Extraterrestrial Civilizations - Isaac Asimov [76]
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