Broca's Brain - Carl Sagan [99]
These tiny “animalcules” had never before been seen by any human being. Yet Leeuwenhoek had no difficulty in recognizing them as alive.
Two centuries later Louis Pasteur developed the germ theory of disease from Leeuwenhoek’s discovery and laid the foundation for much of modern medicine. Leeuwenhoek’s objectives were not practical at all, but exploratory and adventuresome. He himself never guessed the future practical applications of his work.
In May of 1974 the Royal Society of Great Britain held a discussion meeting on “The Recognition of Alien Life.” Life on Earth has developed by a slow, tortuous step-by-step progression known as evolution by natural selection. Random factors play a critical role in this process—as, for example, which gene at what time will be mutated or changed by an ultraviolet photon or a cosmic ray from space. All the organisms on Earth are exquisitely adapted to the vagaries of their natural environments. On some other planet, with different random factors operating and extremely exotic environments, life may have evolved very differently. If we landed a spacecraft on the planet Mars, for example, would we even be able to recognize the local life forms as alive?
One theme which was stressed at the Royal Society discussion was that life elsewhere should be recognizable by its improbability. Take trees, for example. Trees are long skinny structures, above ground fatter at the top than at the bottom. It is easy to see that after millennia of rubbing by wind and water, most trees should have fallen down. They are in mechanical disequilibrium. They are unlikely structures. Not all top-heavy structures are produced by biology. There are, for example, pedestal rocks in deserts. But were we to see a great many top-heavy structures, all closely similar, we could make a reasonable guess that they were of biological origin. Likewise for Leeuwenhoek’s animalcules. There are many of them, closely similar, highly complex and improbable in the extreme. Without ever having seen them before, we correctly guess they are biological.
There have been elaborate debates on the nature and definition of life. The most successful definitions invoke the evolutionary process. But we do not land on another planet and wait to see if any nearby objects evolve. We do not have the time. The search for life then takes on a much more practical aspect. This point was brought out with some finesse at the Royal Society discussion when, after an exchange remarkable for its rambling metaphysical vagueness, Sir Peter Medawar rose to his feet and said, “Gentlemen, everyone in this room knows the difference between a live horse and a dead horse. Pray, therefore, let us cease flogging the latter.” Medawar and Leeuwenhoek would have seen eye to eye.
But are there trees or animalcules on the other worlds of our solar system? The simple answer is that no one yet knows. From the vantage point of the nearest planets, it would be impossible to detect photographically the presence of life on our own planet. Even from the closest orbital observations of Mars made to date, from the American spacecraft Mariner 9 and Viking 1 and 2, details on Mars much smaller than 100 meters across have remained invisible. Since even the most ardent enthusiasts of extraterrestrial life do not anticipate Martian elephants 100 meters long, many important tests have not yet been performed.
At the present time we can only