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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Particle physics


The Origin of Our Species by Chris Stringer (2011)

This is a very demanding and scholarly book. In the last thirty years major leaps forward in DNA science, the technology of dating fossils, our ability to CT scan and analyse old bones and skulls right down to atomic level and other impressive techniques, as well as a steady stream of new finds of the remains of our prehistoric ancestors, have hugely deepened and complicated our knowledge of human ancestry, of the lineage which stretches back 6 million years to when our ancestors split from the ancestors of modern apes. It’s a massive, complicated and ever-changing field of knowledge.

As the blurb on the back points out, Chris Stringer has been closely involved in much of the crucial research into the origins of humankind and sets out in this book to explain all the latest research, techniques, discoveries and theories in the area, which he does comprehensively and thoroughly.

However, the patchiness of the evidence, the changing results given by evolving techniques, the legacy of sharply conflicting theories and interpretations etc, take a lot of explaining and putting into context. As well as the actual finds and the science we use to interpret them, the book slowly opens up a jungle of differences and debate between archaeologists, paleo-anthropologists, psychologists, DNA researchers, ancient historians and so on, at numerous levels, from large-scale over-arching theories to the interpretation of almost every single find and specimen.

Chapter by chapter, Stringer introduces us to all the evidence and all the techniques and all the controversies – but it is a lot to take in. It doesn’t help that the same theories, techniques and finds recur in different chapters, but in the context of different approaches or discussion of different theories or ideas. You need your wits about you. It’s a book to be read at least twice.

Two theories of human origins

In 1988 Stringer co-wrote an article titled Genetic and Fossil Evidence for the Origin of Modern Humans. This sketched out the two main theories about human origins: Recent African Origin (RAO) and Multiregional Evolution.

1. The multi-regional theory dates from the 1930s and believes that Homo erectus (himself descended from Homo habilis and a distinct species by about 2 million years ago) spread out from Africa over 1 million years ago, settling across Eurasia and Africa, and it was these scattered populations who all transitioned to modern man, Homo sapiens, although with variations which explain the different appearance of modern ‘races’.

2. Recent African Origin (also known as the ‘Out of Africa’ theory) agrees that Homo erectus spread across Eurasia by around 1 million years ago (the original or ‘Out of Africa 1’ scenario), but then postulates the separate development of ‘modern’ man (Homo sapiens) around 100,000 years ago, probably in East Africa. These modern humans also spread out beyond Africa (in so-called ‘Out of Africa 2’), superseding (overwhelming, conquering, killing?) their more primitive cousins wherever the two came into contact.

But a) there are numerous other theories which conflict with both the above, starting with an ‘Assimilationist’ theory, e.g. that Homo sapiens bred with Homo erectus rather than wiping them out; and b) almost every year brings new discoveries which throw up new puzzles and complicate the picture. Also c) Homo sapiens himself seems to have undergone a sudden burst of technological, cultural and social complexity around 50,000 years ago, when better tools, cave art, necklaces etc suddenly appear in the fossil record. It was this new, improved Homo sapiens who appears in Europe from 35,000 years ago. How does that fit into the timeline?

Neanderthal Man In Europe a distinct branch of humans was named Neanderthal Man (after the first specimen whose skull and bones were found in the Neander Valley in Germany in 1856). Neanderthal bodies were bigger, more muscly than ours, they had significantly larger brain cases (as Stringer humorously points out, in brains as with other things, size is not everything) but their most notable feature was really thick, heavy, threatening brow ridges over the eye sockets. Neanderthals are generally considered a distinct species, Homo neanderthalensis, and are thought to be descended from a more primitive species, Homo heidelbergensis, itself a branch of Homo erectus. Nenaderthal man became distinct from Heidelberg man around 600,000 years ago. (Typically, some paleoanthropologists disagree with the whole notion of defining these different specimens as distinct species, and consider Neanderthals and all the other ‘types’ which have been found in the past 150 years to be subspecies of Homo sapiens – thus Neanderthals would be Homo sapiens neanderthalensis).

One of the most intriguing questions remains what it was when I was a boy: We have evidence that modern man (often called Cro-Magnon Man in his European incarnation, after the cave in south-west France where the first specimen was found in 1868) and Neanderthal man both inhabited Europe at the same period, around 40,000 years ago (the Neanderthals having been around in Europe for hundreds of thousands of years, modern man being a new thing, fresh out of Africa). Shortly after the arrival of modern man, records of Neanderthals come to an end; there are no specimens more recent than 30,000 years ago.

So, did we wipe Neanderthals out? Archaeologist Nicolas Teyssandier has noted the period of overlap of the last Neanderthals and the first Moderns is characterised by a profusion of different types of spear tip – was there a stone age arms race? Or did ‘we’ interbreed with Neanderthals to become a cross-breed, Neanderthal records stopping because they had been ‘assimilated’ into our line – so that each of us has a little Neanderthal blood in us? Or did Neanderthals die out due to climate or other changes which they were too dim to adapt to, but which we with our super-smart brains managed to survive? The theories have become more intricate as new DNA evidence has emerged – but to this day, no-one knows.

Homo sapiens (left) Homo neanderthalensis (right)

Homo sapiens (left) Homo neanderthalensis (right)

Homo heidelbergensis This is another distinct form of human, that lived in Africa, Europe and western Asia between 600,000 and 200,000 years ago (and named after the first specimen, discovered in 1907 near the German town of Heidelberg). Some paleoanthropologists think that a population of heidelbergensis migrated into Europe and western Asia between 400,000 and 300,000 years ago and evolved into Neanderthal man. A later branch of the same family had evolved into Homo sapiens in Africa by around 130,000 years ago and then also spread into south-west Asia and Europe where, for 100,000 years, both related species lived alongside each other.


The Pleistocene period is said to date from 2.5 million years ago (Ma) to 12,000 years ago.

The Stone Age or Paleolithic period period lasted roughly 3.4 million years and ended between 8700 BC and 2000 BC, with the advent of metalworking (the date varying according to location, since different human groups developed metal work at different dates).

The Lower Paleolithic Period is 2,500,000 to 200,000 years ago. The Middle Paleolithic is the era during which the Neanderthals lived in Europe and the Near East, c. 300,000–28,000 years ago. The Upper Paleolithic dates from 50,000 to 10,000 years ago in Europe, ending with the end of the Pleistocene Era and onset of the Holocene Era at the end of the last ice age.

The Holocene Era is marked by the end of the ice ages around 13,000 years ago, followed swiftly (in the Fertile Crescent of modern Iraq) by the birth of agriculture, in what Jared Diamond calls ‘the Neolithic Revolution’. This saw humans transition from a life of hunting and gathering to one of agriculture and settlement, a transition whose causes and implicatoins Diamond deals with at length in his classic book, Guns, Germs and Steel.

Dating technologies

The modern technology used to date fossils and ancient remains is now bewilderingly complex and dauntingly sophisticated. Here are some terms; if you’re interested, you’ll have to google them for full accounts.

  • ABOX Acid Base Oxidation-Stepped Combustion pretreatment methods for dating charcoal thought to be over 30,000 years old
  • AMS accelerator mass spectrometry – a technique for measuring long-lived radionuclides that occur naturally in our environment
  • CT computerised tomography X-ray scan
  • ESR electronic spin resonance, method of dating
  • OSL – optically stimulated luminescence
  • TL thermoluminscence dating technique

New words and acronyms

I’m a humanities graduate, not a scientist; I get pleasure from new words and from new concepts (even ones I don’t fully understand).

  • Allen’s Law – animals in cold climates have low surface-to-volume ratios; animals in hot climates, the reverse.
  • atlatl – a spear thrower.
  • Biological Species Concept – the notion that species are defined as groups which can interbreed
  • burins – engraving tools.
  • CI Campanian Ignimbrite – debris from a huge volcanic explosion which took place in Campania, central Italy, 39,000 years ago.
  • Doggerland – the area of land that connected Britain to mainland Europe during and after the last Ice Age until it was flooded by rising sea levels around 6,500–6,200 BC.
  • Dunbar’s Number – after researching primate brain size against the size of their social groups British anthropologist Robin Dunbar estimated that humans can only form meaningful relationships with a maximum of 148 (generally rounded up to 150) other individuals.
  • EQ – encephalisation quotient, the ratio of brain volume to body mass.
  • glottology – the history or science of language.
  • Heinrich Event – brief but severe cold events when icebergs break off from northern ice caps and float south chilling the ocean and surrounding lands (pp.93-94)
  • microtephra – dust from a volcanic explosion which is invisible to the eye.
  • morphometrics – measuring shapes.
  • sapropels – dark layers of sediment laid down where the Nile reaches the Mediterranean.
  • survivorship – the proportion of a population surviving to a given age.
  • tang – edge or shoulder of a triangular stone point used to mount it as a projectile on a wooden handle.
  • varves – annually deposited layers in the bottom of deep lakes.


  • Anthropologist Grover Krantz strapped on a fake thick protruding ‘brow ridge’ from a Homo erectus skull, and wore it for months (!) to see what advantages it brought. He discovered that it kept his hair out of his eyes, shielded his eyes from the sun – and scared the daylights out of people he met on dark nights. Stringer takes this last point seriously, saying the heavy brows of our ancestors possibly accentuated their stare, giving them an aggressive attitude which helped them intimidate other males and woo females, in the struggle for existence. (p.32)
  • Apparently, there are rumours in the paleoanthropology world that either the Americans or the Russians or both, in the 1940s and 50s, experimented by injecting human sperm into female chimps, bringing the resulting creatures to birth and experimenting on them. (p.33)
  • Male baboons gently fondle each other’s scrotums as a sign of friendship and trust – a defeated chimpanzee makes submissive noises and holds out its hand to the victor – if accepted the victor will embrace and kiss the supplicant, if rejected, he’ll bite it. (p.131)
  • Fire dates to around 1.6 million years ago in Africa, 800,000 years ago in Israel, 400,000 years ago in Britain. (p.140)
  • The Grandmother Hypothesis developed by James O’Connell and Kristen Hawkes proposes that human evolution favoured older women who lived on after the menopause (something which doesn’t happen in primates) who can help their daughters with child-rearing and food-gathering. (p.141)


The ninth and final chapter presents a conclusion of sorts – which is that, having extensively reviewed the current evidence, Stringer has modified his lifelong adherence to the Recent African Origin thesis in several ways:

  1. The one that surprised me the most has to do with the size of the communities we’re talking about. Up-to-date genetic evidence suggests that the groups which left Africa and moved out to populate Arabia and around the Indian coast, might have numbered in the hundreds. Even within Africa the various species may at any one time have only numbered in the thousands. (‘The long-term effective size of the ancestral population for modern humans might have been only about 10,000 breeding individuals’, p.175, whereas the number of breeding Neanderthal females in Europe might have been as little as 3,500). Given these numbers, the extinction of the Neanderthals is changed from being some kind of war of extermination (as it is sometimes painted) into the dwindling and going defunct of already tiny scattered communities (and the most attractive interpretation Stringer gives for this is the notion that Neanderthals were just bigger, heavier and needed more food than the lighter, nimbler Home sapiens – maybe in the unstable climatic situation in Europe 30,000 to 40,000 years ago, small and clever was simply more adaptable).
  2. The last two chapters bring together evidence which Stringer says can be interpreted, in light of these small numbers, to suggest a new hypothesis – that there were, at any given time, multiple human species living in Africa (he repeats several times that modern-day Africans show vastly more genetic diversity than any other continent – modern DNA evidence suggesting that the populations of Asia, the Far East, the Americas, Australia derive from very small bands of ancestors populations with the genetic diversity of modern populations dropping the further you go from the African source). In other words, the linear model of one species evolving into another species has been replaced by a much more complex scene of multiple species or sub-species flourishing in different places at different times. ‘100,000 years ago Africa may have comprised a collection of separate sub-groups’ (p.244). The evidence now suggests ‘that Africa contained archaic-looking people in some areas when, and even long after, the first modern-looking humans had appeared’ (p.255). In other words, the multiregion theory could be true within Africa, where multiple species, sub-species, varieties and groups of humans evolved along separate lines, developing widely different levels of tools, some isolated, some inter-breeding and leaving behind a patchwork of random relics to puzzle and confuse 21st century paleoanthropologists trying to create one continuous narrative.
  3. A recurrent problem in this new, more complex picture is that ‘superior’ technologies or skills seem sometimes, in some areas, to be replaced by inferior ones. Stringer uses the analogy of fires or beacons flaring up in the immense darkness of Africa for a millennium or so, then going out. Why? The brief answer, as with so much paleoanthropology, is that no-one knows. Climate change? Genetic drift? Drought, famine, conflict? But the stops and starts certainly fit with the newish idea of much greater diversity, variation, and contingency in our evolution than had previously been suspected.
  4. All of which brings Stringer to modify his initial RAO thesis: maybe there wasn’t one, but multiple out-of-Africa events. To me, as a layman, this doesn’t seem that surprising. Pre-human species didn’t have maps: they didn’t know they were ‘leaving’ Africa; they were just roaming, hunting and gathering wherever food could be found. It makes more sense to think there would have been multiple ‘exits’ from Africa. If our theories only posited two until recently, that could be because the archaeological record is so thin and patchy as not to spot the others – or it could be that numerous other ‘exit’ populations went extinct leaving no fossil or genetic trace. We think the exit event which led to us is important because it led to us; but it might have been just one among many, and its survival down to pure chance.
  5. And this leads to perhaps the most unsettling thought, which is all these theories tend to undermine our specialness. Even within scientific communities there has been a consensus that Homo sapiens is special because ‘we’ ended up inventing agriculture, cities, religion, states, navies, trains, rockets and all the rest of it – and therefore a tendency to try and identify the reason for that specialness and the moment when that specialness took hold. (Stringer thinks something happened around 50,000 years ago to change human behaviour, nudging it towards greater inventiveness – climate, size of social groups, who knows; but there are scores of other theories – he mentions the ‘Broad Spectrum Revolution’ theory proposed by Lewis Binford and Kent Flannery, a coming-together of climate, population size and innovation which they date to 20,000 years ago). But what if we’re not that special. What if Neanderthal man or some of the more obscure relics, such as Homo floriensis (the so-called ‘Hobbit’, a short version of modern humans found only in East Asia) or other sub-species and hominins as yet undiscovered, had just as much potential to develop and ‘succeed’ – but existed in such small populations that fairly limited events (drought, volcanic eruption, sudden chilling in an ice age) wiped them out and happened, just happened, to leave the field open to us? What if ‘we’ are only here by the merest luck or fluke but – with the arrogance typical of our species – have taken this as giving us an entirely spurious specialness, giving us the right to lord it over the earth and all the other species, when in fact our lucky ancestors just happened to be in the right place at the right time, or not to be in the wrong place at the wrong time…


The Origin of Our Species by Chris Stringer was published by Allen Lane in 2011. All quotes and references are to the 2012 Penguin paperback edition.

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