A single 30-day span begat much of modern biology
Scientists, as well as vintners, brewers and butchers, well knew that virtually all food or drink left unattended would soon teem with life. In retrospect, the observation that a nutrient broth (like bouillon) left unrefrigerated would quickly be swimming with organisms seems to us self-evident. The prevailing notion, however, that this outcome depended on some feature of the air—some principle or property that gave rise to life in a previously lifeless fluid—seems almost incomprehensible to us today. But at the beginning of the 19th century, the vertiginous rise of chemistry had introduced beguiling notions of crystallization, explosion and spontaneous reaction into the scientific vocabulary. It did not seem that farfetched to imagine that organic molecules could similarly organize, crystallize and react to give rise to living organisms.
Yet several experiments, well known to working scientists in Pasteur’s time, had already been performed in an effort to demystify what appeared to be the spontaneous emergence of life. As early as 1668, Francesco Redi demonstrated that the flies that appeared in decaying meat necessarily came from the eggs laid by earlier flies that had had access to the meat. Definitive as that demonstration appeared, however, its results were contradicted by many other experiments. Most notable were those of John Needham, the 18th-century cleric and scientist whose experiments and theories supported spontaneous generation. He observed that blighted wheat seeds gave rise to “tiny eels,” and that these eels appeared viviparous and hence could not be the result of eggs being laid on the seeds. In collaboration with an eminent naturalist, Georges-Louis Leclerc, Comte de Buffon, he also boiled mutton broth in a sealed flask, only to find that the broth still became cloudy with organisms. Needham went on to propose a “universal vegetative force,” which, in an “exalted state,” could account for the emergence of life in broths, flour and water mixtures, and blighted seeds.
Needham’s ideas did not receive an enthusiastic response. No less caustic and influential a skeptic than Voltaire dissected Needham with his wit, and no less august a scientist than Lazzaro Spallanzani spent a decade contradicting Needham’s experiments. Experimental and philosophical objections to the Needham-Buffon worldview accumulated. The experimentalists soon showed that the extent to which broths were boiled mattered. They suggested that Needham and Buffon had simply not managed to get rid of preexisting life in the flasks—and hence had not really demonstrated spontaneous generation. But criticism also came from those, Voltaire included, who feared that theories of spontaneous generation foreshadowed an ever-smaller role for God as the source of life. If, after all, life emerged as the result of some inherent properties of matter, what role did theistic explanation have to play?