Review | Extinctions: Living and Dying in the Margin of Error

G.T. Dempsey


Michael Hannah Extinctions:  Living and Dying in the Margin of Error (Cambridge University Press, 2021), pp. xxi, 240  ISBN: 9781108843539  $24.95

     We are now living in the Anthropocene, the newest period of geological time, established by the geological community in recognition of the ‘extraordinary impact’ we humans have had – and continue to have – on our planet.  As the author observes, ‘Future geologists will examine rocks of Anthropocene age and, preserved in them, will recognize the massive changes to the planet’s ecosystem brought about by human activity.’  These geologists of the future, via ‘sophisticated geo-chemical analyses,’ will demonstrate how Earth underwent rapid warming and immense changes in land use and – the subject of this book – ‘a massive loss of biodiversity across all biotic groups’ as a consequence of deteriorating environmental conditions.

     Are we, indeed, facing such a mass extinction event?  What can we learn from past such extinction events in Earth’s history to predict our future?  We know, from the fossil record, that there were periods when biodiversity exploded and periods when much of life was wiped out.  By comparison, today’s biotic crisis has not yet reached the level of mass extinction, but we are in crisis, with current parallels to past mass extinctions (The ‘Big Five’) profoundly worrying.  We are pushing the Earth System out of equilibrium.

Cover of Michael Hannah's book "Extinctions:  Living and Dying in the Margin of Error".
Michael Hannah Extinctions:  Living and Dying in the Margin of Error (Cambridge University Press, 2021).

     The current estimate is that there are, on Earth, 8.7 million species alive today.   Some 3.7 billion years ago, there was one.  From these figures, it would seem that the total biodiversity of our planet is in flourishing condition.  But the palaeontologists tell us that ‘of all the species that have ever existed on the planet, over 99% are extinct.’  Thus, our current 8.7 million species represent ‘less than 1% of the number of species that have evolved and gone extinct since life first appeared on Earth.’  Sobering numbers.  But not absolute — the margin of error for the total numbers is given as ±130 million species.  As the author puts it, ‘from the perspective of deep time, we, and every other organism on the planet, are living and dying in that margin of error.’   Also, it is highly likely that millions of the species that have existed and gone extinct have left no record.  Still, there is a fossil record which has allowed palaeontologists ‘to piece together, often in fine detail, the history of life on Earth.’  

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     This book sets out, in some nine chapters, both the ‘turbulent’ journey of that one species living 3.7 billion years ago to the 8.7 million today and the human activity threatening that biodiversity: ‘Today, humans are warming the planet: the ice sheets are melting…the chemistry of the oceans is changing – they are becoming more acidic and areas of low oxygen concentrations are spreading.’  Land use is being ‘radically changed’ in order to feed and house our ever-expanding population, radically ‘reducing the natural habitats that support much of Earth’s biota.’  We need to understand how we got into our current situation, if we are to know what we can expect in the future and, crucially, how we can change that impending future.  

     The author is an associate professor in the School of Geography, Environment and Earth Sciences at Victoria University in New Zealand.  He specializes in palaeontology and biostratigraphy (the dating of rocks through fossils) and has participated in major Antarctic drilling projects, deciphering ancient changes in climate and the history of the Antarctic ice sheets.  Throughout, his exposition is masterful, engagingly written, and amply illustrated with charts and tables and diagrams.  The first two chapters set the stage:  defining the Anthropocene and establishing both the value of the established fossil record and its limitations:  ‘a biased record [but] biased in a very predictable way….  I like to compare the fossil record to a chain-link fence [which] is strong, durable, and full of holes.’  The next two chapters (chapters 3 and 4) consider the origin of animals ‘as single-celled organisms develop into multicellular organisms, fundamentally changing the planet and heralding the appearance of the modern Earth System’ and set the parameters of ancient biodiversity.  Chapters 5 to 8 detail the history of past mass extinctions, covering the debates over the interpretation of the evidence.  Cyclical or cataclysm-driven?   We are all aware of the fate of the dinosaurs and the meteor impact of the end-Cretaceous event, some tens of millions of years ago.  But just some 50,000 years back – well within the time-frame of the appearance of our own species – giants roamed the Earth:  kangaroos three meters tall, seven-meter long monitor lizards, wombats the size of hippos, mastodons and mammoths, with rhinos and giant Irish elks in Europe.  By eleven thousand years ago, most of these giant mammals and other megafauna were extinct.  The cause?  The end of the last ice age, global warming, it’s all ‘still under intense debate.’

     The final chapter:  ‘Surviving the Anthropocene.’    Will we?

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A review copy of this book was received from the publisher. 

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About the author
G.T. Dempsey
G.T. Dempsey is a Research Associate in the history of Late Antiquity at the University of California at Davis and, as a retired American career diplomat, he is also a commentator on American foreign policy.