Extraterrestrial Civilizations - Isaac Asimov [85]
The Sun’s stay upon the main sequence will be some 12 billion years, and we might consider this average for Sunlike stars. That means that the Earth (and, on the average, habitable planets generally) will last 12 billion years as the abode of life. If, then, life appears on the Earth after one billion years, it does so after only 8 percent of its lifetime has elapsed.
We can assume that (by the principle of mediocrity) habitable planets in general gain life after some 8 percent of their lifetimes as habitable planets has passed.
Suppose we assume that stars have been forming at a steady rate here in the outskirts of the Galaxy, once the first flurry of star formation in the infancy of the Galaxy had passed.
This is not entirely an assumption. There is evidence that stars have been born in recent times, at least. The giant stars of spectral classes O and B must have been formed a billion years ago or less, or they would not still be on the main sequence. And if stars could form in the last billion years, they must have been forming all along and still be forming now. They must at least be doing so in those galactic regions where clouds of dust and gas (the raw material of stars) are plentiful, and those regions are precisely in the outskirts of galaxies, which, we have already decided, are the only places life can exist.
Moreover, we need not depend entirely on reason to tell us that stars are still being formed today. It is possible we are actually witnessing the process. In the 1940s, the Dutch-American astronomer Bart Jan Bok (1906–) drew attention to certain dust clouds that were opaque, compact, isolated, and more or less spherical in shape. He suggested that these clouds (now called Bok globules) are in the process of condensing into stars and planetary systems. The evidence since then tends to show he is right. Sagan estimates that in our Galaxy, ten stars are born each year on the average.
Assuming, then, a steady rate of star formation, we can say that x percent of the habitable planets have not yet expended x percent of their lifetime. In other words, 50 percent of the habitable planets have not yet expended 50 percent of their lifetime; 10 percent of the habitable planets have not yet expended 10 percent of their lifetime; and so on.
This means that 8 percent of the habitable planets have not yet expended the 8 percent of their lifetime that they should need to form life; that is, they are less than a billion years old.
The converse is that 92 percent of the habitable planets are old enough to have had life develop upon them. That gives us our ninth figure:
9—The number of life-bearing planets in our Galaxy = 600,000,000.
MULTICELLULAR LIFE
Though life may have come to exist on Earth early in its history, its advance was very slow for a long time.
For the first 2 billion years during which life existed on Earth, the dominant forms may have been bacteria and blue-green algae. These were small cells, considerably smaller than the cells that make up our bodies and those of the plants and animals familiar to us. Furthermore, the bacterial cells and the blue-green algae did not have distinct nuclei within which the deoxyribonucleic acid (DNA) molecules that controlled the chemistry and reproduction of the cells were confined.
The difference between these two kinds of cells was that the blue-green algae were capable of photosynthesis (the use of the energy of sunlight to convert carbon dioxide and water into tissue components) and the bacteria were not. Bacteria, without photosynthetic ability, were forced to break down already