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Chapter 1: The Sixth Extinction
The ancestors of the frogs crawled out of the water around 400 million years ago. 250 million years ago, frogs were the earliest representation of what would become the modern amphibian orders. Amphibians have been on Earth for longer than mammals or birds; they were even here before dinosaurs. Recently, it has been reported that the extinction rate of frogs were increasing. Based on these alarming rates, we can predict an event of similarly catastrophic nature was headed our way. The Panamanian golden frog is considered a lucky symbol in Panama. A decade ago, the golden frogs were plentiful in numbers and easy to find. However, within the past couple of years, the frogs started to disappear. Kolbert states that studies by the National Zoological Park in Washington D.C and a mycologist at the University of Maine prove the reason for the increased mortality of Panamanian frogs is due to a type of Chytrid fungi. However, Chytrid fungi is not naturally found in Panama. This left a puzzling question: how did the fungi get to Panama if it doesn’t naturally live there? Evidence indicates that humans are actually how the fungi traveled. Kolbert uses frog and fungi as a symbol of how humans are introoducing invasive species to species who would normally have the proper distribution of alleles for their environment.

Chapter 2: The Mastodon’s Molars
Kolbert explains how fossils of the American mastodon (Mammut americanum) shaped Georges Cuvier’s views on catastrophism. According to Cuvier, there was no reason the mastodon should have died out. The mastodon was large enough to avoid predation, had large enough teeth to consume an abrasive diet, and other phenotypes that would increase their survival. Cuvier concluded there must be sudden and violent natural catastrophes that would be causing such mass extinction of viable species. Kolbert uses the mastodon as a symbol for the idea that catastrophe is an important mechanism of extinction.

Chapter 3: The Original Penguin
The great auk was a large flightless bird that lived in the Northern Hemisphere. It has a large, intricately grooved beak. When the first settlers arrived in Iceland, the auk’s population were probably in the millions. However, the settlers found the auk’s to “very good and nourishing meat.” They also used them for fuel, fish bait, and for stuffing mattresses. Kolbert uses the great auk as a symbol of how human overexploitation resources is an important mechanism of extinction.

Chapter 4: The Luck of the Ammonites
Kolbert explains that the main cause of the Cretaceous-Paleogene extinction event was not the impact of the asteroid itself. It was the dust created by the impact. The debris from the impact incinerated anything in its path. She states that it is impossible to give anything near a full account of the various species that died out due to this catastrophe. Kolbert explains that even though amniotes were ‘fit’ for their current environment, a single moment can completely change which traits are advantageous and which are lethal.

Chapter 5: Welcome to the Anthropocene
Kolbert uses the extinction of graptolites and other clades to explain glaciation as a mechanism for extinction. When carbon dioxide levels in the air are high, there typically is an increased level in temperatures and in sea level. Right around the time graptolites went extinct, carbon dioxide levels dropped. Temperatures fell and sea levels plummeted. This caused a change in the chemistry of the ocean. This can cause a devastating impact on life forms. Kolbert states that human activity has transformed between a third and a half of land surface on the planet. We have dammed most of the major rivers of world, increased levels of nitrogen than can be fixed naturally by terrestrial ecosystem, used more than half of the world’s readily accessible freshwater run-off[[, remove more than one third of the[[primary producers of the oceans’ coastal waters, and changed the composition of the atmosphere by deforestation and fossil fuel combustion.

Chapter 6: The Sea Around Us
Since that beginning of the industrial revolution we have been increasing levels of carbon dioxide in the atmosphere at an alarming rate. Studies show we have added approximately 365 billion tons by burning of fossils fuels and an additional 180 billion tons by deforestation. We add another 9 billion tons or so a year, an amount that has been increasing 6% annually. Essentially, we have increased the concentration of carbon dioxide in the air to higher than it has been in the last several million years. Some of this carbon dioxide is being absorbed by our oceans to create carbonic acid. This is lowering the pH of our ocean and killing many or our marine life. Kolbert uses the drastic decline in life forms around the Castello Aragonese as warning sign of what is to come if we continue to increase carbon dioxide levels in the atmosphere.

Chapter 7: Dropping Acid
Coral reefs support thousands of species by providing food and protection. Subsequently, many species have co-evolved with corals. Due to ocean acidification, it is very possible that corals will become extinct by the end of the century. Prior to the Industrial Revolution, under water reefs had an aragonite saturation state between 4 and 5. However, if current emission intensity remain as they are today, by 2060, there will no longer be a region above 3.5. This will lead to an increase in energy needed for calcification. The same energy that will be expended on calcification is vital to recover from being away by marine species and being battered by waves. Corals prove to be a symbol of ocean acidification as mechanism of extinction.

Chapter 8: The Forest and the Trees
Global warming is most commonly seen as a threat to cold-loving species. As temperatures rise, the ice at the North Pole and South Pole will melt. Any animal that depends on the ice will be faced with extreme challenges that could ultimately drive them to extinction. Kolbert surfaces attention to the fact the poles are not the only place affected by global warming. She explains that climate change is a major force is mass extinction.

Chapter 9: Islands on Dry Land
Kolbert points out how everything in life is interconnected. She traces how there is circus-like complexity in the life-forms of many species. For example, there remains a fixed time for butterflies to evolve to feed on bird feces that evolved to follow ants. She uses this example to show that one minor change causes a domino effect in various ecological systems.

Chapter 10: The New Pangaea
Kolbert points out there is an evolutionary arms race in which each species must be equipped to defend against than any of their potential predators and to be more fit than their competition. For example, the new host has no defense if encountered with a new fungus, [virus]], or bacteria. This can be extremely deadly. There was an example of this in the eighteen hundreds. The American chestnut was the dominant deciduous tree in the eastern forests. Then, a fungus (Cryphonectria parasitica) started to cause chestnut blight. It was nearly 100% lethal. The fungus was unintentionally imported to the U.S. This furthers the idea of the first chapter that invasive species are a mechanism of evolution.

Chapter 11: The Rhino Gets an Ultrasound
The Sumatran Rhino was once so abundant in numbers it was considered an agricultural pest. However, as Southeast Asia’s forests were cut down, the rhino’s habitat became fragmented. In the 1900s, the rhino population has been shrunk to just a few hundred. Today, there are only four living rhinos. Kolbert uses the rhino to show habitat fragmentation as a mechanism of extinction.

Chapter 12: The Madness Gene
Europe was the home to Neanderthals for at least a hundred thousand years. Then, about 30,000 years ago, the Neanderthals vanished. Fossil records show that modern humans arrived to Europe 40,000 years ago. Within 10,000 years, Neanderthals were bred out. Due to molecular sequencing, we have found that there is 1-4 percent Neanderthal DNA in all non-African humans. This would mean humans and Neanderthals reproduced. Then those hybrids reproduced. The pattern continued until Neanderthals were literally bred out. Kolbert states there is every reason to believe that Neanderthals would still exist if it weren’t for humans.

Chapter 13: The Things with Feathers
Kolbert concludes with the hope in humanity. Whether meaning to or not, we are deciding which evolutionary pathways are will be shut off forever and which can be left open to flourish.