Mammoths and sabertooth cats are but two icons of an assemblage of large animals, or megafauna, that disappeared between roughly 50,000 to 12,000 years ago. As with all mass extinctions, several explanations have been put forward, but one man and his idea take centre stage in these discussions: Paul S. Martin’s overkill hypothesis. In End of the Megafauna, palaeomammalogist Ross D.E. MacPhee carefully scrutinises this idea, weighs up the arguments for and against, and explains its enduring allure. To quote Huxley, is this another example of “the slaying of a beautiful hypothesis by an ugly fact”?
Paul S. Martin (1928-2010) was a palaeoecologist who developed the overkill hypothesis in the 1960s: as our ancestors spread over the planet and entered new lands, they hunted the existing megafauna into extinction. Especially in the Americas there seemed to be a close link between humans appearing and megafauna disappearing. He championed this idea in a career spanning five decades, culminating in his 2005 book Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America. A lot of the ensuing academic discussion has been compiled in the edited volumes Extinctions in Near Time: Causes, Contexts, and Consequences (1999) and American Megafaunal Extinctions at the End of the Pleistocene (2009) to which MacPhee contributed, and on which he now draws in this book.
MacPhee first introduces the particular characteristics of our world during the Quaternary (the last 2.6 million years), both its climate with the many ice ages and warmer interglacial periods (see also The Ice Age), and the peculiar megafauna that included giant sloths, enormous flightless birds, and more exotic species such as glyptodonts and gomphotheres, weighing in the hundreds or even thousands of kilograms (see for example Megafauna: Giant Beasts of Pleistocene South America, Horned Armadillos and Rafting Monkeys: The Fascinating Fossil Mammals of South America, Extinct Madagascar: Picturing the Island’s Past, and Australia’s Mammal Extinctions: A 50,000-Year History). Throughout, the book is illustrated with wonderful plates by Peter Schouten that bring to life the landscapes and animals of this time (though some of them have been spread awkwardly over one-and-a-half pages, obscuring some details).
The bulk of the book, however, is dedicated to a careful examination of the two leading explanations for the extinctions; climate change and, foremost, overkill. MacPhee walks the reader through the unfolding discussion as opponents poked holes in these ideas and proponents tried to shore them up again with new data. He takes stock of the main objections and the status of the overkill idea today, and puts forward alternative explanations.
Especially based on the peopling of the Americas (see First Peoples in a New World: Colonizing Ice Age America), Martin envisioned a wave of destruction as humans migrated from north to south, from Alaska to Patagonia, in about 1000 years, killing everything in their path – even likening it to a blitzkrieg. He saw this pattern as a template for recent megafauna extinctions around the globe. Dramatic? Yes. Especially extinctions of island dwellers such as the dodo, which are clearly linked to human hunting, lend the idea credibility (see The Song of the Dodo: Island Biogeography in an Age of Extinctions).
“Martin envisioned a wave of destruction as humans migrated […] from Alaska to Patagonia […] killing everything in their path – even likening it to a blitzkrieg”
But, MacPhee points out, upon closer inspection many things don’t add up. Some species that were not hunted (e.g. horses and camels) went extinct, whereas others that were (e.g. bison), survived (only to be almost exterminated in recent times…). Timing is crucial, but findings from archaeology and ancient DNA (see Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past) increasingly suggest that the Americas were populated before the Clovis people moved in 12,000 years ago. In Africa and Eurasia, meanwhile, humans and megafauna “happily” coexisted for thousands of years. Martin furthermore argued for prey naïveté: not having encountered humans before, megafauna supposedly lacked appropriate anti-predator responses. Though observed in island species, it seems implausible in continental animals. The scale and organisation required from bands of hunter-gatherers equipped with stone-age tools to effect a sustained blitzkrieg strains credibility. Ethnographic studies show that humans might band together shortly for annual hunting expeditions, but normally disperse again afterwards. Finally, there is a lack of any archaeological evidence for mass kill sites.
MacPhee concludes that we have reached an intellectual impasse pursuing this research agenda. Though some strong arguments can be made in favour of overkill, and some extinctions no doubt can be attributed to it, there are many and obvious weaknesses when extrapolating this. He discusses some interesting alternative explanations and highlights what data we need moving forward to come to a better answer. The strong suit of this book is not just that it explores the nitty-gritty biological and palaeontological details, but also that it takes a step back to examine the history and nature of the debate itself.
As MacPhee also remarks, catastrophic, single-cause explanations for extinctions are sexy, and the media love the dramatic storytelling it allows. The discussion surrounding the K-Pg extinction 66 million years ago is a point in case (Meteorite impact? Giant volcanic eruptions? Or a bit of both? – see T. rex and the Crater of Doom and The Ends of the World: Volcanic Apocalypses, Lethal Oceans and Our Quest to Understand Earth’s Past Mass Extinctions). And this is not just a rarefied academic discussion. His insight that the overkill hypothesis has shaped our thinking on the current extinction crisis (see The Sixth Extinction: An Unnatural History) is incisive: staving it off is being cast as a moral duty to redeem ourselves for our past sins of overhunting.
End of the Megafauna is a first-class example of a fantastic popular science book. The combination of a logical structure, accessible writing, shrewd observations, and beautiful illustrations make it a pleasure to read and impossible to put down. Even the glossary makes for an informative read! When I recently reviewed When Humans Nearly Vanished: The Catastrophic Explosion of the Toba Volcano, another example of a catastrophic explanation for observed extinctions, I mentioned being disappointed that Prothero spent so few pages actually discussing the arguments pro and contra. What really makes this book stand out is that MacPhee does it just right: he takes his time to engage with the idea and provides an accessible overview of the arguments for and against. This is, hands-down, one of the best palaeontology books I have read this year and I expect it will be the go-to reference on this topic for years to come.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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