Reviewed 6/20/2015

The Sixth Extinction, by Elizabeth Kolbert

An Unnatural History
Elizabeth Kolbert
New York: Picador, January 2015




ISBN-13 978-1-250-06218-5
ISBN-10 1-250-06218-7 319pp. SC/BWI $16.00

The history of science is in large part a series of bad guesses about the natural world. Often these guesses are defended with vigor, even with rancor. One such battle occurred over meteor craters, which until the early twentieth century were held to be volcanic in origin.1

An even more heated and protracted battle involved catastrophism, the theory that Earth's geological history contained sudden disasters, versus uniformitarianism, the belief that change always occurred gradually over vast periods of time. It is this battle that Elizabeth Kolbert explores in her latest book. A French scientist named Cuvier first understood that fossil bones were of animals that had gone extinct. To explain this, he proposed the idea of catastrophes in prehistory.

"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," Cuvier said. "But what was this primitive earth? And what revolution was able to wipe it out?"

– Page 30

Cuvier was met with scorn. But the dispute, as with all disputes in science, was finally resolved in favor of the actual evidence. That showed that both views have merit, but neither is entirely correct: Earth has undergone long periods of gradual change punctuated by abrupt upheavals. Best known among them is the event that left what we call the Chicxulub Crater east of the Yucatan Peninsula. This was a strike 65 million years ago2 by a large asteroid; it left a crater 100 miles across and probably killed 75 percent of all species then living.3

Yet it is not the worst extinction we know about.4 Such extinction spikes are known from the fossil record, where the number of species abruptly plunges and then slowly recovers, with many new forms replacing the previous ones.

Earth's History of Diversity
Source: Concepts of Biology
Plot of extinctions

The story is one with profound implications for our present age. It tells us that rapid changes in environmental conditions can devastate whole ecosystems. Humanity has now become a geological force; we are overrunning the surface of the land, overharvesting the life of land and sea, and altering the composition of the atmosphere. The changes we are making will not be as sudden as an asteroid impact, but they are still rapid in comparison to the ability of most species to adapt. The implications for biodiversity are ominous.5

"Today, amphibians enjoy the dubious distinction of being the world's most endangered class of animals; it's been calculated that the group's extinction rate could be as much as forty-five thousand times higher than the background rate. But extinction rates among many other groups are approaching amphibian levels. It is estimated that one-third of all reef-building corals, a third of all freshwater mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles, and a sixth of all birds are headed toward oblivion. The losses are occurring all over: in the South Pacific and in the North Atlantic, in the Arctic and the Sahel, in lakes and on islands, on mountaintops and in valleys. If you know how to look, you can probably find signs of the current extinction event in your own backyard."

– Pages 17-18

As in her previous work, Elizabeth Kolbert provides here a well-written and compelling account of what has been learned about extinction events of the past, and what researchers — in the mountains of Peru, on the Great Barrier Reef, down inside caves in the Adirondacks — are learning about our ongoing extinction event.

Along with an informative text, she gives us black-and-white pictures from her travels. There are extensive endnotes, a bibliography with 215 entries, and a good index. My rating: Top marks and a definite keeper.

1 A thorough discussion of this meteor controversy is found in Rain of Iron and Ice.
2 The latest research puts it at 66 million years.
3 This impact is now generally accepted, but controversy remains about its timing relative to the massive extinction event.
4 That was the Permian-Triassic Extinction Event and it occurred much earlier, about 250 million years ago. It wiped out 96 percent of marine species and up to 70 percent of land vertebrates.
5 Here is a decent tutorial on mass extinction in general.
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