Extraterrestrial Civilizations (23 page)

Provided, that is, the doctrine of spontaneous generation could withstand close examination—and it couldn’t.

The first crack in the doctrine appeared in 1668, thanks to an Italian physician and poet named Francesco Redi (1626–1697). Redi noticed that decaying meat not only produced flies, but also attracted them. He wondered if there were a connection between the flies before and the flies after, and tested the matter.

He did this by allowing samples of meat to decay in small vessels. The wide openings of some vessels he left untouched; others he covered with gauze. Flies were attracted to all the samples, but could land only on the unprotected ones. Those samples of decaying meat on which flies landed produced maggots. The decaying meat behind the gauze, upon which the foot of fly never trod, produced no maggots at all, although it decayed just as rapidly and produced just as powerful a stench.

Redi’s experiments showed plainly that maggots, and flies after them, arose out of eggs laid in decaying meat by an earlier generation of flies. There was no spontaneous generation of flies, just the normal process of birth from eggs (or seed).

Even as Redi was working out his demonstration, a Dutch biologist, Anton van Leeuwenhoek (1632–1723), was riding his hobby and grinding perfect little lenses (primitive microscopes, actually) through which he could look at tiny things and magnify them to easy visibility.

In 1675, he discovered living things in ditch water that were too small to be seen by the naked eye. These were the first “microorganisms” known, and those that van Leeuwenhoek first discovered are
now called protozoa, from Greek words meaning
first animals
. In 1680, van Leeuwenhoek discovered that yeast was made up of tiny living things even smaller than most protozoa, and in 1638 he observed still tinier living things, which we now call bacteria.

Where did these microscopic living things come from?

Broths were invented in which microorganisms could multiply. It turned out not to be necessary to seek microorganisms to place in these broths. A broth might be boiled and filtered until there was nothing in it that the lens of a microscope could detect. If one waited a while and looked again, the broth was inevitably swarming with life. (What’s more, it was microorganisms that caused meat to decay even when no microorganisms were placed in the meat.)

Perhaps spontaneous generation did not take place in the case of those species visible to the unaided eye. In the case of the microorganisms—bits of life far simpler than the familiar plants and animals of everyday life—spontaneous generation might well be possible. In fact, it seemed established.

But then, in 1767, came the work of an Italian biologist, Lazzaro Spallanzani (1729–1799). He not only boiled broth, but he sealed off the neck of the flask containing it. The broth, boiled and sealed, never developed any form of microscopic life. Shortly after the seal was broken, however, life began to swarm.

A sealed neck, keeping out the air, acted like Redi’s gauze, and the conclusions had to be similar to Redi’s conclusions. There are microscopic and unseen creatures all about us in the air that are smaller and harder to observe than even the eggs of flies. These airborne bits of life fall into any broth left open to the air, and there they multiply. (Spallanzani isolated a single bacterium and watched it multiply by simply splitting in two.) If those bits of life are kept out of the broth, no life originates.

In 1836, a German biologist, Theodor Schwann (1810–1882), went even further. He showed that broth remained sterile even when open to air, provided the air to which it was exposed had been heated first in order to kill any forms of life it might contain.

Advocates of the doctrine of spontaneous generation pointed out that heat might kill some “vital principle” essential to the production of life out of inanimate matter. Heating broth and sealing it away would in that case fail to produce life. Exposing heated broth to air
that had likewise been heated was no better.

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.

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 not that very simple form of life finally be generated?

The key lay in that phrase
long periods of time
. The hit-or-miss random processes of evolution took a long time (even the evolutionists admitted that) and the question was whether there was time enough for the simple life form to be generated and all the myriad of complex life forms to be developed afterward

In Darwin’s time, scientists had abandoned the notion of a planet that was no more than 6,000 years old and spoke freely of Earth’s age as being in the millions of years, but even that didn’t seem long enough for evolution to do its work.

In the 1890s, however, radioactivity was discovered, and it was found that uranium turned to lead with almost stupefying slowness. Half of any sample of uranium would turn to lead only after 4,500,000,000 years. In 1905, the American chemist Bertram Borden Boltwood (1890–1927) suggested that the extent of the radioactive breakdown in rock would be an indication of the length of time since that rock had solidified.

Radioactive changes of all kinds have been used to measure the age of various parts of the Earth, of meteorites, and, recently, of Moon rocks, and the general consensus now is that the Earth, and the Solar system in general, is about 4,600,000,000 years old.

Hints of this vast age were already available in the early decades of the twentieth century, and it began to appear that there was enough time for evolution to do its work, if life could somehow start spontaneously.

But could that spontaneous start take place?

Unfortunately, by the time the extreme age of the Earth came to be understood, the extreme complexity of life also came to be understood, and the chance of spontaneous generation seemed to shrink further.

Twentieth-century chemists learned that protein molecules, which are molecules peculiarly characteristic of life, were made up of long chains of simpler building blocks called amino acids. They found that every protein had to have every one of thousands of different atoms (even millions in some cases) placed just so if it was to do its work properly. Later on, they discovered that an even more fundamental type of molecule, those of the nucleic acids, were even more complicated than protein molecules. What’s more, different nucleic acids and different proteins, along with smaller molecules of all kinds, intermeshed in complicated chains of reactions.

Life, even the apparently simple life of bacteria, was enormously more complicated than had been imagined in the days when the matter of spontaneous generation was being squabbled over. Even the simplest form of life imaginable would have to be built up out of proteins and nucleic acids, and how did
those
come to be formed out
of dead matter? The origin of life on Earth, despite evolution, seemed more than ever a near-miraculous event.

Some scientists gave up and, in effect, washed their hands of the problem. The Swedish chemist Svante August Arrhenius (1859–1927) published a book,
Worlds in the Making
, in 1908, which took up the matter of the origin of life. In the book, Arrhenius upheld the universality of life and suggested that it was a common phenomenon in the Universe.

He went on to suggest that life might be, in effect, contagious. When simple living things on Earth form spores, the wind carries them through the air to burgeon in new places. Some by the blind force of the wind might be wafted upward high into the atmosphere and actually, Arrhenius speculated, out into space. There they might drift for millions of years through vacuum, pushed by the Sun’s radiation, protected by a hard, impervious pellicle and fiercely retaining the spark of life inside. Eventually, a spore would encounter some suitable planet without life and from it life would start on that planet.

In fact, Arrhenius suggested, that was how life on Earth got its start. It was vitalized by spores from space; spores that had originated on some other world that might remain forever unidentified.

Several points can be used to argue against this notion. One can calculate how many spores must leave a world in order that even one might have a reasonable chance to meet another world in the course of the lifetime of the Universe, and the amount is preposterously high.

Then, too, it is unlikely that spores can survive the trip through space. Bacterial spores are highly resistant to cold, even extreme cold; they might also be expected to survive vacuum. It is doubtful that even the hardiest spores could exist for the length of time it would take to drift from planetary system to planetary system, but we could stretch a point and suppose that at least some could. What we do know, though, is that spores are very sensitive to ultraviolet light and other hard radiation.

They are not subjected to this on Earth, where the air forms a blanket that filters out the Sun’s more energetic radiation; nor was Arrhenius, in his time, aware of the extent to which energetic radiation fills the Universe. The radiation from any star anywhere in its ecosphere would be enough to kill wandering spores that were originally
adapted to life within a protective atmospheric blanket. Cosmic-ray particles would kill them even in the depths of space.

Arrhenius thought that radiation pressure would propel spores away from a star and through space. We now know the Solar wind is much more likely to do so. In either case, whatever propels the spore away from a star and toward others in the first place would repel the spore as it approached another star and prevent it from landing on a planet within the ecosphere.

All in all, the notion of Earth’s having been seeded by spores from other worlds is exceedingly dubious.

Besides, of what use is it to explain the origin of life on Earth by calling upon life on other planets for help? One would have then to explain the origin of life on the other planet. And if it could form on any planet by some natural and nonmiraculous means, then it could form on Earth in the same fashion.

But how? Even as late as the 1920s, biologists were at a loss for a natural mechanism.

One objection to the spontaneous generation of life on Earth is this: If life could be formed out of nonlife in the far past, it should happen periodically in later times, even right now. Since no such formation is ever observed in the present day, ought we not to conclude that it did not happen in the far past, either?