Extraterrestrial Civilizations - Isaac Asimov [77]
In 1864, however, the French chemist Louis Pasteur (1822–1895) produced the clincher. He boiled a meat broth until it was sterile, and did so in a flask with a long, thin neck that bent down, then up again, like a horizontal 5. Then he neither sealed it off nor stoppered it. He left the broth exposed to cool air.
The cool air could penetrate freely into the vessel and bathe the broth. If it carried a “vital principle” with it, that was welcome. What did not enter, however, was dust and microscopic particles generally. These settled at the bottom of the curve of the flask’s neck.
As a result, the broth did not breed microorganisms and did not show any signs of life. Once Pasteur broke off the swan-neck, however, and allowed dust and particles to reach the broth along with the air, microorganisms made their appearance at once.
With that, the notion of “spontaneous generation” seemed dead, once and for all.
ORIGIN OF LIFE?
Once it was clearly established that spontaneous generation did not take place and that all life (as far as human beings were able to observe) came from previous life, it became very difficult to decide how life originated on Earth—or on any other planet.
The changeover was rather like the one that took place in the theories concerning the origin of planetary systems. As long as one clung to an evolutionary theory such as Laplace’s nebular hypothesis, it was easy to suppose that planetary systems were common and that every star was accompanied by one. The nebular hypothesis, in a way, preached the spontaneous generation of planets.
A catastrophic theory of planetary formation, however, involved an event that was so rare that planets themselves had to be regarded as excessively rare, and it became tempting to think that our own planetary system was not to be duplicated elsewhere.
In the same way, the defeat of spontaneous generation and the new suggestion that life came only from previous life, which came only from still earlier life and so on in an endless chain, made it seem that the original forms of life couldn’t possibly have arisen save through some miraculous event. In that case, even if habitable planets were as plentiful as the stars themselves, Earth might yet be the only one that bore life.
Even as Pasteur was knocking the pins out from under spontaneous generation, however, the situation was being eased a little bit. In 1859, the English biologist Charles Robert Darwin (1809–1882) published a book for which The Origin of Species is the commonly used title.
In it he presented exhaustive evidence in favor of an evolutionary theory in which the various species of living things were not separate and distinct from the beginning. Instead, under the pressure of increasing populations and of natural selection, all living things gradually changed; new, and presumably more suitable, species developed from old. In this way, several different species might have a common ancestor and, if one went back far enough, all life on Earth may have developed from a single very primitive ancestral form of life.
The theory met with much opposition, but biologists came to accept it in time.
What it meant was that one no longer had to account for the separate creation of each of the millions of species of living things known. Instead, it would be sufficient to account for the creation of any form of life, however simple. This original simple form, produced by spontaneous generation, could then by evolutionary processes give rise to all other forms of life, however complex—even human beings.
Of course, if spontaneous generation were really impossible, the production of one form of life was as much a miracle as the production of a million forms.
On the other hand, all that biologists had done was to show that known forms of life could not be generated spontaneously in the short periods of time available in the laboratory. Suppose we dealt with a very simple form of life, much simpler than any known, and suppose we had long periods of time and a whole planet at our disposal; might