In Volume 03: Flourish/Collapse, writer Tim McDonnell journeys through the six major extinction events that have shaped life on Earth. Since the oldest fossil evidence of Homo sapiens dates to ~300,000 years ago, humans have flourished and spread around the planet, dramatically altering the landscape. Throughout the twentieth century, however, the global extinction rate has kept pace with a skyrocketing human population. And, as if we’re heading back in time, up to half of all species living today may be gone by 2100.
Five hundred million years ago, life on Earth existed almost entirely underwater. It was a world dominated by invertebrates: tiny, colony-forming graptolites; scurrying trilobites; shell-encased brachiopods. Around this time, the earliest terrestrial plants—mosses and liverworts—began to creep onto the southern supercontinent Gondwana. Plant life flourished, dramatically altering the atmosphere by sucking in carbon dioxide and replacing it with oxygen. Temperatures collapsed, glaciers grew, and sea levels subsided. Marine species accustomed to a hot, watery world began to die off on a massive scale. By the end of the Ordovician period, at least 85 percent of marine species were extinct.
Almost a hundred million years later, another explosion of plant life triggered fatal conditions for marine life. This time, the culprits were the descendants of those first mosses: early trees and ferns. Their roots broke up the rocky surface, creating the planet’s first soil and unlocking a torrent of minerals and nutrients. Those found their way into the ocean, fueling massive blooms of algae, which in turn soaked up oxygen in the water and made it increasingly difficult for marine life to breathe. The trees on land, swallowing ever more carbon from the atmosphere, tipped off another round of global cooling and sea-level decrease. Marine organisms were suffocating, poisoned, and freezing—causing up to 75 percent of species to vanish.
By 300 million years ago, land—reorganized into the supercontinent Pangaea—was richly populated with flora and fauna, including conifers, 10-foot-long reptiles, and the first warm-blooded creatures. The planet’s atmospheric chemistry was about to undergo another massive shift. But this time geology, not plants, would be to blame. A series of massive volcanic eruptions in what is now Siberia filled the air with carbon dioxide and methane, trapping heat and acidifying the air and oceans. Over a period of about 200,000 years, 96 percent of species were killed—the largest mass extinction in history. All life on Earth that has existed since descended from the remaining four percent of species.
Among those that survived were some therapsids, which formed an evolutionary link between reptiles and mammals. Other survivors were giant amphibians and the earliest forms of the lizards, turtles, and crocodiles that persist to this day. From this motley assortment rose the first dinosaurs. Then, Pangaea began to disintegrate, exposing massive volcanoes that again filled the air with greenhouse gases. By 201 million years ago, 80 percent of species were extinct—clearing the way for the age of dinosaur domination.
Pangaea continued to drift apart. Rising ocean-floor ridges pushed sea levels to their highest point ever, up to 820 feet above their level today. Ichthyosaurs, plesiosaurs, and mosasaurs plundered the seas alongside the earliest whales. The earliest cats, cows, and primates hid from triceratops, velociraptors, and tyrannosaurus in forests of ferns and conifers. The earliest ants, butterflies, and grasshoppers visited the earliest flowers. By 65 million years ago, volcanic activity, this time in India, again began to disrupt the climate. A meteor that made impact in the Yucatán Peninsula was the six-mile-wide nail in the coffin. All non-avian dinosaurs were lost, as were the marine reptiles and the flying pterosaurs. Eighty percent of species went extinct.
The oldest fossil evidence of Homo sapiens dates to about 300,000 years ago. Since then, as humans flourished and spread around the planet, they dramatically altered the landscape through the use of fire, simple tools, agriculture, and animal domestication. Throughout the twentieth century, the global extinction rate has kept pace with the skyrocketing human population. Amphibians, fish, and insects are particularly vulnerable. It’s hard to know how many species go extinct in a typical year—we hardly know how many species exist to begin with—but estimates range as high as 100,000, the highest rate of loss in history. If current trends of pollution, overfishing, land clearing, and climate change continue, up to half of all species living today may be gone by 2100.