Creatures Born of Mud and Slime The Wonder and Complexity of Spontaneous Generation

Spontaneous generation, the idea that life can arise out of non-living matter, is both alive and dead today. Current science accepts the idea that at some point in the distant past, complex self-replicating molecules arose, which formed the starting point of billions of years of unicellular life. But there is an obsolete side to this theory. For millennia, philosophers and scientists believed that all sorts of creatures could arise spontaneously from the mud and slime this book refers to. In the late 1850s, The French microbiologist Louis Pasteur performed experiments that definitively put the nail in the coffin for this idea.

Quite a few books have been written about the later discussions around this theory and its eventual demise (contemporary examples are John Farley's The Spontaneous Generation Controversy from Descartes to Oparin or James Strick's Sparks of Life: Darwinism and the Victorian Debates over Spontaneous Generation). Based on a three-part lecture series, this purposefully short book, which is not intended as a complete history, gives a whistle-stop tour of spontaneous generation from Antiquity to 1769. Are you ready?

Though we now understand spontaneous generation to be incorrect, it was a carefully reasoned and supported idea that stood the test of time. Lehoux goes back to the sources and provides largely a textual analysis of Aristotle synthesizing work, liberally quoting Greek passages to show how Aristotle carefully indicated the trust he placed in his own and other's observations and ideas. According to him, non-living matter contained something called pneuma or vital heat, a sort of potential for life, with spontaneous generation being the process to realise this potential. Note that Aristotle was aware of the proverbial birds and bees. He just wasn't so sure about actual bees having a sex life.

The Roman poet and philosopher Titus Lucretius Carus (99-55 BC) and his didactic poem De Rerum Natura espoused a surprisingly materialist explanation in the form of Atomism: the idea that the world was made up of only two things: indivisible particles called atoms and the void they move in. Side note: these philosophical atoms are not quite the same atoms that 19th-century physicists described, though they did decide to name the particles they discovered atoms, as a nod to this school of thought. In a case of unfortunate historical prematureness, atoms later turned out to be divisible into smaller particles after all, but I will not lead the reader further down the path of particle physics. So, anyway, Lucretius was effectively throwing the idea out there that spontaneous generation is nothing more than his atoms coming together in just the right configuration for life to emerge, no pneuma needed.

Fast forward to the North African theologian and philosopher Augustine of Hippo (354-430) who tried to reconcile the too-atheist ideas of Atomism and the Aristotelian constant-generation of life with the biblical account of creation in Genesis and the idea that God had created all life once only. His solution was that God wove into the fabric of the cosmos something called "rationes seminales", which Lehoux translates as "seminal principles". Life was only created once, but each individual's generation required their seminal principles to become actuated for which suitable conditions needed to arise. This allowed him to explain both sexual and spontaneous generation.

Lehoux then moves on to German Catholic friar and bishop Albertus Magnus (~1200-1280) who gives an astrological spin to spontaneous generation in his work De Animalibus, with the stars having generative power. One that is so powerful that not only can it engender human and animal forms in living matter, it can even engender such shapes in stone, which was one explanation in circulation at the time for fossils! The Italian physician and philosopher Fortunio Liceti (1577-1657) wrote a massive book on the topic in 1618 and his take on it combines Aristotelian thinking about activation-by-vital-heat with Augustine's divine creation.

Then there is the sometimes slightly misinterpreted Italian scholar Francesco Redi (1626-1697) who experimentally showed that if flies were kept away from dead organic matter, maggots would not emerge. Spontaneous generation disproved? Not quite, as he simultaneously was convinced that spontaneous generation *was* occurring in *living* organic matter. The final debate Lehoux tackles is the English John Needham vs. the Italian Lazzaro Spallanzani (both biologists and priests) in 1769. Needham was adding organic broths to (what he considered sufficiently) sealed and sterilized vials and still saw life emerge. He published on this in 1748. Spallanzani was way more thorough in his sterilization procedures and did not see life emerging, and published on this in 1765. Needham's retort came in 1769. The interesting thing that Lehoux highlights here is that both had some ideas right, and some ideas wrong. Needham believed in spontaneous generation, but also in epigenesis: the idea that life develops from an undifferentiated mass (we now know this to be true, cells differentiate and specialize during embryonic development), Spallanzani believed that gametes (sperm and eggs) had to be involvled, but also believed in preformationism (Lehoux makes up his own contrasting neologism here: protogenesis): the idea that organisms develop from already fully formed miniature versions of themselves in the form of what were called homunculi or animalcules (see also Clara Pinto-Correia's apparently hilarious The Ovary of Eve).

I will admit that this book wasn't quite what I expected. I expected it to cover the whole story of spontaneous generation, right up to Pasteur's final disproval of it, and wasn't prepared for the liberal quoting of source material in Greek, Latin or Italian. Lehoux says in his introduction that it's easy in hindsight to be dismissive of these ideas as gullible superstitions, but we must look at them through the lens of what was known then. In my opinion, Lehoux succeeds admirably in showing that all of these scholars were working at the cutting edge of their time. None of these scholars could have possibly foreseen future developments in our knowledge and understanding. And Lehoux reminds us that nor can we. We try to make our current theories the best they can be with the tools and historical knowledge available to us today. The (rather intractable) quote "Half of what we will teach you now will later turn out to be wrong – we just don't know which half yet" applies. All in all, Lehoux provides a whistle-stop tour of the idea of spontaneous generation that, though not intended to be exhaustive, makes for no less interesting a book.