The Sixth Extinction: An Unnatural History by Elizabeth Kolbert (2014)

Back in the late 1980s and 1990s there was a fashion for popular science books, and I read as many as I could, becoming better informed about the three major subjects which dominated the lists – cosmology, paleontology with an emphasis on human origins, and environmental biology.

Among them were a number of books by E.O. Wilson, particularly the brilliant Diversity of Life (1992), which gives an unparalleled sense of the wonder and diversity of the natural world, and Richard Leakey’s book, The Sixth Extinction (1995). This latter is an often quite technical account of discoveries and debates in paleontology and environmental biology which, taken together, suggest that the rate at which humanity is killing off species of animals, plants, fish and other fauna amounts to a holocaust, a global extermination, which ranks with the other Big Five mass extinction events that have punctuated the 500-million year story of life on earth – hence the title.

Now, 20 years later, comes a book with the same title by American journalist Elizabeth Kolbert. I was interested to compare the books, not only in terms of what’s changed in our understanding and the plight of nature, but in style and approach.

The situation’s got worse, of course. One third of all reef-building corals, a third of all fresh-water molluscs, a third of sharks and rays, a quarter of all mammals, a fifth of reptiles and a sixth of all birds are headed towards extinction. (p.17) (The radio news today informs me that 7 honey bees have placed on the US endangered species list, as colony collapse disorder continues to decimate hives.)

Kolbert approaches the issue through thirteen chapters, each devoted to a specific species, combining its history, her personal trips and visits to museums or rainforests, along with profiles of key contributors to the history of ecology, and ideas in evolution or conservation thrown up by each story.

The chapters

Thus she opens by visiting a research institute in Panama devoted to trying to save the Panamanian golden frog (Atelopus zeteki). It explains how the fungus Batrachochrytium dendrobatadis is wiping it out, along with scores of other frog species around the world – and so the chapter introduces and explains the notion of the historic mass extinctions.

The second chapter considers discoveries in the 1700s of large bones in America and Europe, specifically of what came to be named Mammut americanum, and how it led the French naturalist George Cuvier to develop and publish a theory of species being wiped out in sudden catastrophes (in an essay published in 1812) although the term ‘catastrophist’ (someone who believes the history of life on earth is marked by long periods of stasis broken by sudden catastrophes in which entire faunas are wiped out and entire new ones replace them) wasn’t coined until 1832, by William Whewell, president of the British Geological Society.

Kolbert contrasts Cuvier’s catastrophism with the ‘uniformitarianism’ of the great geologist Charles Lyell, whose epic work on geology inspired and underpins Darwin’s thinking. It was Lyell who for the first time gave a thorough sense of the profound age of the earth and showed how it had been formed over hundreds of millions of years by slow unrelenting forces. It was this rhythm and metaphor which helped the young Darwin grope his way towards a theory that life on earth had also changed in a slow but unrelenting way due to the process he called ‘natural selection’. The key to both is a nice steady uniform speed of geological and biological processes.

We learn this in chapter three, where it is tied into the history of the great auk (Pinguinis impennis) which went extinct in the 1840s. Kolbert takes a trip to Iceland to visit a nature centre and then go by boat out to the remote island where, supposedly, the last breeding pair of great auks were caught and killed before being sold for £9. This chapter is used to point out that Darwin must have known about man-made extinctions because he witnessed them wherever he went on his epic voyage round the world in HMS Beagle (1831-36).

Chapter four tells the story of Luis Alvarez’s discovery of a layer of iridium at the geological boundary between the Cretaceous and Tertiary periods, the so-called Cretaceous–Tertiary (K-T) boundary about 66 million years ago. Along with other scientists he interpreted this as meaning that the end-Cretaceous extinction, which saw about 70% of species wiped out, was caused by an asteroid or meteorite hitting earth. This chapter recounts the fierce opposition from most paleontologists who were wedded to one form or another of Lyell and Darwin’s uniformitarianism, and so harshly criticised Alvarez’s findings when they were published in 1980. As usual, Kolbert ties this account into a trip she took with paleontologists to a secret location in New Jersey where the K-T boundary is easily accessible and where they hunt for ammonite fossils.

Chapter five explains how ‘neo-catastrophism’ has become the new orthodoxy – i.e. that long periods of uniformity punctuated by disasters, have shaped the story of life and the nature of the current biosphere. This is told via a visit to Dobb’s Lyn, a mountainside stream in Scotland in heavy rain to look for glyptolites, followed by a warm dinner at a local B&B. Here the fossil hunters accompanying Kolbert explain the history of the term ‘Anthropocene’, first suggested in 2000 and now widely used.

Just as organisms are divided into kingdoms, phyla, families, genera and species, so geologists divide the entire history of the earth into eons, themselves divided into eras, which are in turn divided into periods, epochs and ages. Thus we are in the the Phanerozoic Eon, which dates from the beginning of multicellular life some 530 million years ago. This eon is divided into three eras: the Paleozoic Era, the Mesozoic Era and the Cenozoic Era, where ‘zoe’ is Greek for ‘life’ and paleo means old (Old Life Era), meso means ‘middle (Middle Life era) and ceno is from ‘koinos’ which means new = new life era.

Each of these eras is sub-divided into periods: the Paleozoic into the Cambrian Period, Ordovician Period, Silurian Period, Devonian Period, Carboniferous Period and Permian Period; the Mesozoic into the Triassic Period, Jurassic Period and Cretaceous Period; and the Cenozoic Era into the Paleogene Period, the Neogene Period and the Quaternary Period. And these periods are further divided into epochs: thus the most recent period, the Quaternary Period, is sub-divided into the Pleistocene and Holocene epochs, the Pleistocene dated 3 million years ago to around 13,000 years ago i.e. until the end of the last ice age; the Holocene dating from around 13,000 years ago to the present day.

Over the last twenty years or so there have been growing calls from some biologists, paleontologists and archaeologists to define the epoch we’re living in as a new geological epoch, the Anthropocene epoch, because human interaction with the environment is creating unprecedented changes to the entire planet.

I already knew from books and articles about the calls for our age to be named the Anthropocene – but I had never properly processed the full implications of the fact that, not only are we driving species instinct at an unprecedented rate now, in the present – but that all future life on earth will only be able to evolve and cope with changing conditions, from the smaller and smaller and smaller starting base that we are creating. It is not just the present or our children’s world that we are diminishing – but all future possibilities for life on the planet – forever.

Right now, in the amazing moment that to us counts as the present, we are deciding, without quite meaning to, which evolutionary pathways will remain open and which will be forever closed. No other creature has ever managed this, and it will, unfortunately, be our most enduring legacy. (p.269)

I had never grasped the deep historical implications of our greed and arrogance and destructiveness.

Chapter six records Kolbert’s trip to Kastello Aragonese, an islet near Ischia. The island is home to volcanic vents which release a steady stream of CO2 into the sea. Kolbert meets scientists who are researching the impact of rising CO2 levels in seawater: basically it prevents calcifiers, that is all animals which create shells, from being able to do so – starfish, barnacles, clams, oysters, and scores of thousands of other species. Never in the history of the Earth has so much CO2 been injected into the oceans so quickly. Sea life hasn’t time to adapt.

Chapter seven takes this forward via a trip to One Tree Island off the Great Barrier Reef. Here, in a rough and ready research centre, she meets an international team of scientists who say the future for all coral reefs in the world, and all the species they support is ‘grim’. By 2050 they may all be dead. The Chief Scientist of the Australian Institute of Marine Science has said, that he is

‘utterly humbled to have spent the most productive scientific years of my life around the rich wonders of the underwater world, and utterly convinced that they will not be there for my children’s children.’ (quoted p.138)

She times her trip to observe the wonderful and weird sight of the annual ‘spawning’ of the corals. How many more years will it take place?

Chapter eight takes us to the rainforest of Manú National Park in southeastern Peru where scientist Miles Silman shows Kolbert around the 17 plots, each at a different altitude, which he and his assistants have marked out to explore different tropical communities. They were laid out in 2003. It incorporates the research done by Chris Thomas and colleagues from York Uni which estimate that, with worst case rates of global warming, up to 33% of Earth’s species will be exterminated. Back in Silman’s forest, Kolbert describes their research which shows that, as the climate warms up, species are in fact moving up mountains slopes to continue living in the temperature ranges they’re used to. But only so many species can even move (trees are not so mobile) and not many have mountain slopes to move up, but the real killer is speed – scientists think previous changes occurred over millions of years; we are changing the Earth’s climate in a matter of decades.

One of the defining features of the Anthropocene is that the world is changing in ways that compel species to move, and another is that it’s changing in ways that create barriers – roads, clear-cuts, cities – that prevent them from doing so. (p.189)

Chapter nine sees her in the Amazon, visiting some of the squares of rainforest left standing among areas decimated for farmland, as an ongoing scientific experiment. Lots of numbers. There are about 130 million square kilometres of land which are ice free. Of this around 70 million have been drastically remodelled by man; of the remaining 60 million three-fifths is forest. (Another study, by Erle Ellis and Navin Ramankutty divides the world’s surface into 18 ‘anthromes’, or types of human land-use, which says that 100 million have been altered by human hand, leaving 30 million of wildlands – Siberia, northern Canada, the Sahara, Gobi, central Australian desert.)

Kolbert is taken into the rainforest by her hosts to look for birds, incidentally observing the mad profusion of trees, plants and insects, including a huge column of soldier ants (learning that up to 300 species of animals are dependent on soldier ants and the changes they create). At the base she meets Tom Lovejoy, now in his 70s, credited with putting the phrase ‘biological diversity’ into circulation.

Chapter ten The separation of ecosystems on different continents, islands, archipelagos etc has been one of the key drivers of speciation i.e. diversity. Man began to mess that up with his ocean going journeys from about 2,000 years ago as humans sailed out across the Pacific islands, with the Maori arriving 1,000 years ago in New Zealand and devastating its wildlife. But the real ecological mixing began in the Age of Discovery, which was kicked off when Magellan sailed round the world and Columbus discovered America – the introduction of thousands of Old World species to the New World is now referred to as the ‘Columbian Exchange’.

Nowadays human transports are criss-crossing the globe in mind-boggling volumes, transporting flora, fauna and diseases to every last nook and cranny. Kolbert quotes the estimate that in any given 24 hour period some 10,000 species are being moved around the planet just in ships’ ballast water. So it’s no surprise that diseases once restricted to tiny parts of the world can now travel widely, for example the disease killing off the Panamanian frogs we met in chapter one, and the fungus killing bats in Massachusetts – white-nose syndrome – which we meet here. She follows the catastrophic decline in bat populations in Vermont which have collapsed since the fungus was first identified in 2007. In less than a decade bats have gone from flourishing to endangered, and will probably go extinct in the next decade.

Chapter eleven A visit to see Suci, a captive Sumatran rhinoceros at Cincinnati zoo, is the peg for a review of the catastrophic decline of big mammals (elephants, rhinos, lions, tigers, pandas) over the last century. This leads on to a visit to Big Bone Lick, where 19th century naturalists found fossils and bones of huge animals which once roamed North America but which were completely extinct by the 1800s.

It was American ecologist Paul Martin who popularised the Overkill Hypothesis, which is that megafaunas were hunted to extinction wherever prehistoric man went – in Australia 40,000 years ago, in America from 13,000 years ago, in New Zealand 700 years ago and so on. Kolbert presents the counter-arguments of scientists who are not convinced that handfuls of technologically primitive peoples could wipe out entire continents full of big dangerous animals; and then the counter-counter arguments educed from mathematical models, which show that, given enough time, even killing only one big beast a month could wipe out entire species in a few hundred years – which is what appears to have happened.

The conclusion of this line of thinking is that man has never lived in harmony with nature but has massacred large animals and triggered major ecological change wherever he has gone.

Chapter twelve Kolbert visits the centre in the Neander Valley in Germany where Neanderthal Man was discovered (though the cliffs and cave where he were discovered were long ago demolished for construction material). Neanderthal man (Homo neanderathlensis) existed as a branch of the Homo genus for at least 10,000 years from 130,000 to 30,000 years ago. All the evidence is that, wherever populations of the more ‘advanced’ Homo sapiens appeared, Neanderthal man soon after disappeared. As Chris Stringer discusses in his book, The Origin of Our Species, was he pushed or did he jump? Was it environmental change which did for the Neanderthals or some form of warfare with our ancestors or both which led to his extinction?

The chapter is titled ‘the madness gene’ because one scientist contrasts Neanderthals with Homo sapiens – particularly in regard to adventurousness. As far as we can tell Neanderthals made the same stone tools without any development or improvement for 100,000 years, whereas modern man’s culture evolved quickly. The cave paintings in the Dordogne region of France were made by modern man, whereas nothing comparable exists for Neanderthals. Above all, modern man spread far and wide, and the ‘madness’ idea comes in when you consider the urge, the adventurousness, the recklessness of the peoples who set off in primitive ships 2,000 years ago into the vast empty seas of the Pacific with no maps and no guides and no certainty of finding anything but ended up populating Hawaii and all the other Pacific islands, thousands of miles from the mainland. What is that if not reckless adventurism bordering on madness!

Chapter thirteen features the last trip, to San Diego Zoo which has a facility for deep freezing remains of nearly or extinct species – nicknamed the Frozen Zoo. Kolbert views vials full of deep frozen organic matter from various defunct species and wonders – is this what it will come to, will thousands and thousands of life forms survive only as sketches, photos and tubes of frozen gunk? And the reader who has followed her this far on her deeply depressing journey is forced to answer, Yes.

She pays lip service to the good intentions of the millions of nice people who support the Worldwide Fund for Nature or the National Wildlife Federation or the Wildlife Conservation Society or the African Wildlife Foundation and so on and so on. In this she makes what I regard as the classic liberal error of believing most people are like her, or us, educated middle-class, concerned, white people. As the election of Donald Trump and the Brexit vote in Britain should have shown these kind-hearted liberals – most people are not like them. Most people in the West did not go to private school or attend university and didn’t study the humanities and don’t work in white collar professional jobs. Many are struggling to put food on the table or keep a roof over their heads.

And that’s without going further afield into the Developing World where the majority of the population lives in dire poverty, without access to clean water, sewage facilities or nourishing food, and don’t give a damn about the future of the Panamanian frog or the greater mouse-eared bat or the black-faced honeycreeper, let alone the thousands of insect and plant and fungi species Kolbert’s scientists are so concerned about.

There is no great conclusion. Read it and weep. In the book’s last pages she gives a few token reasons for hope and briefly references those sad people who think it will all be OK in the end because humankind can always go off and colonise the moon, or Mars, or other solar systems. Right. She doesn’t even comment on such expensive fatuousness. a) All attempts to live in artificial atmospheres or biomes have failed because we underestimate the complexity of the ecosystem which keeps us alive. b) We can’t even run this planet, what gives anyone the idea we’d do better somewhere else. c) Are we all leaving for Mars, then? All 7 billion of us?

Words and ideas

  • Hibernacula – a place (cave, mineshaft) where creatures seek sanctuary from the winter, often to hibernate.
  • The Latitudinal Diversity Gradient – the closer to the Tropics, the more species are found in ecosystems, thus the tropical rainforest is the most varied and densely speciated environment on earth. There are some thirty different theories why this might be. The Latitudinal Diversity Gradient
  • Psychrophile – a cold-loving fungus.
  • The Signor-Lipps effect – since the fossil record of organisms is generally incomplete, this makes it hard to be confident about the ends or beginnings of taxa or families. In practice it makes what may have been sudden extinction events look long drawn out. The Signor-Lipps effect
  • The Species-Area relationship – the larger an area you sample, the more species you find. The Species-Area relationship


At first I thought it was a gimmick that each chapter focuses on one particular species and goes to one particular location (sometimes two) where she meets one or more scientists working on a particular aspect of the massive issues raised.

But after a while I realised how cleverly Kolbert was dovetailing into each chapter not only snapshots of current research, but also key moments in the history of the discipline, going back to explain the early theories of a Cuvier or Lamarck, a Darwin or Humboldt, to give her reporting a historical dimension and to explain how theories about life on earth arose and have developed over the past century or two.

And I ended up respecting and admiring the skill with which the narrative moves forward on these multiple levels at the same time – all leavened with a dry American sense of humour and an eye for evocative similes (the thin layers of slate at the K-T boundary which she is shown how to handle, fall apart like the pages of an old book; stroking the tough hide of Suci the rhino is like running your hand over tree bark, and so on.)

If you’re new to the subject, this is an excellent, very readable, fascinating, wide-ranging and first-hand account of work going on all around the world. That said, most of us are by now very familiar with this subject. And all of us know in our hearts that things will only get a lot, lot worse.


The Sixth Extinction: An Unnatural History by Elizabeth Kolbert was published by Bloomsbury Publishing in 2014. All quotes and references are to the 2015 paperback edition.

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