilization was in full bloom, in some parts of the world at any rate.
By the time civilization appeared, the Earth was 4,600,000,000 years old and had completed roughly 40 percent of its lifetime. That means, if we follow the principle of mediocrity, that 40 percent of the habitable planets in existence are not old enough to have developed a civilization and 60 percent are old enough.
That gives us our twelfth figure:
12—The number of planets in our Galaxy on which a technological civilization has developed = 390,000,000.
In other words, one star out of 770 in the Galaxy today has shone down on the development of a technological civilization.
We can go a little bit further. Our own civilization, if we count from the invention of writing to the first venture into space, has lasted 5,000 years. If we want to be glowingly optimistic about it, we can suppose that our civilization will continue to last on Earth as long as the Earth can support life—for another 7.4 billion years—and that our level of technology will advance in all that time.* Suppose we say, then, that the average duration of a civilization is 7.4 billion years (we’ll have more to say about that later on in the book) and that space flight is reached in the first 5,000 years. That means that only 1/1,500,000 of a civilization passes before space flight is developed, and all the rest of it progresses to technological levels above and beyond that. Or, to put it another way, only 1/1,500,000 of the civilizations in our Galaxy are so unadvanced that they are barely at the brink of spaceflight or have not yet reached it. All the rest are beyond us.
That means that of the 390 million civilizations in our Galaxy, only 260 are as primitive as we are—an inconsiderable number. All the rest (meaning just about all of them) are more advanced than we are.
In short, what we find ourselves to have been doing is to have worked out not merely the chances of extraterrestrial intelligence but the chances of superhuman extraterrestrial