The Human Journey

Turning Points in the Development of Contemporary Society

The Sixth Extinction: An Unnatural History

Vintage, 2014

Available for purchase here

About the Author: Elizabeth Kolbert has been a staff writer for the New Yorker since 1999. Prior to that she was a reporter for the New York Times. She received the American Association for the Advancement of Science's magazine writing award for the New Yorker series on which this book is based. The Sixth Extinction: An Unnatural History draws on the work of scores of researchers in half a dozen disciplines–geologists who study deep ocean cores, botanists who follow the tree line as it climbs up the Andes, and marine biologists who dive off the Great Barrier Reef. The author accompanies many of these researchers into the field, and introduces the reader to a dozen species–some already gone, others facing extinction.
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At times long stretches of the sea off the west coast of Africa turn a milky white, then an iridescent shade of jade green. Any marine creature that can do so crawls onto land to escape the water which has become laced with hydrosulfuric acid. Hydrogen sulfide gas – heavier than air, foul, and poisonous – bubbles out of the sea. The air for miles around fills with a noxious odor. Around 225 million years ago, during the largest extinction event in Earth’s history, this was the condition of all of the world’s oceans.

Atlantic Ocean and Sulfied Gas
A recent aerial photograph of the iridescent ocean off the west coast of Africa.

This deadly chemical change, caused by the depletion of most of the oxygen in the sea water, lead to the “end-Permian” mass extinction event, sometimes referred to as “The Great Dying.” Oceans absorbed huge quantities of carbon dioxide from the atmosphere triggering a world-wide bloom of anaerobic, sulfur-producing organisms. It was more cataclysmic even than the later extinction of the dinosaurs.

The Great Dying is one of five mass extinctions since the “Cambrian Explosion” of about 500 million years ago. Extinctions great and small happened before the Cambrian period, but conditions on Earth then were so different that they’re not relevant here. By contrast, the five mass extinctions since then – when at least 75% of all species on land and in the seas disappeared – are the subject of active research.

A far-reaching extinction event is in progress right now. The Sixth Extinction, by Elizabeth Kolbert, places the present extinction in line with the previous five. It’s an account of the author’s personal witness to this loss, of the studies undertaken to understand its causes, and of some of the attempts being made to save some species.

While saving a species in captivity prevents its annihilation, it takes an extraordinary effort to return it to the wild. California condors have been brought back from the brink, but before being released they need to be inoculated against bird flu, trained not to fly into power lines, and are then often treated for lead poisoning from swallowed gun pellets. Similar efforts will not be possible for the more than 1,200 other bird species in danger of extinction. For some animals, “extinct in the wild” means extinct, period.

A California condor over the Grand Canyon. Note the tracking tags on its wings.

Darwin believed that extinction is a gradual process. In contrast, Kolbert recounts how the early French paleontologist Georges Cuvier understood that extinction could be sudden and catastrophic. Cuvier’s insight into the nature of the enormous fossils that were turning up led him to theorize that the world was once dominated by giant reptiles, and that these creatures were destroyed by a flood of biblical proportions. There was a catastrophe: 66 million years ago, when the dinosaurs died within a few years of the impact of an asteroid the size of Mt. Everest, in Chicxulub, near the Yucatan peninsula.

But the asteroid didn’t wipe out the dinosaurs – they lived on the other side of the world to the Yucatan. The lethal climate following its impact killed them all, along with almost anything else that lived. Asteroid impacts were not the cause of the other mass extinctions but climate change appears always to have been involved. Although not instantaneous like a collision with an asteroid, the Earth’s climate changed in all the other mass extinctions many times faster than any species’ ability to adapt.

Mass extinctions do create opportunities for new species to develop, but the recovery of biodiversity takes millions of years. As a gruesome analogy, think of a terrorist bomb planted at the finish line of a marathon. Depending on the timing of the explosion, the runners bringing up the rear might be unharmed, and the best runners wiped out.

The present extinction has these same random and rapid properties, but it’s unique in that it’s caused entirely by the actions of a single species – humans. This “Holocene Extinction” is named for the present geological epoch that began after the last Ice Age ended around 12,000 years ago. The impact that Homo sapiens’ proliferation has had on the natural world has been rapid and profound. Of the planet’s 50 million square miles of ice-free land, ½ has been converted to agriculture, industry, and housing. We have altered the composition of the atmosphere, and consequently altered the Earth’s climate in ways that are yet to be determined. 

The Most Deadly Predator

The hallmark of the Holocene extinction is the vanishing of flourishing species: the passenger pigeon, the American bison, and some species of whale, to name but a few. Kolbert describes the extinction of the Great Auk, which was once so numerous that they were even burned for firewood. At one time this bird was hunted for its feathers used to adorn clothing, and, it turns out many were not killed outright, but plucked and left to die of exposure! Some species today are killed for similarly obtuse reasons: the rhinoceros has been thriving for 15 million years, but will certainly be poached to extinction in the wild because its horn is erroneously believed to be an aphrodisiac.

The Last Great Auk, stuffed
One of the two last Great Auks. Along with its mate, it was killed in its nest to be sold for exhibition.

Archaic humans grew in numbers and expanded their territory in the same manner as the Earth’s other large species. Neanderthals lived in Europe for more than 100,000 years and had no more impact on the landscape than any big mammal. But modern humans and megafauna have never been in balance together; Neanderthals may well have been the first species eliminated by Homo sapiens. Our newly evolved cognitive abilities tipped the scales of competition fatally in our favor, both for prey species and for other humans.

Megafauna, such as the wooly mammoth and saber tooth tiger, disappeared in every part of the world soon after Homo sapiens arrived.  There is some evidence of wholesale slaughter, but wanton overkills were not necessarily what finished the mammoths. Studies have found that a small band of hunters taking just one adult megabeast per year will clean a continent of that species in a couple of thousand years. Early humans had no way of knowing this: the megafauna were too large to be prey; their low birth rates were a liability once human hunters arrived.

You don’t know what you’ve got til it’s gone

Predation is the most dire threat facing many large animals, but it is not the most significant factor in the present extinction. Habitat destruction is, and above all in tropical rainforests. The rate at which these forest are being lost, primarily to logging and clearing for agriculture, is difficult to determine and varies widely by location – only about 5% of the original forests remain in south Asian countries, for example. The number of species rainforests support is even less known, though it certainly dwarfs other habitats, and it’s thought that millions of plants and animals, mainly insects, are yet to be discovered.

Rainforests benefit humans most directly as an irreplaceable source of medicine. Aspirin, quinine and many other drugs were first discovered in tropical plants. Most chemotherapy medication is made from plants that only exist in tropical rainforests. Many drugs studied in the search for a cancer cure are derived from tropical plants. A drug discovered in a type of periwinkle found only in Madagascar increased survival rates of childhood leukemia from 20% to 80%. This plant is now extinct. Species in the rainforest habitat are becoming extinct faster than they can be discovered.

Current Global Forest Distribution
Current Global Forest Distribution in 2009 (courtesy of World Resources Institute).

The Brazilian government in 1979 set aside tracts of land in rainforests that were slated to be cleared for agriculture. This ongoing program has provided naturalists with a wealth of information, including how species respond to shrinking habitats. The pattern of habitat loss is important to this understanding – roads, power lines and the like create an enclosure effect even in preserves. Some species –  plant as well as animal – move easily across gaps in the landscape, while others have evolved to remain bounded in their immediate habitat.

All Earth’s species descend from those that survived numerous ice ages and the rapid warming periods that followed. They survived by migrating to new areas similar to the climate and ecology in which they evolved. Today some plants and animals are already relocating in response to climate change, but for many species the globe may be warming too fast – it’s happening faster than any previously known warming – and the habitat may already be too fragmented to accommodate similar migrations. As Kolbert puts it, “One of the defining features of the Anthropocene is that the world is changing in ways that compel species to move, (but) ... prevents them from doing so.”

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