The Sixth Extinction

Book notes for “The Sixth Extinction”, An Unnatural History by Elizabeth Kolbert

Quick review:

Great book. Reminds me of “Last chance to see”.

I enjoyed the writing, and the fact that there are a list of sources and resources at the end of each chapter. The book feels deep and well structured while still being very readable. Full of fascinating facts and stories, I especially enjoyed the history of many theories we take as granted today, that were only proposed 200 years ago.

The rest of the book shows us how much we have already lost. It leaves you depressed and angry and frustrated. The book ends with a tiny note of hope. We are never good at predicting the future, it may well be that we are able to stop the trend. We could: stop eating meat (good chance?), use cleaner energy sources (very good chance?), reduce our footprint by living in more compact cities, reducing our road networks, restoring detroyed habitats (low chance?), have hugely more control over our environment thanks to new technologies (good chance?)

Questions raised:

What are we going to do about this! What can I do about this?

Insights, lessons learnt:

We have already lost so much, and it may not be “fixable” on anything but geological timescales.


No creature has ever altered life on the planet in this way before, and yet other, comparable events have occurred. Very, very occasionally in the distant past, the planet has undergone change so wrenching that the diversity of life has plummeted. Five of these ancient events were catastrophic enough that they’re put in their own category: the so-called Big Five. In what seems like a fantastic coincidence, but is probably no coincidence at all, the history of these events is recovered just as people come to realize that they are causing another one. location 70

(phrasing) Golden frogs have a distinctive, ambling gait that makes them look a bit like drunks trying to walk a straight line. They have long, skinny limbs, pointy yellow snouts, and very dark eyes, through which they seem to be regarding the world warily. At the risk of sounding weak-minded, I will say that they look intelligent. location 143

Until recently, when both of them went extinct, there were two species of frogs, known as gastric-brooding frogs, that carried their eggs in their stomachs and gave birth to little froglets through their mouths. location 182

In ordinary times—times here understood to mean whole geologic epochs—extinction takes place only very rarely, more rarely even than speciation, and it occurs at what’s known as the background extinction rate. location 242

For what’s probably the best-studied group, which is mammals, it’s been reckoned to be roughly .25 per million species-years. This means that, since there are about fifty-five hundred mammal species wandering around today, at the background extinction rate you’d expect—once again, very roughly—one species to disappear every seven hundred years. location 246

One theory has it that Bd was moved around the globe with shipments of African clawed frogs, which were used in the nineteen-fifties and sixties in pregnancy tests. (Female African clawed frogs, when injected with the urine of a pregnant woman, lay eggs within a few hours.) location 283

Either way, the etiology is the same. Without being loaded by someone onto a boat or a plane, it would have been impossible for a frog carrying Bd to get from Africa to Australia or from North America to Europe. This sort of intercontinental reshuffling, which nowadays we find totally unremarkable, is probably unprecedented in the three-and-a-half-billion-year history of life. location 289

In 1812, Cuvier published a four-volume compendium of his work on fossil animals: Recherches sur les ossemens fossiles de quadrupèdes. Before he’d begun his “researches,” there had been—depending upon who was doing the counting—zero or one extinct vertebrate. Thanks for the most part to his own efforts, there were now forty-nine. location 562

Before 1800s there was no concept of extinction! It was assumed that all animals that currently exist, still exist.

In fact, the American mastodon vanished around thirteen thousand years ago. Its demise was part of a wave of disappearances that has come to be known as the megafauna extinction. This wave coincided with the spread of modern humans and, increasingly, is understood to have been a result of it. In this sense, the crisis Cuvier discerned just beyond the edge of recorded history was us. location 668

After the initial heat pulse, the world experienced a multiseason “impact winter.” Forests were decimated. Palynologists, who study ancient spores and pollen, have found that diverse plant communities were replaced entirely by rapidly dispersing ferns. (This phenomenon has become known as the “fern spike.”) Marine ecosystems effectively collapsed, and they remained in that state for at least half a million, and perhaps as many as several million, years. (The desolate post-impact sea has been dubbed the “Strangelove ocean.”) location 1204

On land, every animal larger than a cat seems to have died out. The event’s most famous victims, the dinosaurs—or, to be more precise, the non-avian dinosaurs—suffered a hundred percent losses. location 1210

Everything (and everyone) alive today is descended from an organism that somehow survived the impact. But it does not follow from this that they (or we) are any better adapted. In times of extreme stress, the whole concept of fitness, at least in a Darwinian sense, loses its meaning: how could a creature be adapted, either well or ill, for conditions it has never before encountered in its entire evolutionary history? location 1255

until us! We have broken free of evolution and are able to adapt using science/tech (up to a point)

In 1949, a pair of Harvard psychologists recruited two dozen undergraduates for an experiment about perception. The experiment was simple: students were shown playing cards and asked to identify them as they flipped by. Most of the cards were perfectly ordinary, but a few had been doctored, so that the deck contained, among other oddities, a red six of spades and a black four of hearts. When the cards went by rapidly, the students tended to overlook the incongruities; they would, for example, assert that the red six of spades was a six of hearts, or call the black four of hearts a four of spades. When the cards went by more slowly, they struggled to make sense of what they were seeing. Confronted with a red spade, some said it looked “purple” or “brown” or “rusty black.” Others were completely flummoxed. The symbols “look reversed or something,” one observed. “I can’t make the suit out, whatever it is,” another exclaimed. “I don’t know what color it is now or whether it’s a spade or heart. I’m not even sure now what a spade looks like! My God!” location 1268

it revealed how people process disruptive information. Their first impulse is to force it into a familiar framework: hearts, spades, clubs. Signs of mismatch are disregarded for as long as possible—the red spade looks “brown” or “rusty.” At the point the anomaly becomes simply too glaring, a crisis ensues—what the psychologists dubbed the “ ‘My God!’ reaction.” location 1279

Even intense global warming and ocean acidification seem inadequate to explain losses on such a staggering scale, and so additional mechanisms are still being sought. One hypothesis has it that the heating of the oceans favored bacteria that produce hydrogen sulfide, which is poisonous to most other forms of life. According to this scenario, hydrogen sulfide accumulated in the water, killing off marine creatures, then it leaked into the air, killing off most everything else. The sulfate-reducing bacteria changed the color of the oceans and the hydrogen sulfide the color of the heavens; the science writer Carl Zimmer has described the end-Permian world as a “truly grotesque place” where glassy, purple seas released poisonous bubbles that rose “to a pale green sky.” location 1428

(A recent study of pollen and animal remains on Easter Island concluded that it wasn’t humans who deforested the landscape; rather, it was the rats that came along for the ride and then bred unchecked. The native palms couldn’t produce seeds fast enough to keep up with their appetites.) location 1463

Crutzen wrote up his idea in a short essay, “Geology of Mankind,” that ran in Nature. “It seems appropriate to assign the term ‘Anthropocene’ to the present, in many ways human-dominated, geological epoch,” he observed. Among the many geologic-scale changes people have effected, Crutzen cited the following: • 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 plants 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. Most significantly, Crutzen said, people have altered the composition of the atmosphere. Owing to a combination of fossil fuel combustion and deforestation, the concentration of carbon dioxide in the air has risen by forty percent over the last two centuries, while the concentration of methane, an even more potent greenhouse gas, has more than doubled. location 1493

The best explanation anyone has come up with for the end-Permian extinction is a massive burst of vulcanism in what’s now Siberia. But even this spectacular event, which created the formation known as the Siberian Traps, probably released, on an annual basis, less carbon than our cars and factories and power plants. location 1712

The Great Barrier Reef extends, discontinuously, for more than 2,600 kilometres, and in some places it is a hundred and fifty metres thick. By the scale of reefs, the pyramids at Giza are kiddie blocks. location 1787

A paper published in Nature by the former head of the One Tree Island Research Station, Ove Hoegh-Guldberg, predicted that if current trends continue, then by around 2050 visitors to the Great Barrier Reef will arrive to find “rapidly eroding rubble banks.” location 1795

he once calculated that cutting down all the world’s forests and replacing them with grasslands would have a slight cooling effect. (Grasslands, which are lighter in color than forests, absorb less sunlight.) Other calculations of his show that to keep pace with the present rate of temperature change, plants and animals would have to migrate poleward by about ten metres a day, and that a molecule of CO2 generated by burning fossil fuels will, in the course of its lifetime in the atmosphere, trap a hundred thousand times more heat than was released in producing it. location 1825

It is estimated that at least half a million and possibly as many as nine million species spend at least part of their lives on coral reefs. location 1932

Honeybees warm themselves by contracting the muscles in their thorax. Wood storks cool off by defecating on their own legs. (In very hot weather, wood storks may excrete on their legs as often as once a minute.) location 2204

It is now generally believed that ice ages are initiated by small changes in the earth’s orbit, caused by, among other things, the gravitational tug of Jupiter and Saturn. These changes alter the distribution of sunlight across different latitudes at different times of year. When the amount of light hitting the far northern latitudes in summer approaches a minimum, snow begins to build up there. This initiates a feedback cycle that causes atmospheric carbon dioxide levels to drop. Temperatures fall, which leads more ice to build up, and so on. After a while, the orbital cycle enters a new phase, and the feedback loop begins to run in reverse. The ice starts to melt, global CO2 levels rise, and the ice melts back farther. location 2217

To use an extreme example, an island might be home to a single breeding pair of birds of species X. One year, the pair’s nest is blown out of a tree in a hurricane. The following year, all the chicks turn out to be males, and the year after that, the nest is raided by a snake. Species X is now headed toward local extinction. If the island is home to two breeding pairs, the odds that both will suffer such a string of fatal bad luck is lower, and if it’s home to twenty pairs, it’s a great deal lower. location 2499

In the nineteen-fifties and sixties, bird-watchers kept meticulous records of every pair that bred on Bardsey Island, off Wales, from common house sparrows and oystercatchers to much rarer plovers and curlews. In the nineteen-eighties, these records were analyzed by Jared Diamond, who at that time was working as an ornithologist, specializing in the birds of New Guinea. Diamond found that the odds that any particular species had gone missing from the island could be plotted along a curve whose slope declined exponentially as the number of pairs increased. Thus, he wrote, the main predictor of local extinction was “small population size.” location 2505

What distinguishes islands—and explains the phenomenon of relaxation—is that recolonization is so difficult, in many cases, effectively impossible. (While a land-bridge island may support a small remnant population of, say, tigers, if that population winks out, new tigers presumably aren’t going to paddle over.) The same holds true for any sort of habitat fragment. Depending on what surrounds the fragment, species may or may not be able to recolonize it once a population has been lost. location 2513

Army ants—there are dozens of species in the tropics—differ from most other ants in that they have no fixed home. They spend their time either on the move, hunting for insects, spiders, and the occasional small lizard, or camped out in temporary “bivouacs.” (Eciton burchellii “bivouacs” are made up of the ants themselves, arrayed around the queen in a vicious, stinging ball.) location 2546

around the turn of the century, Cryphonectria parasitica, the fungus responsible for chestnut blight, was imported to the U.S., probably from Japan. Asian chestnut trees, having coevolved with Cryphonectria parasitica, were easily able to withstand the fungus, but for the American species it proved almost a hundred percent lethal. By the nineteen-fifties, it had killed off practically every chestnut in the U.S.—some four billion trees. Several species of moths that depended on the tree disappeared along with it. location 2832

Before Europeans arrived, New England had no earthworms of its own; the region’s worms had all been wiped out by the last glaciation, and even after ten thousand years of relative warmth, North America’s native worms had yet to recolonize the area. location 2855

By transporting Asian species to North America, and North American species to Australia, and Australian species to Africa, and European species to Antarctica, we are, in effect, reassembling the world into one enormous supercontinent—what biologists sometimes refer to as the New Pangaea. location 2879

The process that turns an organism’s long strands of DNA into fragments—from a “text” into something more like confetti—starts pretty much as soon as the organism expires. A good deal of the destruction is accomplished in the first few hours after death, by enzymes inside the creature’s own body. After a while, all that remains are snippets, and after a longer while—how long seems to depend on the conditions of decomposition—these snippets, too, disintegrate. Once that happens, there’s nothing for even the most dogged paleogeneticist to work with. “Maybe in the permafrost you could go back five hundred thousand years,” Pääbo told me. “But it’s certainly on this side of a million.” Five hundred thousand years ago, the dinosaurs had been dead for about sixty-five million years, so the whole Jurassic Park fantasy is, sadly, just that. On the other hand, five hundred thousand years ago modern humans did not yet exist. location 3368

“Chimps do a lot of incredibly smart things,” Michael Tomasello, who heads the institute’s department of developmental and comparative psychology, told me. “But the main difference we’ve seen is ‘putting our heads together.’ If you were at the zoo today, you would never have seen two chimps carry something heavy together. They don’t have this kind of collaborative project.” location 3441

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.” location 3467