The Sixth Extinction Book Summary, by Elizabeth Kolbert

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The Sixth Extinction: An Unnatural History (2014) is a nonfiction book about the development, impact and extinction of various species throughout the history of the world. The author, an American journalist and Pulitzer Prize winner Elizabeth Kolbert, argues that we are currently in the process of experiencing a “Sixth Extinction” caused almost entirely by human beings. As proof of this phenomenon, Kolbert travels to the farthest reaches of the globe to research and examine examples of past mass extinction events and connect them to one she believes is now occurring. Further evidence for her theory comes from profiles on different species whose populations are dwindling or have already become extinct because humans have interfered with their habitats.

In this book, Elizabeth Kolbert presents various cases of animal species that have gone extinct because of human activities. She starts by visiting Panama to study the golden frog and its effect on the environment. The humans who introduced a fungus to their ecosystem caused it to die out. In Chapter Two, she discusses how hunting an animal for its meat and feathers is not enough; humans also need to hunt during mating season in order to completely wipe out a species like the American mastodon.

Chapter Three focuses on great auks, which were entirely wiped out by man’s actions alone; they had no chance of surviving when we hunted them during mating season.

Chapter 4 covers the ammonites, who survived a meteor impact during the end of the Cretaceous Period. It compares that event with what’s happening today due to deforestation and industrialization. Chapter 5 looks at how graptolites died out after changes in ocean chemistry as well as rising sea levels and temperatures. The author also shows evidence that similar ecological effects are occurring today, perhaps heralding an upcoming Sixth Extinction Event. In Chapter 6, Kolbert examines how the Industrial Revolution affected our planet and led to an alarming increase in carbon dioxide in our atmosphere.

In chapter 7, Kolbert writes about the coral reefs and their importance to the ocean ecosystem. She says that within 30 years, they may go extinct because of ocean acidification. This would mean all organisms dependent on them will die off as well. In chapter 8, she discusses global warming and how it affects the tropics; these areas have a diverse range of species living in them that are at risk for extinction due to shifting temperatures around the world. Plants are especially vulnerable because they can’t move or adapt as easily as animals can to changing environments. Chapter 9 focuses on mini-ecosystems like rainforests which contain many interdependent species that could lead to a domino effect that causes an entire ecosystem’s extinction if one goes extinct.

Chapter Ten goes back to the past and examines how the continents drifted apart, and how human activity is causing a reverse reaction today. Kolbert uses the Chestnut blight to explain how new Pangea will cause extinction of those species that cannot defend against or adapt to new species or pathogens. In Chapter Eleven, she examines attempts to save nearly-extinct species such as the Sumatran rhino. On its last legs, the species was nearly destroyed by deforestation and poor captive breeding programs; now it’s being saved by humans who caused these dire straits in the first place.

Chapter 12 looks at climate change from an economic perspective—the cost of doing nothing versus investing in sustainable energy sources for future generations. The author also discusses geoengineering (intervention into Earth’s environment) as a potential solution if we continue on our current path of fossil fuel consumption without reducing emissions significantly enough over time. She believes that geoengineering should be used only as a last resort because it could have disastrous consequences for Earth’s ecosystems and biodiversity.

In Chapter 12, Kolbert examines the Neanderthal. The human species introduced a new organism that upset the entire ecosystem. This resulted in extinction of other organisms. She expresses hope for humanity in Chapter 13. Scientists are working hard to preserve endangered species and she hopes that we will continue to learn how we change the world and make good decisions about our own future as well as preservation of other species on Earth.

Full Summary of The Sixth Extinction

Overall Summary

Elizabeth Kolbert learned about the sixth extinction of species and how it’s caused by humans in her book The Sixth Extinction. She also visited Panama to study the golden frog, which inspired her to learn more about extinction as well as its place in science history.

Writer Elizabeth Kolbert learned that, for most of the history of science, humans didn’t understand that some animals went extinct. It was Georges Cuvier, the influential naturalist who first proposed that some species are no longer alive. Scientists later discovered fossils of large mammals further legitimating his theory. Even after Charles Darwin published On The Origin Of Species in 1859 scientists didn’t fully grasp how human beings can influence the environment to cause extinction. It wasn’t until Alfred Newton tried to preserve a bird population in Iceland and realized humans could play a role in preserving species at risk of going extinct much earlier than other scientists did so.

A major milestone in understanding extinction was the paper written by Walter Alvarez and Luis Alvarez. They argued that a large asteroid killed off all of the dinosaurs, rather than dying out gradually over time. The theory became accepted because it reinforced another theory—that many species go extinct at almost the same time, instead of dying out slowly over thousands of years. One reason for this is that traits that were once useful may not be useful anymore after some event occurs, like an asteroid striking Earth. For example, ammonites—a prehistoric nautilus-like creature—were abundant because their eggs could travel across oceans easily on ocean currents. But after an asteroid struck Earth, their eggs couldn’t survive so they went extinct as well.

Humans have been altering the Earth for thousands of years, and now we are living in a time period called the Anthropocene. This is an era that will be defined by how humans change their environment, which has caused many species to die out or become endangered. Humans have altered the oceans by increasing ocean acidity levels and temperatures, which means that sea creatures will need to adapt to survive or they’ll go extinct.

Kolbert travels to the Great Barrier Reef and learns from scientists that higher temperatures and acidity have dramatically reduced coral reef diversity. Warmer waters will cause algae and plankton populations to increase, which in turn will reduce available nutrition for larger animals. She also visits tropical rainforests of South America where she sees how much biodiversity there is. In recent years, scientists have found evidence that different species are forced to migrate so they can find a suitable environment similar to the one they’re used to. At the same time, deforestation by farmers has decreased the amount of land available for wild creatures while some rainforest species will not survive environmental changes facing our planet. Furthermore, extinction of even a few rainforest species would greatly affect overall biodiversity since these species depend on each other very closely.

Another factor that contributes to the sixth extinction is human travel. Humans have been traveling around the world since prehistoric times, and species have been confined to different ecosystems. However, modern transportation has led to organisms from one ecosystem being introduced into another environment. This upsets their equilibrium as they are no longer isolated from other environments (for example, when non-native species are brought into a new area). The introduction of an invasive species tends to reduce biodiversity in the long run.

Scientists have been trying to figure out ways to help endangered animals, such as the rhinoceros, survive. Some scientists believe that in order for large mammals like the rhino and other creatures to survive they need more people working on their reproduction. However, it’s only because humans hunted them so much that these animals are at risk of extinction today. Humans have been hunting large mammals for thousands of years and many prehistoric creatures went extinct because early human beings killed them off centuries ago before the Industrial Revolution even began.

In this passage, Kolbert talks about the Neanderthal—an early humanoid species that went extinct thousands of years ago. Paleontologists believe that they were technologically advanced for their time and had larger brains than modern humans. Scientists also discovered that some human beings have Neanderthal DNA because there was interbreeding between them. Perhaps one of the reasons why we survived extinction is because we are more curious than other species and explore our world with a sense of purpose.

Kolbert acknowledges that her book paints a bleak picture of the human race. However, it’s important to keep in mind that humans have the power to preserve endangered species and nurture them so they can survive. Humans also need to realize that their actions will affect life on Earth for many years into the future, even if they stop now.


The story begins with a new species that can name things. However, there aren’t many of them and most live in eastern Africa. The species isn’t strong or fast but they’re very resourceful and capable of exploring different environments, crossing oceans and deserts, as well as hunting animals. They travel to Europe where they meet other humans who look like them but have different features such as red hair.

As time goes on, the species continues to kill off different animals. Soon, it can be found in every corner of the Earth and begins to change its environment. This forces other species to migrate away from their homes or die out completely.

Humans are the only species that have changed the planet to such a great extent. However, there were other times when extinctions occurred and they were caused by different factors. We need to be aware of this because it will help us appreciate how special our time is now. In her book, Elizabeth Kolbert talks about thirteen species including mastodons and dinosaurs and discusses why they became extinct in order for readers to understand how important it is for humans not to repeat history.

Chapter 1

The Panamanian town of El Valle de Antón is located in a volcanic crater. Until very recently, golden frogs were extremely common in the area and known to be poisonous. When they began disappearing, few people saw it as a crisis.

The author of this text found out about the golden frogs in a children’s nature magazine. The article described how biologists were trying to save them by building a facility; meanwhile, they captured some golden frogs and kept them in an area where they could be protected. At around the same time, Kolbert came across an article that said there was another mass extinction happening right now—the sixth one ever recorded. This would have devastating consequences for amphibians and many other species on Earth. There had been five previous mass extinctions throughout history, including the famous Cretaceous period when dinosaurs died out.

In El Valle, Kolbert visits the El Valle Amphibian Conservation Center (EVACC) that’s focused on saving golden frogs from extinction. The center is run by Edgardo Griffith who claims that we’re losing frog species before people even know they exist. This claim is supported by the fringe-limbed tree frog which was discovered in 2005 and went extinct a few years later.

Frogs are particularly interesting because they’re very resilient animals that can survive in many different environments. Frogs have been around for a long time, since the Earth was one land mass called Pangaea. There are over 7,000 species of frogs today and they live everywhere from the Arctic Circle to the Mojave Desert.

Why are frogs going extinct in the 21st century? One might think that it’s because of human activity, but the fact is that even frogs living in areas with no humans around have also started to die off. Scientists discovered a fungus called Bd, which was killing off most of the golden frogs in El Valle. The same fungus has spread to Colombia and Australia as well, killing many species of frogs there.

There is a difference between mass extinction and background extinction. Background extinction happens over time, but during a mass extinction many species die in an extremely short period of time. One could say that life on Earth consists of long periods without much change (background extinction), with occasional panics (mass extinctions).

Amphibians are the most endangered animals in today’s times. They’re going extinct at a rate 45,000 times higher than normal. However, other classes of life like mollusks and birds are also going extinct at about the same rate.

In the next section, Kolbert talks about why Bd has spread so quickly. Some believe that humans unintentionally spread it around the world in the 1960s by using it as a pregnancy test ingredient. Others think that North American bullfrogs were introduced to Asia and Africa, which caused Bd to spread. In both cases, however, human travel is responsible for this unprecedented reshuffling of life on Earth.

One day, Kolbert accompanies Griffith to the El Valle area to look for frogs. They drive through rainforest and wait until nightfall so they can see more frogs. After a while, Griffith spots a San Jose Cochran frog that he puts in gloves and swabs to test if it has chytrid fungus (Bd). He continues by swabbing many other species of frog. Two of them are put into glass tanks at EVACC: a blue-bellied poison frog and an unidentified pale salamander.

Chapter 2

Extinction is a concept that children learn about early on. They play with toy dinosaurs and come to the realization that they’re long extinct. This may seem strange, but it’s actually more advanced than what scientists knew thousands of years ago. Aristotle wrote a treatise on animals without considering extinction as an option, while Carl Linnaeus didn’t address the possibility in his animal taxonomy studies either. Even today some people believe that no animals have ever gone extinct or that those who did drowned in Noah’s flood (as described in Genesis).

Scientists first proposed the concept of extinction in the late 18th century. A naturalist named Georges Cuvier studied fossils and concluded that all such creatures must have died out long ago. His ideas were criticized at the time but are now praised for being ahead of their time.

European explorers probably discovered mastodon teeth in the early 18th century. Then, in 1739, Charles le Moyne, the Baron de Longueuil, discovered mastodon bones while exploring present-day Ohio. His men carried the bones back to New Orleans where they were shipped back to King Louis XV in France. Louis kept the mastodon bones in his museum for most of the 18th century. Scientists argued about what kind of animal it belonged to and whether or not it was a new species altogether. Even Thomas Jefferson wrote an essay on this subject because he wanted to find out if there was such an animal living somewhere on American soil.

Cuvier lived in Paris at the end of the 18th century. He worked as a lecturer for the Natural History Museum and gave a lecture on April 4, 1796. In his lecture, he discussed mastodon fossils from Ohio and Russia that were similar to each other but different from elephants. Cuvier concluded that they belonged to two new animal species—lost creatures—that had died out over time. He based this conclusion on his own research into those remains as well as animal fossils found in Argentina. Based on these findings, Cuvier concluded that there must be many more lost species than previously thought; however, he did not understand how they could have disappeared over time.

Kolbert meets with Pascal Tassy, the current director of the Paris Museum of Natural History. Tassy shows Kolbert a display case full of elephant, mammoth and mastodon remains, including the mastodon teeth that Longueuil found in Ohio. Mastodon teeth are huge and brownish but they have the same basic structure as human teeth; dentin surrounded by brittle enamel. Tassy also shows Kolbert a fossilized sea monster known as “the Maastricht animal.”

Cuvier’s lectures popularized the theory of extinction. To prove that extinct species existed, he needed to find more fossils. He investigated quarries around Paris and eventually identified twenty-three new species as being extinct. Cuvier traveled all over Europe with his fossils, exhibiting them in a showman-like manner. His most famous discovery was the pterodactyl (a flying reptile), which he correctly identified as an extinct species.

The theory of extinction is a popular topic in the early United States. It can be attributed to Thomas Jefferson, who was influential in his time and had many notable fossils discovered on American soil. Charles Willson Peale established a natural history museum where he displayed many notable fossils from North America, including an elephant-like mammal called mastodon that was named “mammoth” because of its size. The popularity of the so-called mammoth (mastodon) led to restaurants and taverns with names similar to it. Later, scientists discovered another extinct animal like mammoths but named them after their scientific name: mammoth.

Cuvier was a hero to scientists and aristocrats. He believed that species go extinct, and although he died before he could prove it, his ideas were proven right. Fossil hunters discovered the remains of huge reptiles in England and France, which helped him come up with his theory about extinction. The deeper fossils are buried underground, the older they are—they belong to animals that have gone extinct.

Cuvier, a famous French naturalist of the 18th century, believed that animals were perfectly suited for their environment and diet. He thought this was because they did not change over time or evolve. Cuvier’s rival in the same museum was Jean-Baptise Lamarck who argued that animals could slowly adapt to their environment by changing their bodies over time. For example, he believed giraffes’ necks became longer as they reached higher into trees for food. Cuvier disagreed with Lamarck’s theories since he saw no evidence of evolution in ancient Egyptian cats even though thousands of years had passed.

Cuvier had concluded that some species would die out over time. However, he still needed to explain why this was happening. He initially thought that one catastrophic event in the distant past caused all of these extinctions, but later changed his mind and argued for multiple catastrophic events instead. In 1812, Cuvier wrote an influential essay on the possibility of ancient cataclysms. Although Cuvier cited multiple religious texts in his essay, the mostly Anglican staff at Oxford and Cambridge ensured that it favored a Christian interpretation when translated into English. This way, people could use Cuvier’s research as “proof” for Noah’s flood.

Many of Cuvier’s ideas have been disproven, but his basic idea that species go extinct over time has led to further research. He pointed out the American mastodon went extinct about 13,000 years ago; however, he thought this was due to a flood or other natural disaster. It probably went extinct because human beings hunted it to extinction instead.

Chapter 3

In 1832, William Whewell coined a word: catastrophist. This word has taken on different meanings since then, but in the beginning it meant that someone who believed that sudden and global events caused large numbers of species to go extinct. The opposite is uniformitarianism, which means gradual change over time.

One geologist named Charles Lyell studied the rocks of different regions and found that there was a slow, gradual process of erosion in the planet’s geological structure. He also found no evidence for mass extinctions. His ideas were popularized through his lectures and writings.

Charles Lyell was an influential person in the scientific community. He had a major impact on Charles Darwin, who was also an influential scientist. Lyell inspired Darwin to think about his theories of evolution and the Galápagos Islands.

Although Lyell believed in gradualism, he didn’t believe that evolution occurred. Darwin disagreed with him and stated that if there were no extinctions, then there would be no new species either. He argued that life forms on Earth had to compete for limited resources of food and shelter. This was the case because some species are better at finding food than others, or have characteristics which make them more likely to survive in their environment than other species do—and this is why they become dominant over time. Although it takes a long time for new species to appear (thousands of years), we don’t see any new ones being born today because they take so long to develop—this is what Cuvier’s theory about fossilized cats disproved; his findings suggested that animals could not change into different types of animals over time because they never lived long enough to evolve from one type into another.

Writer Elizabeth Kolbert visits the Icelandic Museum of Natural History to learn about an extinct bird that was once a prominent figure in North America, Europe and even Iceland. The great auk, which resembles a penguin, is now extinct because it was hunted by humans for food and fuel. It could swim well but couldn’t fly; its wings were too small. The great auks lived in places as different as Norway, Italy and Florida (Newfoundland used to be full of these birds). They were excellent swimmers who spent most of their life in water.

The final home of the great auk was probably the island of Eldey in Iceland. There are lots of gannets there now, which look like big white seagulls. The birds were first discovered by fishermen who went to the island and saw them, but they didn’t know what they were at that point. In 1844, three fishermen rowed out to Eldey Island and found two last great auks on Earth. They chased them around until they caught them; then an egg from one of their nests broke open and spilled everywhere so they couldn’t eat it for dinner because it was rotten or something like that. Anyway, these guys sold off pieces from their catch to some Icelandic dealer (I think he might have been related) who sold it to someone else—a naturalist named John Wooley or Alfred Newton (the author isn’t sure). He ended up going back over there years later trying to find more great auks only to realize none existed anymore!

At the same time that Newton and Wooley were returning from their trip to Iceland, Charles Darwin was publishing a paper on natural selection. He had already encountered this phenomenon in the Galapagos Islands while he was there. The whalers and fishermen ate all of the tortoises, which caused them to become extinct within ten years of his visit. In his book, Origin of Species, he noted that species became rare before they went extinct; however, he did not mention humans as a cause for extinction until later in life when he befriended Newton.

Although Darwin was aware of human-caused extinction, he didn’t seem to think it was a problem. He thought that humans were subject to the same laws of natural selection as other animals and even recognized that intelligence was an evolutionary adaptation like the lion’s claws or finch’s beak. Yet, he didn’t realize that humans were unique in one way: they were the only animals who caused others to go extinct. Furthermore, if humans could cause extinctions then Cuvier may have been right about cataclysmic events playing a role in extinction.

Chapter 4

A hundred miles north of Rome is a small town called Gubbio. It’s known for the limestone that was mined there, and in the 1970s, scientists discovered an asteroid which hit Earth during the Cretaceous period, causing mass extinction of dinosaurs.

Walter Alvarez had come to Italy to study plate tectonics. He found layers of marine fossils, but he also noticed that there were thick clay layers imbedded in the limestone of Gubbio. Back in California, his father suggested that Walter test for iridium because it’s a radioactive element. In fact, Walter found that the clay contained huge amounts of iridium and argued that this was evidence of an asteroid striking Earth at the end of the Cretaceous period (killing dinosaurs).

Biologists and paleontologists believed that the extinction theory was wrong because it wasn’t gradual. They pointed out that scientists had known about sudden decreases in fossils for more than a hundred years, but they believed that there would be more fossils discovered eventually. Most of them didn’t believe in a sudden global change at the end of the Cretaceous period—instead, they took a uniformitarian view of the distant past.

Although the Alvarez theory was unpopular, evidence continued to mount for an asteroid strike at the end of Cretaceous period. The first important piece of supporting evidence was shocked quartz that dated back to the same time. Another clue came in the form of sandstone apparently caused by a huge tsunami wave that also matched up with this time frame. Finally, scientists discovered an enormous crater in present-day Mexico which contained melted rock from around this same period.

Kolbert meets with a paleontologist named Neil Landman. He specializes in ammonites, which are prehistoric creatures that look like nautiluses. They have a thick layer of iridium-rich rock and soil at the site where they live.

Ammonites were a type of sea creature that are now extinct. They had spiral shells, and it’s unknown if they had tentacles or not. They probably evolved their distinctive spiral shells because the shells could withstand intense water pressure. Ammonites may have lived for millions of years, and fossils can be found in different parts of the world today.

The Alvarezes argued that the true killer of the dinosaurs wasn’t the asteroid itself but rather the dust it left behind. The dust was extremely hot and moved around Earth at a fast rate, either burning or suffocating most living things. Large creatures such as dinosaurs were first to go, followed by sea creatures and mammals. The concept of preservation potential is one way scientists measure how likely a species will become extinct. A high population, many places where they live, and thick shells are all characteristics of higher preservation potential for mollusks; hollow-boned birds have less preservation potential than them because their bones are more fragile.

Back in New Jersey, Kolbert watches Landman and his peers investigate the ammonite fossil site. They explain that during dinosaurs’ extinction, ammonites died out very quickly. However, nautiluses survived because they produced eggs that were resistant to sudden changes in temperature or water pressure.

The survival of the nautiluses is an important point because it shows that every single life form on earth today has survived a mass extinction. However, just because modern animals are descended from species that survived past extinctions does not mean they will survive future ones. The rules for surviving change constantly, and a creature who survives one extinction might not be able to survive another. For example, ammonites had high preservation potential before dinosaurs went extinct due to their small eggs that floated throughout the ocean (which increased their number), but after the sudden changes in habitat, this became a disadvantage for them.

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Chapter 5

In 1949, an experiment was conducted in which students were asked to identify playing cards as they flipped them over. Some of the cards had been doctored, but the student’s mistakes were due to their own speed and not necessarily because of the card itself. The experiment proved that people who are forced into a new environment will adapt by using old strategies first before learning new ones.

Kuhn’s ideas about paradigms and “paradigm shifts” are a good lens through which to study the history of evolutionary science. For example, when scientists studied fossils in the past, they tried to argue that these fossils belonged to living species; it took Cuvier to introduce the new paradigm: some animals go extinct. The same is true for uniformitarianism: for a century, scientists tried to explain the “gap” in fossil layers by citing the old paradigm that species gradually go extinct. It took Alvarez with his theory of an asteroid that wiped out dinosaurs, introducing a new paradigm of mass-extinction. Today most scientists believe that species usually go extinct gradually except when there’s sudden catastrophic event during which many species go extinct very quickly.

Kolbert travels to a cliff called Dob’s Linn, located in the Scottish Highlands. There, she meets with a stratigrapher named Jan Zalasiewicz who shows her layers of rock dating back 445 million years to the era immediately following the extinction of dinosaurs. During this era, sea life grew rapidly and coral reefs formed for the first time ever. Midway through that era moss-like plants appeared on land but at the end of it almost all sea life went extinct (85% or so).

Zalasiewicz’s specialty is the graptolite, a marine animal that used to be common in the ocean. The V-shaped body of the graptolite was strong and resilient to changes in temperature and pressure. However, something must have happened around 444 million years ago that made it an evolutionary disadvantage for these organisms.

After the Alvarezes published their paper on the asteroid that wiped out the dinosaurs, scientists began to rethink some of their ideas about extinction. Some scientists argued that periodic gaps in the fossil record could be explained by a companion star. However, they haven’t been able to back up this thesis with much empirical evidence.

The most common theory of the Ordovician mass extinction is that it was caused by glaciation. Around this time, carbon dioxide levels in the atmosphere dropped and temperatures fell. Sea levels went down as well, which had a devastating effect on marine life. It’s not clear why carbon dioxide levels went down so quickly, but there is some evidence to suggest that mossy plants drew the carbon dioxide from the atmosphere—meaning that in short, these plants were responsible for killing off many marine animals.

Scientists have argued that the mass extinction 252 million years ago was caused by an increase in carbon dioxide levels. The average ocean temperature went up by 18 degrees, water became more acidic, and the amount of oxygen in the water decreased, suffocating many species. As a result, about 90% of species went extinct. It’s likely that increases in temperature also triggered growth of poisonous bacteria accelerating the extinction process. In general it appears that each mass-extinction is different than one before it with no general theory for how so many species can go extinct at once.

In Scotland, Kolbert learned about Zalasiewicz’s theory of giant rats. He argues that in the future, rats will take over the planet; they’ll reproduce and get bigger and bigger. While this seems unlikely at first, it’s undeniable that humans have rearranged ecosystems wherever they go. Rats follow them on ships to new places such as Australia, China and South America. As a result, rats are among the most resilient creatures on Earth and are likely to survive if there is a mass extinction event.

The premise of Zalasiewicz’s theory is that humans have ushered in a new era of mass-extinction. The most popular term for the “age of humans” is Anthropocene. Paul Crutzen, who coined the term in 2000, argued that humans have fundamentally changed their planet by 1) dramatically transforming almost half of the world’s land surface; 2) either damming or diverting most rivers; 3) increasing nitrogen levels in the atmosphere due to agriculture; 4) eliminating more than one third of marine life from coastal waters; 5) consuming more than half of available fresh water and 6) changing atmospheric composition by releasing carbon dioxide and methane gas. On these six points, Crutzen argued that humans have brought about a new epoch in Earth history called Anthropocene. Moreover, he argued that this result will be a new mass-extinction event on Earth.

Zalasiewicz is one of the most vocal supporters of Crutzen’s Anthropocene argument. He lobbied for it to be recognized as a geological epoch before Kolbert visited him in Scotland, and if he succeeds “every geology textbook in the world immediately will become obsolete.”

Chapter 6

There is a small island in the middle of the Tyrrhenian Sea called Castello Aragonese. It was formed due to pressure between African and Eurasian tectonic plates, which sometimes release carbon dioxide gas. Kolbert travels there during winter in order to investigate how much carbon dioxide is released into the surrounding waters.

In Castello Aragonese, Kolbert meets Jason Hall-Spencer and Maria Cristina Buia. They take Kolbert scuba diving to show her the vents in the sea floor that are releasing huge amounts of carbon dioxide into the water. The water near these vents is very acidic with little marine life, so Kolbert predicts that if humans continue to release billions of tons of carbon into our atmosphere, we can expect dramatic changes in our oceans and marine ecosystems.

On land, Hall-Spencer and Buia show Kolbert some of the animals they’ve rescued from the ocean. They have a starfish with a missing arm and many other sea creatures that are endangered by carbon dioxide emissions. The more erosion on their shells, the closer they lived to a vent in the ocean. Based on this data, entire ecosystems will disappear if nothing is done about climate change.

In 2008, Hall-Spencer published a paper on the acidification of the ocean. Since then there has been considerable interest in this topic by other scientists and research projects which have confirmed his basic point: carbon dioxide emissions will greatly decrease the quantity and variety of life in the oceans. Many small bacteria and plankton will thrive in the newly acidic oceans, consuming more nutrients and depriving larger creatures of nutrition. Acidification probably played a large role in at least two of Earth’s “Big Five” mass extinctions, including one that killed 90% of all species on Earth 250 million years ago (at the end of Permian period), as well as another extinction that occurred about 252 million years ago (end Triassic).

Ocean acidification is harmful. It can damage marine animals’ shells, which are made out of calcium. Acidification causes a slow erosion of the creatures’ exoskeletons, and affects photosynthesis by killing plankton, which is necessary for many organisms to survive.

Carbon dioxide emissions are dangerous for ocean life because oceans absorb about a third of the carbon dioxide that humans pump into the atmosphere. The problem isn’t just that we’re adding carbon dioxide to the ocean, but also how fast we’re doing it. If humans continue at their current rates, then this will be one of the most cataclysmic events in planetary history.

Chapter 7

At the southernmost tip of the Great Barrier Reef in Australia, there’s a place called One Tree Island. On the island, there is a research station affiliated with the University of Sydney. Scientists from all over study different chemicals on nearby coral reefs.

The Great Barrier Reef was discovered by Captain James Cook in 1770. He didn’t understand how it was formed. Charles Darwin visited the reef in Tahiti and concluded that it had become submerged due to rising sea levels. Scientists now know that coral reefs are “part animal, part vegetable, and part mineral.” The Great Barrier Reef is made up of many calcifying creatures—in fact, a coral reef itself is largely composed of a calcium-rich structure with many different plants, animals, and minerals. Like human cities, coral reefs grow over time but they add new life forms rather than destroying them. In 2050 the Great Barrier Reef will be eroded into a “rubble bank.”

Kolbert met a scientist named Ken Caldeira on One Tree Island. He is an expert in ocean acidity and global temperature changes, but he has also studied the chemical composition of forests and how they have changed over time. On one diving expedition, Kolbert felt mystified by octopi like Cook must have been when he first saw the Great Barrier Reef more than two centuries ago.

The first evidence that carbon dioxide may destroy coral reefs came from an experiment called Biosphere in Arizona. It was a huge glass structure designed to be self-sustaining, but the people who tried to live there got altitude sickness because of the high levels of carbon dioxide. The project yielded some interesting conclusions about how increased levels of carbon dioxide eroded the composition of coral inside it. In recent years, scientific studies have shown that coral reefs erode significantly when CO2 rises in surrounding water, meaning they could “dissolve” within 50 years at current rates.

Kolbert went snorkeling with the scientists stationed on One Tree Island and saw beautiful sea life—sting rays, turtles, sharks, fish, and coral. Coral is particularly amazing because tropical waters are usually low in nitrogen and phosphorous. This means that theoretically they should be barren; however this is not the case. Charles Darwin was one of those who first noticed this puzzle which scientists have yet to solve.

Coral reefs have come and gone throughout history. They disappeared in the Triassic Era, but they were also threatened by overfishing and pollution. Now, coral reefs face an even greater threat: carbon emissions are warming ocean water, which is causing algae to grow more abundantly than usual. This has a negative effect on food availability for larger coral species because of less plankton available as food.

During her visit to One Tree Island, Kolbert observes the mating of birds during their mating season. She also looks up at the stars and feels a sense of insignificance. Later she notes that it was ironic for her to feel insignificant when writing about how humans affect nature.

Once a year, coral reefs spawn. The spawning is an event when corals reproduce sexually by releasing eggs and sperm into the water. One of the most common species in the Great Barrier Reef is Acropora millepora, which releases bundles containing many dozens of eggs and thousands of sperm. Scientists have studied samples from this kind of coral to see how acidification impacts it. They’ve found that acidification can destroy these bundles, but they witnessed something else when Kolbert was there late at night—a stream of bubbles that looked like pink fireworks coming out of the reef.

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Chapter 8

In Peru, Kolbert visits with Miles Silman. He’s a forest ecologist who studies the tropical ecosystems of South America. Global warming is often perceived as being most dangerous for animals in cold climates, but global warming poses an equally severe threat to tropical life forms.

Imagine that you’re standing at the North Pole and decide to go south. You might walk through Greenland, Quebec, and then a huge forest in the United States. Keep walking south until you reach Peru where there are more than 1,000 species of frogs and trees.

One theory suggests that the reason tropical climates are so diverse is because animals reproduce faster in those areas. A second theory argues that there’s more biodiversity in tropical climates because, due to relatively low temperature fluctuations, different zones can only harbor life with very specific thermal tolerances. As a result, the theory posits that different animals self-stratify based on temperatures they can withstand and new species gradually emerge over time. Another theory argues that tropical ecosystems are diverse because they have been around for longer than other ecosystems and have had time to accumulate great diversity over time.

In the Peruvian forests, Kolbert sees many different types of trees. Silman takes her to three different levels in the forest that have unique temperatures and humidity. He has been studying seventeen tree plots for more than a decade. The change in temperature has caused plants and animals to move up eight feet per year on average. For example, Schefflera trees replace themselves every year at higher altitudes as the climate changes around them. Other resilient Ilex trees haven’t moved from their original altitude over time despite changing weather patterns around them.

Every species on the planet has evolved to adapt to changes in temperature. The world’s average temperature goes up and down over time, for example 35 million years ago it was much colder than it is today (ice ages). However, scientists didn’t know why until recently. They believe that when Jupiter and Saturn are closest together they pull the sun closer together making less sunlight at northern latitudes causing snow which causes global carbon dioxide levels to drop further leading to a significant decrease in temperature.

During the Pleistocene period of Earth’s history, the planet became significantly cooler. Charles Darwin speculated that animals migrated to warmer climates during times of global cooling, and scientists have confirmed this theory. Today, scientists predict that average temperatures will increase in the next century at a faster rate than ever before in Earth’s history. It is not clear which species will be able to migrate north quickly enough to survive.

Silman is a botanist who takes Kolbert on several expeditions to observe the trees and collect samples. He’s also described as being funny, interesting, and kind. For example, one time he was talking about different types of trees in Peru. He described them as hilarious or clever. Another time, when Kolbert woke up at night because some men were selling coca leaves (used mostly for medical purposes), Silman yelled at them even though it wasn’t wise to do so.

In ecology, there is a well-known rule that the more land an area has, the more species will live in it. There appears to be a strong positive correlation between the two variables, such that as you increase the amount of land available for species to inhabit, you also increase the number of species living there. Therefore, humans decrease biodiversity by decreasing the total amount of land available for other species to inhabit. This principle can help us understand how much impact humans have on biodiversity—according to this principle 30%-50% of all worldwide species could die out within 50 years. However, some scientists believe that many animals and plants will be able to migrate northward in order to find temperatures they can survive in; however these migrations may not work for every animal or plant because not every animal or plant can move north and find climates similar enough for them to survive at those new locations.

Back in Peru, Silman shows Kolbert a tree species called Alzatea verticillata. The tree has bright green leaves and tiny pale flowers. Some species are able to adapt to the changing temperatures while others can’t.

One day, Kolbert and Silman went to a forest. There they saw different birds that were cold-adapted species. These birds survived the last Ice Age because of their unique traits. However, many other species will die out due to rapid climate change in the next fifty years.

Chapter 9

There is a square-shaped area in Brazil near the Venezuelan border called Reserve 1202. It consists of 25 acres of untouched rainforest, and all around it there are remnants of cut down trees known as “scrub.” There are many other similar reserves in the area controlled by an organization founded by Tom Lovejoy, who wanted to protect certain areas from farmers and ranchers cutting down trees. He presented his plan to the Brazilian government, and since then he has been given grants to study these preserves. His research involves making comparisons between these tiny reserves and the main rainforest miles away.

There are about fifty million square miles of land on the surface of Earth that do not contain ice. Humans have developed more than 25 million square miles in the last century, and it’s one of the defining characteristics of our era. It can be difficult to decide what does and doesn’t count as development—for example, is a tropical rainforest with a pipeline through it developed or not? The remaining undeveloped land probably looks like Reserve 1202—an island surrounded by development.

Kolbert goes to Reserve 1202 with an ornithologist named Mario Cohn-Haft. He takes Kolbert out into the forest late at night to listen to birdcalls and explains that, over the years he’s noticed a decline in both species diversity and biodiversity.

Islands are not as diverse in species as continents. One reason for this is that there isn’t enough space on islands to support many different species, and they’re more likely to go extinct if the island becomes smaller. As a result of relaxation, biodiversity declines over time due to fewer species and less space.

Kolbert is taken to a different reserve, where she sees more plants and animals. She learns that the rainforest is an extremely diverse ecosystem, with many species of plants and animals. One creature in particular—the army ant—is very aggressive and travels constantly. Army ants are unlike any other creatures on Earth; they provide food for hundreds of species of living things around them.

Scientists have been studying the number of insects in rainforests for decades. One scientist, Terry Erwin, estimated that there are 30 million species of arthropods in the world’s rainforests. Other scientists disagree with his estimate but agree that there are a lot of different life forms in these forests. The biodiversity is so high because many different species can live together and support each other. Because we’re destroying our rainforest habitats at such an alarming rate (some five thousand species per year), it’s important to understand how much we stand to lose if we don’t do something about it soon.

Throughout the nineties and 2000s, it was argued that a rainforest species went extinct every day. Later studies have shown that this estimate exaggerated the extinction rate. It is possible that the effects of reducing available land take time to set in as well as possible for areas deforested by humans to regrow over time. Another possibility is that human beings aren’t very good at counting how many vanished species there are on earth.

Lovejoy is still involved with the BDFFP (Biodiversity and Development Foundation of Papua New Guinea) today, though he’s in his seventies. His main priority is building support for the organization, as well as environmentalism in general, by educating people about what they’ve discovered through their research on biodiversity. He argues that many species have vanished from his reserves over the last few decades, suggesting that fragmentation of rainforest ecosystems results in rapidly shrinking biodiversity.

One night, Kolbert wakes up to see an army ant parade. She was told that she would be able to see the ants marching through the forest, but they didn’t appear. Cohn-Haft explains that the army ants were going into statue phase and wouldn’t march for a while. This is like how birds are affected when one species disappears from the rainforest: hundreds of other species are also affected by this disappearance.

Chapter 10

In 2007, a team of scientists climbed into caves in Albany, New York in the dead of winter expecting to see bats hibernating. What they saw instead was thousands of bats lying on the ground covered with a white powdery substance. They investigated further and found that this fungus was killing off all the bats in the area. Because it’s cold-loving, it spread to other states as well

The ideas of Darwin are useful in understanding the bat die-off. The theory of evolution argues that animals can’t travel long distances due to their inability to do so. However, this is not true because many organisms have been able to migrate across oceans and other natural barriers due to human intervention. This has never happened before in history, which makes it a unique event.

Kolbert lives near the Albany bat caves, where a fungus was killing off bats. She discovered that the fungus had spread to West Virginia. Kolbert met with Al Hicks, one of the scientists who first discovered it in Albany. He took her into the Adirondacks and showed her his research on endangered species of bats with white fungus growing on them.

Species move around the world in a process similar to Russian roulette. Most of the time, nothing happens when a species encounters a new environment. However, if it survives and proliferates in that new environment, then its population explodes. It’s not clear why some species are better at adapting and proliferating than others—perhaps their success is as random as Russian roulette. Scientists have hypothesized that when a species moves to a new environment it often has fewer evolutionary rivals and predators, which allows its population to explode.

When a new species is introduced to an ecosystem, the change can sometimes be devastating. For example, when humans brought brown tree snakes from Australia to Guam in the 1940s, many of Guam’s bird species became extinct due to predation by the brown tree snake. New diseases spread quickly and have also caused extinctions; for instance, chestnut trees were almost completely wiped out in America because of a Japanese fungus that was accidentally imported on plants from Asia at around the turn of the 20th century. One disease that has killed off bats in Albany is also an example of how dangerous new pathogens are.

It’s likely that you can see a species that humans have introduced to the environment, and there are many of these invasive species. For example, in Brisbane, Australia, the cane toad has wiped out hundreds of plant and insect species. In a way, this is like going back to prehistoric times when all landmasses were one supercontinent called Pangaea. There are no natural barriers anymore because humans have made it possible for any species to move anywhere in the world.

The authors travel to the Aeolus Cave in Vermont, where they discover a dead pile of bats. The scientists proceed to put the bats in plastic bags and test them later on. When did this “New Pangaea” come into existence? Humans have crossed natural barriers for many thousands of years, but particularly since nautical travel became more common after Columbus came to America around 1492, species exchange has increased enormously. One group estimates that California alone acquires a new invasive species every sixty days. In the short-term, diversity increases because there is one extra species added to an ecosystem; however, biodiversity decreases as a result of dominance by these invasive species over time.

Invasive species are a hot topic. They’re all over the news, and many people have strong views on them. The study of invasive species began with Charles Elton’s work in the 1950s. He compared invasive species to gases that are mixed together (molecules). When two different gases mix, they form new chemicals (new combinations of molecules). Eventually, these new chemicals reach an equilibrium in which there is less molecular variety (fewer types of gas) than before the mixing occurred. In other words, if you don’t add more chemical components into this mixture for a long time, it will eventually return to its original state and continue to remain at that level unless you add more ingredients or stir up the system again by creating some sort of change within it. This might be similar to what happens when human commerce ceases—the world reaches an equilibrium where there are fewer organisms than before humans appeared on Earth, but once humanity disappears from existence, nature may begin growing again.

A biologist named Kolbert studies a cave where bats used to live. A fungus known as Geomyces destructans wiped out most of the bat population and forced them to relocate elsewhere.

Chapter 11

Kolbert travels to the Cincinnati Zoo to meet Suci, an enormous rhinoceros. Roth tells Kolbert that Suci is a Sumatran rhinoceros and one of only five species left on the planet. As such, she has been trying to artificially inseminate her with no success.

Sumatran rhinos used to live in the Himalayas, as well as on Sumatra and Borneo. They were once common but are now headed for extinction. Recognizing that they’re going extinct, a conservation group decided to send them to American zoos so their species could be preserved. However, five of these rhinos died almost immediately because of disease spread by flies. Then several others captured in Borneo also died from tetanus and other injuries. By this time there were only three Sumatran Rhinoceros left in the US Roth’s job is to impregnate one of these Rhinos so its species can survive forever

Roth tried to inseminate a rhino named Emi. She had many false attempts at getting pregnant, but eventually she gave birth to Suci and Harapan. These are the only Sumatran rhinos born in captivity for over 30 years. This is also true of other large mammals such as elephants, jaguars, pandas, and cheetahs—they exist mostly or entirely in zoos or preserves because scientists have been unable to get them to mate successfully.

Kolbert observes Suci eating and is later permitted to pet Suci. Many of the largest mammals in the world are peaceful herbivores; their size keeps them safe from dangerous predators like tigers and jaguars. At the end of Ice Age, many animals were huge—cave bears, giant elk, mastodons etc. Biologists have posed an important question—why is it that so many extinct species are exceptionally large by modern-day standards? And similarly why did so many go extinct?

To answer the question of whether humans are causing mass extinction, Kolbert travels to Big Bone Lick. Here, 19th century fossil-hunters discovered some of the world’s most famous fossils. Some 19th century thinkers argued that a great modification in climate killed off creatures like the mastodon. Charles Darwin agreed with this idea and wrote that an Ice Age must have caused extinction. Other scientists disagreed and said it was because human beings hunted them down for food or sport. This theory suggests that modern times might actually be responsible for mass extinction—mass extinctions started during the last Ice Age when human beings were around to witness it first hand.

There are three good reasons to conclude that humans were responsible for the mass extinction of large mammals. First, it’s likely that there was a pulsing effect in which they went extinct over time and not all at once. Second, these mammals survived droughts before humans came along so environmental factors couldn’t be ruled out as the cause. Third, excrement samples reveal that they didn’t die of malnutrition and this points to human beings being the most likely candidate for causing their demise.

There are some scientists who argue that humans didn’t wipe out the large mammals of the late Ice Age because humans were not dangerous enough at that time. But John Alroy, a scientist who tested this hypothesis, found that “humans could have done in” the large mammals with only modest effort. It’s likely then that we really began affecting our environment thousands of years ago instead of just 200 years ago when industrialization started.

In the beginning, large mammals were dominant. They were peaceful and easy to hunt. The presence of humans changed that. Humans hunted them for food and other resources, which led to their extinction. We can say that there was never a time when humans lived in harmony with nature because they always hunted other creatures for food or sport.

Chapter 12

In Germany, there is a small valley known as the Neander Valley. In it, in the mid-19th century, workers stumbled upon some remains of an ancient human ancestor that they named the Neanderthal. Since then, scientists have found more remains of this species in other parts of Europe and the Middle East. The Neanderthals had sophisticated tools for hunting and keeping themselves warm. Then about 30 000 years ago, they vanished from existence without leaving any descendants behind. Some researchers argue that environmental changes wiped out the Neanderthals while others claim that Homo sapiens killed them off by interbreeding with them until their genetic line was completely lost.

Kolbert talks to Svante Pääbo, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig. He’s trying to map Neanderthal DNA so that humans can compare their genetics with those of Neanderthals.

Scientists thought that Neanderthal bones were from regular humans. However, some specialists pointed out the unusual parts of the skeletons. Over time, more Neanderthal remains surfaced and showed that they had large skulls and bowed femur bones. Scientists in early 20th century portrayed them as brutish creatures who could barely stand up straight due to their uncivilized nature. After World War II, anatomists re-examined these remains and made some striking conclusions: Neanderthals didn’t walk with a slouch; weren’t hairy; looked like modern humans; buried their dead and planted flowers on graves.

DNA is often viewed as a blueprint for the structure of an organism. Humans have billions of lines made up of four chemicals (adenine, thymine, guanine, and cytosine) in their nuclei that make up DNA. Since humans die quickly after they pass away, it’s hard to find any genetic information about them from ancient times. However, scientists have found evidence of DNA in Neanderthal bones. The analysis reveals that Neanderthal DNA is very similar to human DNA with Europeans and Asians having more similarities than Africans do.

The most popular theory about how humans evolved is the “Out of Africa” hypothesis. This theory pertains to a small group of modern humans who had left Africa 200,000 years ago and migrated to other parts of the world, including Middle East, Europe, Asia, Australia and America. Before they left Africa in this migration event known as “out-of-Africa”, Neanderthals were already living in Eurasia.

Some people think that all living humans have the same genetic overlap with Neanderthals, but in fact some people’s DNA has more in common with Neanderthal DNA than others. This is due to a slight modification of the Out of Africa theory—the “leaky-replacement hypothesis,” which states that early human beings interbred with Neanderthals when they first encountered them. Furthermore, it seems as if half-human, half-Neanderthal children were cared for rather than being scorned or hated.

What makes humans different from other animals? One might say that it’s intelligence, but apes are intelligent too. Scientists have shown that primates can solve complex puzzles and make inferences. However, human children beat out apes in tests designed to measure their ability to read social cues. Perhaps what makes us human is the ability to engage in collective problem-solving – solving a problem by communicating with others about it.

Neanderthals are believed to have made stone tools and buried their dead. They also seem to have been injured, perhaps from hunting or other activities. Neanderthal remains show signs of serious injuries, but they survived them, suggesting that they took care of each other. Interestingly, it seems like Neanderthals spread across Europe without building boats for crossing bodies of water.

Pääbo has a theory about Neanderthals. He believes that some of the qualities which make humans unique are genetic, and they’re responsible for our success as a species. For example, we have ambition to conquer new territory and wipe out other species—qualities that Neanderthals may not have had. Pääbo also found evidence in human fossils of interbreeding with other hominid species like Denisovans. It’s possible that Denisovans went extinct because their reproductive rates were too low, just like apes today are going extinct due to their slow reproduction rate. If this trend continues then it’s possible that all hominids will go extinct eventually, including chimpanzees and gorillas.

Kolbert drives to La Ferrassie, a French site where the largest recorded assemblages of Neanderthal remains were discovered 100 years ago. She watches a team of paleontologists at work and imagines what life had been like for Neanderthals. While she finds a beautiful hand-ax, almost perfectly symmetrical, there’s no evidence that they designed their tools to be beautiful or made art.

Kolbert visited another archeological site in France, Grotte des Combarelles. There are paintings on the walls that indicate that humans once lived there. It is strange to think of how far humans have come since then, with all their madness and ambition to explore caves armed only with fire and axes. If it weren’t for human beings, Neanderthals might still be alive today.

Chapter 13

Kolbert visits the Institute for Conservation Research (ICR) in San Diego to learn about what they do. There, she sees a collection of vials that contain the DNA of endangered species, such as birds and animals. She also learns that there are other collections like this one available.

Kolbert wonders if the beautiful plants and animals need to go extinct for humanity to survive. It’s true that humans can be destructive, but they can also be altruistic in their efforts to save endangered species. The Endangered Species Act of 1974 was a step in the right direction, as it protected many animals from extinction. Scientists and conservationists have worked hard to prevent more species from being wiped out so we don’t have a bleak future where there are only plastic vials left of all life on Earth.

Next to the ICR, there’s a veterinary hospital that serves both the ICR and nearby San Diego zoo. Barbara Durrant is a reproductive physiologist at this hospital. She tells Kolbert about Kinohi, a Hawaiian crow she was trying to get to reproduce but had no success with it. It’s indicative of how seriously humans take extinction that they are willing to spend thousands of hours getting crows, rhinos, and other species to bear offspring.

Throughout the book, Kolbert has been talking about the Anthropocene era or Sixth Extinction. It’s not clear if biodiversity will continue to plummet because humans have a capacity for solving problems and behaving selflessly.

The American Museum of Natural History has a small exhibit about the Sixth Extinction. It shows some extinct animals and implies that humans could become extinct, too. When we study life on Earth, it becomes clear that past performance is no guarantee of future results—humans have survived for so long but don’t know if they will continue to survive in the future.

Some people believe that humans will always find a way to survive, while others argue that we’re causing the extinction of many species. However, Kolbert believes it’s more important to focus on life as it is now rather than what might happen in the future. We can’t change what has already happened but we can try and help save endangered species by changing our behavior right now.

The Sixth Extinction Book Summary, by Elizabeth Kolbert

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