The Sixth Extinction: An Unnatural History

Read on Amazon

3 Sentence Summary

There have been five great extinctions in the earth’s history, and now we are witnessing a sixth. The rate of biodiversity is shrinking today on an unprecedented scale due to human activity. This award-winning book is a straightforward, entertaining, and sobering account of what may very well be mankind’s lasting legacy.

5 Key Takeaways

1. We are in the middle of a sixth mass extinction caused by human activity.
2. Humans have been over-hunting species to extinction starting with the megafauna of the ice age.
3. Human migration and world trade have spread non-native species at an accelerated pace, leading to decreasing global diversity.
4. Industrialization and deforestation are contributing to climate change at an unprecedented rate. Many species will be unable to adapt quickly enough.
5. Our future survival depends upon responsible stewardship of our natural resources and protecting bio-diversity.

The Sixth Extinction Summary

1) The Sixth Extinction

The five major extinction events on earth:

1. Ordovician-silurian Extinction: 450 million years ago. When most living things were confined to water.
2. Devonian Extinction: 365 million years ago.
3. Permian-triassic Extinction: 250 million years ago. Most devastating.
4. Triassic-jurassic Extinction: 210 million years ago.
5. Cretaceous-tertiary Extinction: 65 million years ago. Wiped out the dinosaurs.

Amphibians

• Amphibians have been around longer than dinosaurs.
• Amphibians is from the Greek meaning “double life.”
• Amphibians are found on every continent except Antarctica.
• Frogs have been disappearing from habitats all around the world at an alarming rate.
• A new species of chytrid fungi—Batrachochytrium dendrobatidis (Bd)—kills frogs by preventing them to absorb essential electrolytes through their skin. It has spread around the globe seemingly impossible to stop.
• Amphibians today are the world’s most endangered class of animals.
• The amphibians extinction rate is currently estimated at 45,000X higher than the background rate.

Extinction Rate

• In normal times, we lose species to extinction at a “normal” rate. This is called the background extinction rate.
• Mass extinctions are different. These are events that eliminate a significant proportion of the world’s biota in a geologically insignificant amount of time.
• 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.
• Intercontinental reshuffling of plants and animals due to human activity is unprecedented in the 3.5 billion year history of life.

2) The Mastodon’s Molars

• Extinction was not an understood phenomena until it finally emerged as a concept in revolutionary France.
• With great trepidation, France’s leading naturalist, Georges Buffon, was the first to propose that the giant unknown tooth came from a creature that seemed to have disappeared (the only land animal ever to have done so.)
• Thomas Jefferson partially agreed, but believed the animal was still alive and roaming the unexplored American territories.

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.

Thomas Jefferson

The Father of Paleontology

• The French naturalist Georges Cuvier was the first to fully embrace the idea of extinction. He started paleontology—the branch of science concerned with fossil animals and plants.
• In the early years of the nineteenth century, fossil collecting became very popular among the English upper classes.
• Cuvier hypothesized that the globe was wracked periodically by cataclysms which resulted in mass extinctions.

3) The Original Penguin

• The great auk once ranged from Norway over to Newfoundland and probably from Italy to Florida, and its population probably numbered in the millions.
• The flightless bird was easy prey for Iceland’s medieval inhabitants.
• A 1622 account by a captain named Richard Whitbourne describes great auks being driven onto boats by the hundreds.
• The birds were used as fish bait, plucked feathers were used for stuffing mattresses, and their oily bodies used as fuel for the fire.
• They were hunted to extinction in North America by the year 1800. The last known surviving pair killed on Eldey Island in 1844.

Darwin

• Darwin recognized that just as the features of the inorganic world—deltas, river valleys, mountain chains—were brought into being by gradual change, the organic world similarly was subject to constant flux.

It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers.

Charles darwin

• According to Darwin, extinction and evolution were two sides of the same coin.

The theory of natural selection is grounded on the belief that each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of less favored forms almost inevitably follows.

Charles Darwin

• Darwin believed that extinction occurred exclusively by natural selection, and without world-altering catastrophes.
• This hypothesis concluded that extinction and evolution must happen at roughly the same rate. But this was simply not true.

4) The Luck of the Ammonites

• Foraminifera, or “forams” for short, are the tiny marine creatures that create little calcite shells, or tests, which drift down to the ocean floor once the animal inside has died.
• Forams tend to be widely distributed and abundantly preserved, and this makes them extremely useful as index fossils. Their presence helps geologists determine the rock’s age.

• Walter Alvarez observed a sudden disappearance of many species of forams in the layer of limestone that coincided around the time when the last of the dinosaurs were known to have died off.
• Late-Cretaceous clay sediment samples from around the world all have astronomical levels of iridium, which is an element very rare on earth but common on meteorites.

On an otherwise ordinary day sixty-five million years ago, an asteroid six miles wide collided with the earth. Exploding on contact, it released energy on the order of a hundred million megatons of TNT, or more than a million of the most powerful H-bombs ever tested. Debris, including iridium from the pulverized asteroid, spread around the glob. Day turned to night, and temperatures plunged. A mass extinction ensued.

• A hundred-mile-wide crater was discovered beneath the Yucatan Peninsula. Now named the Chicxulub crater.
• Cores drilled from the crater site were found to contain a layer of glass—rock that had melted, then rapidly cooled—right at the Cretaceous-Tertiary (K-T) boundary.

Ammonites

• Ammonites construct spiral shells that are divided into multiple chambers.
• The animals themselves occupied only the last and largest chamber; the rest were filled with air.
• Most ammonites could fit in a human hand; some grew to be the size of kiddie pools.
• Ammonites were wiped out by the meteor impact even though their cousin nautiluses survived. This had everything to do with luck and nothing to do with being better suited to survive a cataclysmic event.

The Impact

• The main cause of the mass extinction was not the impact itself, or even the immediate aftermath. The truly catastrophic effect of the asteroid was the dust.
• When the asteroid slammed into the Yucatan Peninsula, it was moving at forty-five thousand miles per hour.
• A vast cloud of searing vapor and debris raced over the North American continent, incinerating everything in its path.
• The asteroid blasted into the air more than fifty times its own mass in pulverized rock.
• As the ejecta fell back through the atmosphere, the particles incandesced, lighting the sky everywhere at once and generating enough heat to, in effect, broil the surface of the planet.
• The dust, rich with sulfur, blocked the sunlight and depressed global temperatures for years.

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?

5) Welcome to the Anthropocene

• When presented with disruptive information, our first impulse is to force it into a familiar framework.
• The history of the earth is long periods of boredom interrupted occasionally by moments of extreme panic.
• Asteroids crashing into the earth are not thought to be the cause of every major extinction event. The current theory for the cause of the Ordovician extinction is glaciation.
• The end-Permian extinction also seems to have been triggered by a change in the climate. Massive amounts of carbon were released into the air and temperatures soared.
• The Anthropocene will be marked by a unique “biostratigraphical signal,” a product of the current extinction event on the one hand and of the human propensity for redistributing life on the other.

Geological-Scale Changes Caused by Humans

• 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.
• People have altered the composition of the atmosphere.

6) The Sea Around Us

• As we pump CO₂ into the air, it gets absorbed into the sea.
• CO₂ dissolves in water and turns it acidic.
• The oceanic ecosystem starts to crash at a mean pH of 7.8, which is predicted to happen by 2100.
• Ocean acidification played a role in at least two of the Big Five extinctions (the end-Permian and the end-Triassic) and quite possibly it was a major factor in a third (the end-Cretaceous).
• By burning through coal and oil deposits, humans are putting carbon back into the air that has been sequestered for tens—in most cases hundreds—of millions of years. In the process, we are running geological history not only in reverse but at warp speed.

If current trends continue, CO₂ concentrations will top five hundred parts per million, roughly double the levels they were in preindustrial days, by 2050. It is expected that such an increase will produce an eventual average global temperature rise of between three and a half and seven degrees Fahrenheit, and this will, in turn, trigger a variety of world-altering events, including the disappearance of most remaining glaciers, the inundation of low-lying islands and coastal cities, and the melting of the Arctic ice cap.

7) Dropping Acid

• Coral reefs are organic paradoxes—obdurate, ship-destroying ramparts constructed by tiny gelatinous creatures. They are part animal, part vegetable, and part mineral, at once teeming with life and, at the same time, mostly dead.
• The Great Barrier Reef extends, discontinuously, for more than fifteen hundred miles, and in some places, it is five hundred feet thick.
• It is likely that reefs will be the first major ecosystem in the modern era to become ecologically extinct.
• Coral reefs are like the rainforest of the ocean. 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.
• Zooxanthellae are microscopic plants that live in coral polyps and give them their color. When water temp rises, the zooxanthellae produce dangerous concentrations of oxygen radicals, and the polyps respond by expelling them. This is what causes coral bleaching.

• The corals of the Great Barrier Reef observe a lunar calendar. Once a year, after a full moon at the start of the austral summer, they engage in what’s known as mass spawning—a kind of synchronized group sex.

8) The Forest and the Trees

• Global warming won’t just adversely affect penguins and polar bears. It may have an even greater impact in the tropics.
• As a general rule, the variety of life is most impoverished and the poles and richest at low latitudes.
• Plants can migrate to cope with temperature changes. Some better than others.
• In its magnitude, the temperature change projected for the coming century is roughly the same as the temperature swings of the ice ages. But at a much faster rate.
• Warming today is taking place at least 10X faster.

9) Islands on Dry Land

• Islands tend to be species-poor.
• Small populations are statistically at risk for dying off.
• Recolonization of new species on islands is almost impossible.
• This phenomenon on islands is also true of habitat fragmentation (land islands). Species may or may not be able to recolonize once a population has been lost.
• Tropics have high species diversity but also low population density, and that’s a recipe for speciation—isolation by distance. These isolated, small populations have an increased vulnerability to extinction.
• There’s a dark synergy between fragmentation and global warming, just as there is between global warming and ocean acidification. Species trapped by fragmented land islands can’t migrate to escape rising temperatures.
• Human activity has created an obstacle course for the dispersal of biodiversity.

10) The New Pangaea

• The body temperature of a hibernating bat drops by fifty or sixty degrees Fahrenheit, often to right around freezing.
• The process of remixing the world’s flora and fauna, which began slowly, along the routes of early human migration, has, in recent decades, accelerated to the point where in some parts of the world, non-native plants now outnumber native ones.
• When a new species is introduced, it either goes unnoticed and dies off, or it survives and thrives.
• Roughly 5-15% of new invasive species will succeed in establishing themselves. 1% of those will cause immense ecological change.
• The pet industry and global trade accounts for many species swaps.
• Local diversity has been increasing, while global diversity is decreasing.

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.

11) The Rhino Gets an Ultrasound

• There used to be many more large mammals than there are now.
• Humans are most likely the cause, beginning all the way back to the middle of the last ice age. We’ve been an over-killer from the start.
• The megafauna extinctions occurred in pulses around the globe that coincide with the sequence of human settlement. The chronology of extinction matches the chronology of human migration.
• Large mammals have long gestation periods. They cannot reproduce quickly.
• Their large size protected them from other natural enemies. But not against humans who have no constraint on what we can eat.
• Even a very small initial population of humans—a hundred or so individuals—could, over the course of a millennium or two, multiply sufficiently to account for pretty much all of the extinctions in the record.
• For the people involved in it, the decline of the megafauna would have been so slow as to be imperceptible.
• Though it might be nice to imagine there once was a time when man lived in harmony with nature, it’s not clear that he ever really did.

12) The Madness Gene

• The Neanderthals lived in Europe for at least a hundred thousand years. For the most part, this was a time of cold.
• Modern humans arrived in Europe around forty thousand years ago, and again and again, the archaeological record shows, as soon as they made their way to a region where Neanderthals were living, the Neanderthals in that region disappeared.

• 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.
• Neanderthals were our closest relatives, but they were not human.
• The Neanderthals lived in Europe for more than a hundred thousand years and during that period they had no more impact on their surroundings than any other large vertebrate.
• The Neanderthals dispersed across Europe and Asia, stopping when they reached water or some other significant obstacle. But during that same time period, modern humans were venturing out on the ocean where you don’t see land. Part of their ability to do so was technology; you have to have ships to do it. However, there must also be some crazy desire and curiosity to explore the unknown.

With the capacity to represent the world in signs and symbols comes the capacity to change it, which, as it happens, is also the capacity to destroy it. A tiny set of genetic variations divides us from the Neanderthals, but that has made all the difference.

13) The Thing with Feathers

• 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 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.
• Humans have freed themselves from the constraints of evolution, but we still remain dependent on the earth’s biological and geochemical systems.

In pushing other species to extinction, humanity is busy sawing off the limb on which it perches.

Paul Ehrlich

More Books Like The Sixth Extinction

If you enjoyed The Sixth Extinction, then check out these similar book summaries:

• Why We Sleep: Unlocking The Power of Sleep and Dreams
• Sapiens: A Brief History of Humankind
• Food Rules: An Eater’s Manual

Or, browse all of my book summaries.