Categories College Essays

Elizabeth Kolbert’s “The Sixth Extinction”: A Summary Review

Elizabeth Kolbert presents a rare blend of erudition, eloquence and down-to-earth observation and investigation. Her ‘breakthrough’ book was 2006’s Field Notes From A Catastrophe, and The Sixth Extinction has only enhanced her reputation further. She is a staff writer for The New Yorker, and a professor at Williams College, and has won several awards and fellowships, mostly recent the 2015 Pulitzer for non-fiction.

Elizabeth Kolbert’s “Sixth Extinction” surely deserves the Pulitzer it won in 2015. It’s a book that merits the term “hybrid vigor”–appropriately enough for a book so concerned with matters biological. Part science history, part personal reflection, part travelogue, its erudition never becomes dry, and its asides enliven and illumine.

That’s a good thing. The book tackles a topic—the wave of biological extinctions characterizing our time—that is far from cheerful. Nor is Ms. Kolbert afraid to delve into scientific details that could easily excite tedium. But the author keeps us engaged with an artful interweaving of character sketches of scientists past and present, theoretical exposition, wry commentary, and first-person reporting from places as far-flung as Australia’s Great Barrier Reef, Peru’s Manu National Forest, and suburban New Jersey. As you read, it all seems deceptively simple. You may forget that you are learning, but you won’t forget what you are learning.

No summary can really do the book justice, but there is some merit to a synopsis, if only to demonstrate the scope of the work. So summarize we shall.

Each of the thirteen chapters bears the name of a species, living or dead—an emblem for the topic at hand. The first four chapters form a unit, laying out much of the basis for what follows.

For Chapter One, the emblematic species is Panama’s Golden Tree Frog, Atelopus zeteki—a species unexpectedly extinguished in the wild in just a few short years. The culprit turned out to be the chytrid fungus named Batrachochytrium dendrobatidis, or “Bd” for short. It’s not clear whether the source was North American bullfrogs, which have been shipped widely as a food item, or African clawed frogs, used around the world, surprisingly, for pregnancy testing. Both species are commonly infested with bd, but do not become ill, making them perfect carriers of the fungus. But whichever the host species was, its dispersion was clearly tied to the emergence of the ‘global economy’ in the 1980s.

And it wasn’t only the Golden Frog. Numerous species, from Central America to Spain to Australia, fell victim to bd’s unstoppable advance. In fact, the extinction rate for all amphibians—frogs and toads, newts and salamanders, and caecilians—has been estimated to have reached 45,000 times the normal ‘background’ rate. It’s a strange development for a group of creatures that have “been around since before there were dinosaurs.”

But the Golden Frog isn’t yet gone. It has friends and protectors, foremost among whom is Edgardo Griffith, director of the El Valle Amphibian Conservation Center, or EVACC. Here’s Kolbert’s description of him:

Griffith is tall and broad-shouldered, with a round face and a wide smile. He wears a silver ring in each ear and has a large tattoo of a toad’s skeleton on his left shin. Griffith has devoted pretty much his entire adult life to the amphibians of El Valle, and he has turned his wife, an American who came to Panama as a Peace Corps volunteer, into a frog person, too.

At EVACC, the frogs live and breed isolated from the world that once nurtured them: the only mountains are painted murals, and the streams the frogs must have issue from small hoses.

Nothing comes into the building that has not been thoroughly disinfected, including the frogs, which, in order to gain entry, must first be treated with a solution of bleach. Human visitors are required to wear special shoes and to leave behind any bags or knapsacks or equipment that they’ve used out in the field. All of the water that enters the tanks has been filtered and specially treated. The sealed-off nature of the place give it the feel of a submarine or, perhaps more aptly, and ark mid-deluge.

It proves to be a recurrent theme in The Sixth Extinction: humanly induced extinction risk held away by a fingernail-width, thanks to heroic efforts of small groups of humans.

Chapters Two and Three lay out the history of extinction as a concept. Most readers will probably have absorbed the idea as I did, playing with plastic dinosaur figurines whose fearsomeness was made more pleasurable by the knowledge that the real thing was safely relegated to a past millions of years distant. To us, extinction seemed intuitive enough—even obvious.

Yet the idea came late to humanity. The Biblical accounts envisioned the Creation of familiar and unchanging animals and plants. Ancient naturalists like Aristotle or Pliny recognized no creatures that had disappeared from the Earth—although the latter did recognize a few that were purely imaginary. Thomas Jefferson himself, the scientist-President, wrote flatly that “Such is the economy of nature that no instance can be produced of her having permitted any one race of her animals to become extinct; of her having formed any link in her great work so weak as to be broken.”

Ironically, Jefferson was already looking for an extinct creature. The mastodon—confusingly named Mammut americanum—had become a craze, due to the immense size of its bones, dragged from the swamps of Kentucky’s Big Bone Lick and elsewhere. One of Lewis and Clark’s tasks, on their epochal journey of exploration, was to keep an eye out for any mastodons that might have been wandering the unexplored West.

But by the time of Jefferson’s Presidency newer ideas were arising. Georges Cuvier, a young French anatomist, had arrived in Paris in 1795, and by 1796 had demonstrated that Siberian mammoth bones and teeth were not the same as those of living elephants–and moreover that both elephants and mammoths were different from mastodons. Mammoths and mastodons, Cuvier proclaimed, were “lost species.” Soon he added to the list Megatherium, a giant sloth, and the “Maastricht animal,” a reptile we now know to have lived in Permian seas. If four lost species had once existed, must there not be remains of yet more, still to be unearthed?

Cuvier wrote:

All these facts, consistent among themselves, and not opposed by any report, seem to me to prove the existence of a world previous to ours. But what was this primitive earth? And what revolution was able to wipe it out?

By 1812, the list of known extinct creatures had reached forty-nine, and Cuvier was discerning a pattern: more recent layers of rock had more relatively familiar creatures, like the mastodon; deeper, older layers gave up strange beasts like the “Maastricht animal.” The conclusion was clear; there had been not just one ‘lost world,’ but successions of them. Earth was subject to occasional catastrophes, “revolutions” which destroyed enormous numbers of living creatures. This idea would become known as ‘catastrophism,’ and was destined to be highly influential.

As Chapter Three tells us, the term comes from an 1832 coinage by the Englishman William Whewell, who also coined a term for the opposing view: “uniformitarian.” There was really only one uniformitarian of scientific note on Whewell’s horizon: a young geologist named Charles Lyell.

Lyell’s adage was “The present is key to the past,” and the essence of his perspective was that present processes had operated in the same manner throughout time, implying that those processes could account for all observed features of the landscape. He extended this idea to the living world, arguing that extinctions must be gradual, infrequent affairs; the appearance of catastrophe was an artifact of spotty data. Extinctions might not even be final; what arose naturally once, might arise again given the right environment, so that:

…the huge iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyle might flit again through umbrageous groves of tree-ferns… there is no foundation in geological facts for the popular theory of the successive development of the animal and vegetable world.

Lyell’s view would become the dominant one, rendering the term ‘catastrophist’ faintly pejorative. But nowhere would his influence be greater than that he exerted indirectly, through the work of a single disciple—Charles Darwin. The father of the theory of natural selection first read Lyell at twenty-two, reading Principles of Geology “attentively” during his famous voyage aboard HMS Beagle.

Later, as the older Darwin developed his theory, he gave credit to Lyell, and frequently criticized catastrophism. What he failed to notice was that his views held a subtle but deep-seated inconsistency. On the one hand, his Origin of the Species denied humanity any special status; wisdom evolved, just as tusks or flippers, in response to natural factors. Humanity was placed firmly as part of nature. Yet if extinction was a slow and gradual affair, as Darwin asserted, then what of extinctions witnessed during Darwin’s lifetime?

The most notable was the eradication of the Great Auk. Incredibly numerous into the early modern era, populations of ‘the original penguin’ had been reduced inexorably by human predation, until in June of 1844 the last breeding pair was strangled in order that their carcasses could be sold to a wealthy collector of curiousities. This shameful episode did at least help to initiate wildlife conservation efforts, especially in Britain, and expecially on behalf of birds.

So, as Ms. Kolbert sums the matter up:

Either there had to be a separate category for human-caused extinction, in which case people really did deserve their “special status” as a creature outside of nature, or space in the natural order had to be made for cataclysm, in which case, Cuvier—distressingly–was right.

Catastrophism, however, would strike back, as we learn in Chapter 4, The Luck of the Ammonites. (Ammonites were a group of highly successful marine molluscs, one of which, Discoscaphites jerseyensis, serves as the totemic species for the chapter). Between the early 1970s and 1991, researchers Luis and Walter Alvarez uncovered evidence of a truly drastic catastrophe: the K-T extinction. Named after the Cretaceous-Tertiary boundary, it was the end of the dinosaurs, and innumerable other creatures, including the ammonites—quiet, obscure creatures of the sea, highly successful, then abruptly gone.

The Alvarezes published their idea that meteoritic impact had been responsible for the extinction in 1980 in a paper called, appropriately enough, Extraterrestrial Cause for the Cretaceous-Tertiary Extinction. The Lyellian paradigm of the day ensured a spectacular reception: the idea was derided as ‘an artifact of poor understanding’, ‘wrong’, ‘simplistic’ and, colorfully, ‘codswallop.’ The researchers were accused of ‘ignorance’ and ‘arrogance’. But by 1991, the now-famous Chicxlub impact crater had been located, and various lines of evidence for the Alvarez hypothesis had become pretty conclusive. Catastrophes, it seemed, could and did happen.

The fate of the ammonites illustrates an important point: what happens in a catastrophe has nothing to do with classic Darwinian fitness. The ammonites were highly successful—numerous, varied and dispersed. Clearly, they were well-adapted to their environment. As Ms. Kolbert asks, “How could a creature be adapted, either well or ill, for conditions it has never before encountered in its entire evolutionary history?” When conditions radically change, it is a matter of luck how a creature adapted to the old can endure. The luck of the ammonites was bad.

Chapters 5-7 are all sea-haunted in some way.

Chapter 5 takes us to the Scottish Highlands, where a picturesque spot called Dob’s Linn harbors fossilized graptolites—curious sea-creatures of the Odovician period, the traces of whose tiny bodies resemble some exotic script. It appears that they quite suddenly disppeared, roughly 444 million years ago, for reasons not entirely clear. Apparently carbon dioxide levels crashed, causing widespread glaciation, but several possible pathways to the near-extirpation of the graptolites exist. As graptolite expert Dr. Jan Zelasiewicz expressed it in a colorful metaphor, “You have a body in the library and a dozen butlers wandering around looking sheepish.”

It’s not that researchers didn’t search. The Ordovician was the first of the Big Five extinctions, and some thought that a unified theory of extinctions might be possible. But over time, it seems clear that extinctions may be triggered by many different events: global warming as in the end-Permian extinction, global cooling as in the end-Ordovician, or asteroid impact as in the end Cretaceous.

But regardless of cause, the consequences of the extinction remain: the survivors always determine the heritage of all subsequent descendants—and in ways that may not have a lot to do with Darwinian fitness. The new paradigm is called “neocatastrophism.” As Ms. Kolbert puts it, “conditions on earth change only very slowly, except when they don’t.”

But in today’s world the most obvious agent of rapid change is humanity—sometimes abetted by intentional or unintentional commensal species, such as the rats that have always accompanied human sea-voyaging. The latter have been a sort of biological tide, turning much of the biota of numerous island habitats in around the world into “rat protein.” (They may have borne much of the responsibility for the deforestation of Easter Island, for example.)

Direct and indirect human effects inspired Dutch Nobelist Paul Crutzen to suggest that the Holocene epoch is over, supplanted by an epoch he terms the “Anthropocene.” In a paper in the journal Nature he noted that:

  • Human activity has transformed between a third and a half of the land surface of the planet.
  • Most of the world’s major rivers have been dammed or diverted.
  • Fertilizer polants produce more nitrogen than is fixed naturally by all terrestrial ecosystems.
  • Fisheries remove more than a third of the primary production of the oceans’ coastal waters.
  • *Humans use more than half of the world’s readily accessible fresh water runoff.

And, of course, we have increased the concentration of carbon dioxide in the atmosphere by more than 40%.

Dr. Zelasziewicz, intrigued by this research, asked his fellow members of the stratigraphy committee of the Geological Society of London what they thought of this term. Twenty-one of twenty-two thought the idea had merit, and consideration of the term proceeded. At present, a full vote by the International Commission on Stratigraphy on the official adoption of the term “Anthropocene” is expected sometime in 2016.

Chapter 6 looks at another human impact on the planet: ocean acidification. When carbon dioxide concentrations in the atmosphere rise, some carbon dioxide is absorbed by the ocean. It is dissociated, forming carbonic acid. On current trends, by the end of the 21st century oceanic pH will have dropped from 8.2 to 7.8, which under the logarithmic scale used means that it will be 150% more acidic.

The Sixth Extinction examines this phenomenon mostly through the lens of the long-term observational study of the waters surrounding the Castello Aragonese, where a natural vent continually releases CO2. The study began in 2004, when Dr. Justin Spencer-Hall started surveying the biota and taking water samples, initially without any funding whatsoever. He and his Italian colleague, Dr. Maria Cristina Buia, have now been able to show that acidification has devastating biological consequences, wiping out all but a few of the hardiest species. It is unclear just how long CO2 has been bubbling into the sea there, but it is likely more than long enough that biological adaption would have occurred by now if it were possible.

Chapter 7 examines the plight of coral reefs in this context. The world’s coral reefs are home to an incredible variety of creatures, and create the paradox of great biological richness in relatively nutrient-poor waters. But acidification, together with a whole list of other human impacts, is putting the world’s coral at existential risk.

That risk first began to show up in the aftermath of the failure of the Biosphere 2 project. Biologist Chris Langdon, brought in to analyze the failure, found that corals were highly sensitive to what is called the ‘saturation state,’ a property related to acidity:

[There is a] more or less linear relationship between the growth rate of the corals and the saturation state of the water. Corals grew fastest at an aragonite saturation state of five, slower at four, and still slower at three. At a level of two, they basically quit building… Today, there’s almost no place left on the planet… above four… By 2100, none will remain above three.

It’s well to remember that:

…there are ‘reef gaps’ of millions of years following at least three of the five great extinctions.

Apparently we shouldn’t take our coral for granted.

Chapters 8-10 bring us back ashore, and teach some ecological basics.

The scene for Chapter 8 is a research plot high in the Peruvian Andes, in Manu National Park. There, Miles Silman and his collaborators and grad students have laid out a series of altitudinally-sorted forest plots. In each one every tree more than four inches in diameter has been painstakingly tagged and recorded. Since temperature is dependent upon altitude, the researchers can trace the upward migration of species as the climate warms.

But Ms. Kolbert doesn’t take us straight to the Andes. We get there via the North Pole. Even in imagination, that might seem a gratuitous detour; but it serves vividly to illustrate the concept of the “Latitudinal Diversity Gradient”–a puzzling phenomenon first noted by scientific great Alexander von Humboldt.

At the Pole there are, naturally, no trees, just frozen ocean. Five hundred miles south lies Ellesmere Island, where grows the Arctic Willow, a woody shrub which, full grown, will reach your ankle. Another fifteen hundred miles or so brings you first to Baffin Island, where a few more dwarf willow species appear, and then to northern Quebec. Once there, a mere two hundred and fifty miles more brings you to the tree-line, where the great boreal forest begins. There you will find twenty or so species of trees. Slowly, diversity creeps up: by the time you reach Vermont, there are about fifty species of tree; North Carolina boasts more than two hundred. And Dr. Silman’s plots, at about thirteen degrees north latitude, contain at least one thousand and thirty-five.

Ms. Kolbert tells us that there have been more than thirty theories proposed to explain this rule—for it applies not just to trees, but to most kinds of organisms. It turns out to be a consequential relationship, too, even if the exact reasons for its existence remain unsettled.

We also learn of another important relationship that holds across much of the field of biology. That is the “Species-Area Relationship.” It’s usually formulated as an equation:

S=cAz

The “S” stands for “species”, of course, or more precisely the number of species found within the area “A”. “c” and “z” are coefficients that vary according to the characteristics of the particular environment being considered. Basically, as the area drops, the number of species drops, too—slowly at first, but becoming faster and faster.

It seems pretty simple, even banal. But in 2004, a group of researchers used the relationship to do a ‘first cut’ estimate of extinctions to be expected under future warming. It worked like this: they made a sample of one thousand species, of all sorts of creatures, and plotted the temperature characteristics of their ranges. Those ranges were then compared to those generated by simulations of future ranges, and estimates were made of possible adaptive migrations. The result was a new value for “A” in the equation. Taking mid-range values of warming and species dispersal, it turned out that 24% of all species would be at risk of extinction.

It was a blockbuster result, and created a lot of buzz—and hence a lot of criticism. Some subsequent studies concluded that Thomas et al. (2004), as the paper is known had over-estimated the risk, others just the opposite. But as Dr. Thomas says, the order of magnitude appears to be correct. That means that “…around 10 percent, and not 1 percent, or 0.01 percent” of species are at risk.

Chapter 9 delves deeper into the ramifications of the SAR, as they manifest much farther east in the Amazon basin—Reserve 1202, north of Manaus, Brazil, part of the thirty-year experiment known as Biological Dynamics of Forest Fragments Project. In it, ‘islands’ of undisturbed rainforest are left untouched among the cattle ranches now dominating the area. One of the long-term researchers there is Dr. Mario Crohn-Haft, a man capable of identifying any of the thirteen hundred-plus bird species of the Amazonian rainforest solely by its call.

The BDFFP is the flagship experiment in a field that has been dubbed “fragmentology.” As wildlife refuges—natural, or as in the case of Reserve 1202 and the other plots, man-made—first become isolated, biodiversity and abundance may rise, as creatures are concentrated in the remaining wildland. But then attrition sets in, in a process misleadingly termed ‘relaxation’. Species disappear, year upon year and century upon century, gradually approaching supportable levels, in accordance with the SAR. The process may take thousands of years in some cases. But it’s readily observable over the decades during which the BDFFP has been running: 1202 and the other reserves have become increasingly “depauperate”—biologically impoverished.

Crohn-Haft thinks that the effect is exacerbated by the very biodiversity characterizing the region—a diversity that he sees as self-reinforcing. “A natural corollary to high species diversity is low population density, and that’s a recipe for speciation—isolation by distance.” When habitat is fragmented, it’s also a recipe for vulnerability.

While it endures, however, it creates biological marvels. As Crohn-Haft puts it, “These are megadiverse systems, where every single species is very, very specialized. And in these systems there is a huge [survival] premium on doing exactly what you do.”

An example is the ant-bird-butterfly procession seen in the Reserve (and elsewhere). The seemingly endless, ever-moving columns of the army ant Echiton burchelli are followed by birds whose sole feeding strategies involve following the ants in order to snap up the insects they flush out of hiding in the leaf litter. Then there’s a set of butterflies who follow the birds to feed upon their droppings, and various parasitic flies who attack the insects, not to mention several sets of mites which infest the ants themselves. In all, more than three hundred species live in association with E. burchelli.

It’s not unique; Ms. Kolbert calls it a ‘figure’ for the whole logic of the region’s biology: exquisitely balanced, but highly dependent upon existing conditions. When they change, all bets come off.

In Chapter 10 Ms. Kolbert goes home to New England, but finds it to be on its way to becoming part of what she calls the “New Pangaea.” The idea of Pangaea, new or old, is itself fairly new. Charles Darwin had considered the question of geographical distribution, noting that “the plains near the Straits of Magellan are inhabited by one species of rhea, and northwards the plains of La Plata by anotherr species of the same genus, and not by a true ostrich or emu, like those found in Africa and Australia.”

Later, paleontologists began to notice correspondences between certain regions, now widely separated, where similar fossils were to be found. The adventurous Alfred Wegener proposed that the continents must have drifted over time: “South America must have lain alongside Africa and formed a unified block… The two parts must then have become increasingly separated over a period of millions of years like pieces of a cracked ice floe in water.” Unsurprisingly, his theory was widely derided; but the discovery of plate tectonics would largely vindicate his ideas—including the idea of a unified supercontinent he termed Pangaea.

In our time, the biological effects of hundreds of thousands of years of geographic separation are being undone to an amazing degree. As Ms. Kolbert puts it:

One of the striking characteristics of the Anthropocene is the hash it’s made of the principles of geographic distribution [of species]… global trade and global travel… deny even the remotest islands their remoteness. The process of remixing the world’s flora and fauna… has, in recent decades, accelerated to the point where in some parts of the world, non-native species now outnumber native ones.

This was illustrated, painfully, beginning with a disturbing event near Albany, New York, in the winter of 2007. Biologists doing a routine bat census of a cave there were horrified to find “dead bats everywhere.” Survivors “looked as if they had been dunked, nose first, in talcum powder.” At first, it could be hoped that this was a strange anomaly, something that would come and go. But the next winter saw the same horrible events happen at thirty-three different caves in four states. 2009 brought five more states into the mortality zone. As of this writing, twenty-four states and five Canadian provinces are affected—basically everything east of the Mississippi between central Ontario and Quebec south to the mountains in the northern portions of South Carolina, Georgia and Alabama.

The culprit was a European fungus, accidentally imported sometime in 2006. Initially it had no name; because of its devastating effects on North American bats, it was dubbed Geomyces destructans. (Later examination would result in its genus being reassigned, which made it Pseudogymnoascus destructans–harder to pronounce, perhaps, but unfortunately no less deadly than before.)

By 2012, bat fatalities had risen to an estimated 5.7 to 6.7 million. Some populations were reduced by 90% within the first five years, and total extinction was predicted for at least one species. Census efforts continue today, and the indirect effects are also a subject of continuing research; in 2008 the National Forest Service projected that 1.1 million kilograms of insects would survive uneaten as a result of bat mortality, with possible economic impacts to agriculture.

When an invasive species is introduced to a new environment, Ms. Kolbert proposes, the situation can be compared with a multistage version of Russian Roulette. In most cases, the foreign organism die out quite unheeded, since it is not well-adapted to the new environment. That outcome is analogous to an empty chamber in the revolver. But in a few cases, the organism survives to reproduce; after a couple of generations, the species is said to be ‘established.’

Much of the time, nothing much happens; the new species is just a new ‘face in the crowd.’ But in some cases the new environment is not just benign; it’s a bonanza. This may happen because a species’ specific predators have not made the trip—a phenomenon called “enemy release.” But whatever the reason, of every one hundred invasive species, five to fifteen will become established, and one—the ‘bullet in the chamber’—will reach the stage called simply “spread.”

It’s usually a geometric process: the Japanese beetle, for instance, showed up in small numbers in New Jersey in 1916. The next year, three square miles were infested, then seven, then forty-eight. Today it can be found from Montana to Alabama.

North America has certainly had its share of invasives, from chestnut blight and purple loosestrife to the emerald ash borer and the zebra mussel. But the problem is world wide, as the proliferation of invasive species databases attests. There’s the European DAISIE, tracking more than 12,000 species; the Asian-Pacific APASD, FISNA for Africa, not to mention IBIS and NEMESIS.

The seminal work on the topic came out in 1958, when British biologist Charles Elton published his The Ecology of Invasions by Animals and Plants. He realized—counterintuitively, perhaps, given the species area relationship, but the math does work—that “the eventual state of the biological world will become not more complex, but simpler—and poorer.”

Chapters 11-13 turn to humanity and its responses to the crisis underway—to conservation biology, anthropology, and sociology.

Conservation biology comes first, in The Rhino Gets An Ultrasound. The chapter begins by considering the case of the Sumatran rhinoceros, a species considered an agricultural pest in the nineteenth century, but now on the brink of vanishing forever. We meet one of the survivors, a rhino named Suci who lives at the Cincinnati zoo, where she was born in 2004. She’s one of less than 100, and she’s part of a captive breeding program that is trying to save the species. It’s a complex and challenging task, and the program lost more rhinos in the early days than it has since been able to breed up. But there’s no alternative.

The Sumatran Rhino is not unique in this, however: all rhino species are in trouble, and all but one are endangered. But rhinos are not unique in this, either; most large ‘charismatic’ mammals such as big cats, bears and elephants are in serious decline.

Moreover, those species are just the surviving remnants of a global collection still more remarkable—from the mastodons and mammoths, to Australia’s ‘diprotodons’ and New Zealand’s various species of giant moas, and the eight-foot eagles that preyed upon them.

It’s more than possible that all are victims of human predation. The timing of specific losses coincides suspiciously with that of human arrivals (as best as they can be determined for each locale). Other possible causes have in some cases been eliminated, too.

Further, numerical modeling experiments for both North America and Australia show that “even a very small initial population of humans… could, over the course of a millennium or two… account for pretty much all of the extinction in the record… even when the people were assumed to be only fair-to-middling hunters.” The key to this result is that, as biologist John Alroy said, “A very large mammal is living on the edge with respect to its reproductive rate.” Thus, even small additional loss rates can be decisive.

Interestingly, “For the people involved in it, the decline of the megafauna would have been so slow as to be imperceptible”–even though lightning-fast in geological terms.

Chapter 12 turns to anthropology, with a visit to Germany’s Neandertal valley, and a review of the story of humanity’s most famous cousins. Here, too, the record suggests that humans crowded out the competition, though how aggressively or intentionally remains unclear:

Perhaps the Neanderthals were actively pursued, or perhaps they were just outcompeted. Either way, their decline fits the familiar pattern, with one important (and unsettling) difference. Before humans finally did in the Neanderthals, they had sex with them. As a result of this interaction, most people alive today are slightly—up to four percent—Neanderthal.

Perhaps it’s fitting in a way—from the beginning, views of the Neanderthals have been bound up with our views of ourselves. Initially, there was denial that the strange bones that had turned up were anything but human; and fanciful theories were invented to explain away the strange features of the unknown bones. Bowed legs? Due, perhaps, to a Cossack, legs bowed from a lifetime on horseback, escaping from a German battle of the Napoleonic wars.

Later, Neanderthals were caricatured as ape-men, the better to display human refinement; pictured as ‘normal guys’ the better to display human tolerance (or perhaps auctorial sang-froid); and idealized as proto-flower children, the better to support the countercultural narrative of the 1960s.

So what can we say with reasonable certainty about the Neanderthals, given the state of today’s knowledge?

Neanderthals were extremely tough—this is attested by the thickness of their bones—and were probably capable of beating modern humans to a pulp. They were adept at making stone tools, though they seem to have spent tens of thousands of years making the same tools over and over again. At least on some occasions, they buried their dead. Also on some occasions, they appear to have killed and eaten each other… many Neanderthal skeletons show signs of disease or disfigurement… [but several specimens] recovered from their injuries, which means that Neanderthals must have watched out for one another, which, in turn, implies a capacity for empathy.

They may also have lacked art. To be sure, some of their tools may strike modern humans as beautiful; but that does not show that they thought of them as anything more than useful. No unambiguously Neanderthal artifacts are also purely esthetic in purpose.

Ms. Kolbert draws a telling parallel, visiting a Neanderthal site in France, La Ferrasie. There are stone tools and the bones of prey animals, and the remains of Neanderthals and the humans who displaced them. Half and hour’s drive away lies the Grotte des Combarelles, a human site.

Deep inside the narrow, cramped cave lie breathtaking paintings of mammoths, aurochs, woolly rhinos, as well as surviving species such as wild horses and reindeer. What would it have been like to crawl a couple of hundred meters back into the darkness, carrying a torch for light, and a full palette of pigments and binding substances, to make those magical images?

Nowadays we know that it was not only the Neanderthals with whom we once shared the Earth. In 2004 the so-called “hobbits” came to light—a diminutive humanoid species named Homo floriensis, after the Indonesian island where their remains were found. Then, in 2010, DNA analysis of a single finger bone from Siberia turned up a new and unsuspected species, dubbed the Denisovans. Like the Neanderthals, some of their DNA survives in human populations today—up to six percent, in contemporary New Guineans, rather surprisingly, though not in Siberians, or Asians generally for that matter.

Though our ‘sibling species’ are gone, our first cousins survive: chimps, gorillas and orangutans. Their abilities throw an interesting light on ours, Ms. Kolbert proposes. They have been compared with human children, not always to the advantage of the latter:

When researchers from Leipzig performed a battery of tests on chimps, orangutans and two-and-a-half-year-old children… [they] performed comparably on wide range of tasks that involved understanding of the physical world. For example, if a researcher placed a reward in one of three cups, then moved the cups around, the apes found the goody just as often as the kids—indeed, in the case of chimps, more often. The apes seemed to grasp quantity as well as the kids did—they consistently chose the dish containing more treats, even when the choice involved using what might loosely be called math—and also seemed to have just as good a grasp of causality…. And they were equally skillful at manipulating simple tools.

Where the kids routinely outscored the apes was in tasks that involved reading social cues. When the children were given a hint about where to find a reward… they took it. The apes either didn’t understand that they were being offered help, or couldn’t follow the cue. Similarly, when the children were shown how to obtain a reward… they had no trouble grasping the point and imitating the behavior. The apes, once again, were flummoxed… in general, apes seem to lack the impulse toward collective problem-solving that’s so central to human society.

On one hand, collective problem-solving, on the other, art, restlessness—even, perhaps, a kind of madness. Ms. Kolbert quotes Svante Pääbo, head of the team that analyzed the Denisovan finger bone:

[Homo erectus] spread like many other mammals in the Old World. They never came to Madagascar, never to Australia. Neither did the Neanderthals. It’s only fully modern humans who start this thing of venturing out on the ocean where you don’t see land. Part of that is technology, of course; you have to have ships to do it. But there is also, I like to think or say, some madness there. You know? How many people must have sailed out and vanished on the Pacific before you found Easter Island? I mean, it’s ridiculous. And why do you do that? Is it for the glory? For immortality? For curiosity? And now we go to Mars. We never stop.

Whatever the Faustian combination of human traits may be, it has not played out well for our kindred species:

Having cut down our sister species—the Neanderthals and the Denisovans—many generations ago, we’re now working on our cousins. By the time we are done, it’s quite possible that there will be among the great apes not a single representative left, except, that is, for us.

Apparently it is as in the old television show, The Highlander: “There can only be one.”

Chapter 13 is the conclusion, and inevitably, perhaps, its dedicatee species is Homo sapiens–us. It’s less than satisfying, but perhaps that is more an artistic choice than a failure of artistry. Ms. Kolbert resists facile conclusions: humanity’s nature and impact on the world are multifaceted. As yet, there are chapters still to be written by our collective decision-making: will we rein in our growth, our carbon emissions, our toxic pollution? Will we maintain and enhance our efforts to preserve the environment around us, or will our efforts fail over time in the face of climate change, ocean acidification and other environmental impacts which affect our own interests? No one knows—yet.

Ms. Kolbert does not discount human efforts to preserve our biological patrimony, taking us first to the Institute for Conservation Research, where she shows us the cryogenically preserved cells that are all now remaining of the po’ouli, or black-faced honeycreeper, which became extinct in 2004. The “Frozen Zoo” there contains cell cultures of more than a thousand species. Most still exist in the wild, but the proportion is likely to decrease in the future. Similar facilities exist elsewhere, for example Cincinnati’s “CryoBioBank,” or Nottingham’s “Frozen Ark.”

Nor are efforts to protect and preserve other species limited to recent times and high technology:

Time and time again people have demonstrated that… they’re willing to make sacrifices on [other] creatures’ behalf. Alfred Newton described the slaughter that was occurring along the British coast; the result was the Act for the Preservation of Sea Birds. John Muir wrote about the damage being done in the mountains of California, and this led to the creation of Yosemite National Park. Silent Spring exposed the dangers posed by synthetic pesticides, and within a decade, most uses of DDT had been prohibited.

The Endangered Species Act followed just a couple of years later, in 1974. Listed species rescued include the California Condor, of which once only 22 individuals existed; now there are around 400. To achieve this, humans have raised condor chicks using puppets, trained condors to avoid power lines and trash using behavior conditioning, vaccinated the entire population against West Nile virus (notably, no human vaccine yet exists!), and monitor and treat (repeatedly if necessary) condors for lead poisoning resulting from the ingestion of lead shot. Even more heroic have been the efforts on behalf of the whooping crane:

Each year, a team of pilots flying ultralight aircraft teaches a new cohort of captive-raised crane chicks how to migrate south for the winter, from Wisconsin to Florida. The journey of nearly thirteen hundred miles can take up to three months with dozens of stops on private land that owners give over to the birds.

Sometimes rescue efforts can yield tragicomedy. Take the case of the Hawaiian crow, extinct in the wild since 2002. About a hundred individuals exist in captivity, and strenuous efforts are being made to increases the population—though the question raised by the refuge built for the Golden Frog, that is, “Where can the saved species possibly live in the future?”–must surely vex many minds.

So valuable to the limited gene pool is the DNA of each individual that Kinohi, an aberrant male who will not breed with his own species, receives, each breeding season, the attentions of a biologist who attempts to harvest his sperm in hopes of using it to artificially inseminate a female Hawaiian crow. As Ms. Kolbert observes:

Such is the pain the loss of a single species causes that we’re willing to perform ultrasounds on rhinos and handjobs on crows.

Yet this remarkable commitment, more widely shared perhaps than most of us are aware, does not tell the whole story.

When the world changes faster than species can adapt, many fall out. This is the case whether the cause drops from the sky in a fiery streak or drives to work in a Honda. To argue that the current extinction event could be averted if people just cared more and were willing to make more sacrifices is not wrong, exactly; still, it misses the point. It doesn’t much matter whether people care or don’t care. What matters is that people change the world.

…As soon as humans started using signs and symbols to represent the natural world, they pushed beyond the limits of that world… If you want to think about why humans are so dangerous to other species, you can picture a poacher in Africa carrying an AK-47 or a logger in the Amazon gripping an axe, or better still, you can picture yourself, holding a book on your lap.

Of course, this danger is not only limited to ‘other species.’ Richard Leakey warned that “Homo sapiens might not only be the agent of the sixth extinction, but also risks being one of its victims.” After all, we may have “freed ourselves from the constraints of evolution” in some ways, but we are nevertheless still “dependent upon the Earth’s biological and geochemical systems”–or as Paul Ehrlich put it, pithily, “In pushing other species to extinction, humanity is busy sawing off the limb on which it perches.”

Yet Ms. Kolbert suggests that even the understandably concerning possibility of self-induced extinction is not “what’s most worth attending to.” For the paleontological records suggests that humans will not exist forever, regardless of our choices in the current historical moment. But even after we ourselves cease to exist, our influence will continue, in the form of the biology that survives the winnowing we impose:

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 forever be closed. No other creature has ever managed this, and it will, unfortunately, be our most enduring legacy. The Sixth Extinction will continue to determine the course of life long after everything people have written and painted and built has been ground into dust and giant rats have—or have not—inherited the Earth.

I would be inclined to quibble with the idea that ‘no other creature has ever managed this’–for there is some reason to believe that the blue-green algae did just that. Around 2.5 billion years ago their unfettered oxygen emissions caused an atmospheric change dubbed the ‘Great Oxygenation Event.’

This seems to have led to a mass extinction. If so, it would be the first of which we have evidence. It would also be long before the first of the canonical Big Five extinctions, the Ordovician extinction of about 450 million years ago. Call it the zeroth extinction, and read the story as I told it in the Hub Puny Humans. (See sidebar link.)

Yet there is an important difference between the two cases. For the cyanobacteria, there was no alternative: their metabolic processes produced free oxygen, just as a cow’s produces methane today. For the cyanobacteria, as for us or our commensals, it’s breathe or die—obviously.

Not so human behaviors. Their management may be maddeningly refractory, and our choices may be too often perverse and self-defeating, but choose we do. We chose to save British sea birds, the American bison and, later, snail darters, bald eagles, California condors and whooping cranes. We continue to try to save Hawaiian crows and Sumatran rhinos. We even try to save ourselves.

Our choices continue. We can choose to implement the Paris Climate Agreement, which would limit warming from greenhouse gases, and slow the acidification of the ocean. Or we can choose to let it slide, distracted, perhaps, by the politics of insecurity and division. We can also choose, if we deem fit, to ratchet up our efforts, as the agreement provides for, in order to close the ‘ambition gap’ between what we have committed to do, and what we need to do in order to achieve our real aims.

Our choices continue, and will continue. Ms. Kolbert reveals to us that those choices will not only shape our future, but they will shape the entire future of terrestrial life. “Puny humans,” indeed.