What is spontaneous generation?
The concept of Spontaneous Generation has been a scientific controversy in many ways equivalent to anthropomorphic climate change (global warming,) but perhaps lacking some of the political hyperbole that the latter field experiences. Spontaneous generation can be succinctly described as the premise that cells can arise from non-living matter, and apparently conclusive experiments either supporting or rejecting this hypothesis were conducted over the course of centuries. The first chapter of Bauman describes how this concept fell into and out of favor.
The Greeks, beginning with Aristotle, observed that non-living environments such as dry lake beds rapidly demonstrated an abundance of living organisms, following a summer rain shower. It was thought that these organisms arose via a process called abiogenesis, due to the infusion of water stimulating a vital force, resulting in the growth of animals. This way of thinking shaped the scientific understanding of biology for centuries, until the Italian physician Francesco Redi conducted a series of experiments in the late 1600’s that contradicted the premise of spontaneous generation. Redi placed meat into several containers, which were either open to the air or had covers on them. The open containers rapidly became spoiled and maggots appeared on the meat. The closed containers also became spoiled, but flies were kept from the meat, and no maggots appeared in the containers. Redi’s experiments supported the contention that animals could only arise from other animals, but did not however disprove that the microorganisms could not arise by spontaneous generation.
John Needham was a British naturalist of the mid-18th century, and performed a series of experiments to demonstrate that broths contained a “life force” that promoted microorganisms to appear from non-living material. Needham prepared a series of beef infusions, and boiled them extensively. Following the boiling treatment, no living cells were observable, and the containers were tightly sealed by inserting corks. However after several days, the vials were cloudy with the growth of microorganisms, leading him to conclude that bacteria could arise spontaneously.
In 1799, an Italian scientist named Lazzaro Spallanzani readdressed Needham’s experiments. Spallanzani boiled his beef infusions for an hour, then sealed the containers by melting the thin glass necks of his culture flasks. These cultures remained clear until the flask necks were broken, at which point they quickly demonstrated microbial growth. This result appeared to conclusively disprove the hypothesis of spontaneous generation, however critics of Spallanzani claimed that the critical “life force” (possibly oxygen?) was prevented from entering his flasks, and therefore preventing life from arising.
The final experiment which laid this debate to rest was carried out by the father of modern microbiology, Louis Pasteur in the 1860’s. Pasteur conducted an experiment essentially like Spallanzani’s, where he extensively boiled beef extracts in glass flasks. Pasteur however utilized a novel flask, called a “swan-neck” flask, to boil his beef extracts for an hour. These flasks remained open to the air (and available to the mysterious “life force”), but remained clear of all microbial growth once they cooled to room temperature. The bend in the neck trapped microorganisms that fell from the air, and prevented them from reaching the sterilized broth. However, if the flask were tipped, so that broth entered the bend and then returned to the flask, the culture became cloudy within days, demonstrating that the broth still maintained the potential to allow microorganisms to grow. The Pasteur Institute in Paris has an example of his original swan-neck flask that still exhibits no microbial growth 150 years after being sterilized.
Bonus Opportunity: based on what we have learned to date, what do you think is going on with the work of Needham? Why did he observe the results that he did, given that the experiment of Spallanzani was not greatly different?