When it comes to big volcanic eruptions, names such as Vesuvius, Mount Saint Helens, and Krakatau will ring a bell. But all of these are dwarfed by a far larger eruption that few outside of the science community will have heard of. Noted geologist, palaeontologist and author Donald R. Prothero here tells the story of the eruption of Mount Toba in what is nowadays Sumatra, Indonesia, some 74,000 years ago. An eruption so gargantuan that it almost wiped out the human race.

Prothero recounts the discovery of this eruption in his first chapter. Over the course of the 1980s to the mid-1990s various unrelated strands of evidence suggested to scientists that something dramatic happened around this time. Ice cores revealed a spike in atmospheric sulfuric acid, while marine sediment cores showed a large, rapid drop in global temperatures concurrent with an ash layer (the latter work was done by Mike Rampino, who, as you might remember from my review of his book Cataclysms: A New Geology for the Twenty-First Century, is fascinated by the link between disastrous events and the evolution of life). The proverbial smoking gun, that this something was a large volcanic eruption, was found when volcanologist John Westgate studied ash samples retrieved thousands of kilometres apart, all of which could be dated to some 74,000 years ago. It took four more years to find the actual culprit: an ancient caldera (a volcanic crater) in Sumatra that now held the waters of Lake Toba.

With this in place, Prothero then spends most of the book on four other topics that are important to better understand the effects of the Toba eruption: volcanoes, genetics, human evolution and mass extinctions. He vividly recounts the impact of well-known eruptions such as Krakatau (see also Krakatoa: The Day the World Exploded), Tambora (see also Tambora: The Eruption That Changed the World and Tambora and the Year without a Summer: How a Volcano Plunged the World into Crisis), Vesuvius, and the, to me, lesser-known eruption of Mount Pelée in Martinique (which I first encountered in Volcanoes: Encounters Through the Ages). Next to causing huge loss of life, the first two also had very observable effects on the weather, causing widespread crop failure and famine, as well as knock-on effects on political affairs.

Prothero then spends several chapters on the basics of genetics and human evolution, as well as mass extinctions and the importance of volcanism (see also The Ends of the World: Volcanic Apocalypses, Lethal Oceans and Our Quest to Understand Earth’s Past Mass Extinctions). This extensive excursion should result in readers all being on the same page when it comes to understanding the claim that the eruption and its impact on the planet’s climate caused a genetic bottleneck in humans. A bottleneck comes about when the majority of a species dies, leaving few survivors and thus limited genetic diversity in the gene pool from which to repopulate. And this is exactly what happened with humans around this time. Several species went extinct, and for Homo sapiens estimates suggest only 1,000 to 10,000 breeding pairs remaining. And there is a raft of supporting evidence, such as genetic bottlenecks in other species (cheetahs, tigers and orangutans), and palaeoclimatic evidence of global cooling.

“the eruption and its impact caused a genetic bottleneck in humans with estimates suggesting only 1,000 to 10,000 breeding pairs remaining.”

Though I tore through this book, I have two pieces of criticism. One is minor: though plenty of papers and books are mentioned, a list of references is not included. The other is quite major: in the end, Prothero does not write all that much about the actual eruption and its consequences. The eruption is dealt with in chapter 1, and a lot of the supporting evidence in 13 pages in chapter 7. On page 145 he even writes that: “much has been said and written about the Toba catastrophe hypothesis, and we cannot summarize all of the discussion even in a book like this one”. I would argue this book is exactly the place to do just that!

Instead Prothero spends a lot of chapters on the four topics I mentioned earlier. The upside of this is that if you come to this book knowing little about genetics, human evolution, volcanoes or mass extinctions, Prothero makes sure that you will learn all you need to know. The downside is that it feels this is done at the expense of a more in-depth discussion of the evidence supporting the catastrophic nature of the eruption, as well as the criticism levelled against it. And some of the material seems extraneous. I’m not sure that elaborations on the discovery of the structure of DNA, junk DNA or the Piltdown hoax – fascinating as Prothero’s writing on it is – are fully relevant to the story at hand.

This criticism notwithstanding, When Humans Nearly Vanished is incredibly well written. Prothero is a masterful storyteller who makes complex science easy to grasp. Especially when describing past eruptions in combination with eyewitness accounts, his writing is vivid, regularly causing me to gasp as he drove home the scale of these events (all of which were a sneeze compared to Toba). As Prothero mentions, outside of academic circles this topic is relatively poorly known and no popular book on it has been written before. With that in mind, if you have any interest in mass extinctions or volcanoes, you do not want to pass up the opportunity to read this book.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

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