A Series of Fortunate Events Chance and the Making of the Planet, Life, and You

Every one of us is here through a long string of happy accidents that might just as well not have happened. That is the contention behind A Series of Fortunate Events, a short and snappy book by evolutionary biologist Sean B. Carroll. Examining planetary events, evolution, and our personal lives and deaths – and introducing one remarkable French biologist – it read like an appetizer that left me wanting to explore this topic further.

This book can be seen as a complement of sorts to Carroll's previous book The Serengeti Rules. Where that book examined the similarity between rules underlying both physiology and ecology, this book reminds you that that still leaves plenty of opportunity for chance to govern biology. One person that crops up in both books is Jacques Monod (1910–1976), the Nobel-Prize winning French biochemist. In his best-selling book Chance & Necessity he wrote: "chance alone is at the source of every innovation, of all creation in the biosphere" (p. 8). A Series of Fortunate Events can easily be seen as Carroll's tribute to Monod's book. As the subtitle indicates, it explores chance on three levels.

First up is the role of chance in the external world, which, surprisingly, least impressed me. Carroll gives a long and lively description of the immediate and long-term impacts of the asteroid that did the dinosaurs in. So far, so familiar. But what about the role of chance? Carroll is brief: impacts this big are rare and the Yucatán Peninsula was a bad spot, rich in hydrocarbons and sulfur that were thrown into the atmosphere. What struck me as odd was his assertion that the asteroid would have harmlessly splashed into the ocean had it been off by 30 minutes, thanks to Earth's rotation. Given Earth also orbits the Sun, would that not have resulted in a near-miss? He omits any mention of the cataloguing of near-Earth objects and what we have since learned about the risk of cosmic collisions.

Carroll similarly sees the fingerprint of chance in our planetary climate since then. The formation of the Himalayas exposed fresh rock to be weathered, drawing down CO₂ and lowering global temperatures. Carroll considers the timing of the tectonic collision between the Indian and Asian plates a fluke; the former is relatively thin and thus moved relatively fast across the Indian Ocean. I am more inclined to agree that the rapid climatic fluctuations of the last 100,000 years have an unpredictable character to them. Interestingly, Walter Alvarez, who suggested an asteroid killed the dinosaurs, wrote A Most Improbable Journey that looks at highly improbable events in our planet's history, so I am putting a pin in this topic for now.

Far more convincing are the chapters on the role of chance in evolution. Darwin already talked about the accidental nature of variation but did not yet know about DNA and mutations. Point mutations, where the difference is literally one "letter" in the DNA, are one kind and they can happen in several ways. The mechanism Carroll details here was completely new to me. Each of the four DNA bases can occur in one of two forms, so-called tautomers, with the keto-form common and the enol-form rare. They differ in the position of a single hydrogen atom, allowing for example guanine (G) to bind with thymine (T) instead of its regular partner cytosine (C). This shape-shifting is a transient phenomenon lasting just a fraction of a second, but if a base is in the "wrong" configuration exactly at the moment that DNA polymerase is duplicating a DNA strand, a point mutation results. In 1953, Watson and Crick already speculated this could happen, but it took until 2015 before we had the technology to visualise this at the atomic level.

But wait, given that mutations are rare, and that many new traits require multiple mutations, are the odds of new traits thus arising not astronomically small? As explained here, they do not have to happen simultaneously. This is where natural selection comes in. As long as each mutation along the way provides some benefit, the necessary mutations can accumulate in a stepwise fashion. What this also reveals, Carroll adds, is the limitation of both of these mechanisms: "natural selection can't invent anything on its own [while] mutation alone cannot change a population nor produce multiple changes at once" (p. 117). In other words "chance invents, and natural selection propagates the invention" (p. 118). Or at least, natural selection does so when an invention is useful in the population at that time and place. To take his example of antifreeze proteins in Antarctic fish, the mutations for these are unlikely to be selected for when occurring in tropical fish.

This part is another example that left me wanting to immediately read deeper into this topic. I am aware more has been written about randomness and chance in evolution and an interesting counterpoint is raised by the recent Mutation, Randomness & Evolution. Furthermore, Carroll gives a nice definition of contingency: "Chance [...] pertains to an event itself, while a contingency emerges through the benefit of hindsight" (p. 59). In other words, "contingency is the aftermath of chance" (p. 60). However, though his further reading list mentions Improbable Destinies, he sidesteps the whole contingency versus convergence debate, that is, the question of how repeatable evolution is.

The final part looks at chance in our personal lives and deaths. When our body creates sperm and egg cells, there are several mechanisms at play that create an almost limitless amount of genetic variation. The same is true of our immune system. We have millions of different antibodies to recognize foreign invaders, but we do not have millions of genes coding for an antibody each. Instead, a clever mechanism that mixes one of the many variants of each of the several segments that make up an antibody is enough to create the required variation. And chance mutations have a role to play in cancer. Both bad habits such as smoking and the good fortune of our extended lifespan can result in random mutations building up and culminating in cancer.

Carroll has won awards for both his books and documentaries. His skill at science popularisation shines through here, as the writing is amusing, even irreverent, and he draws readers into each chapter with general interest stories. The afterword, an imaginary conversation between comedians and other intellectual heroes of Carroll, some of whom "are quite inconveniently dead" (p. 167), is a nice flourish. There are useful infographics throughout by Kate Baldwin, with additional illustrations by Natalya Balnova. Overall then, this is an accessible and fun book but be forewarned that it might leave you wanting more. Personally, I take that as a good sign.