II.

Mass Extinctions

In its most general sense, the term mass extinction refers to any episode of multiple loss of species. But the term is generally reserved for truly global extinction events—events in which extensive species loss occurs in all ecosystems on land and in the sea, affecting every part of the Earth's surface. Scientists recognize five such mass extinctions in the past 500 million years. The cause or causes of each of these mass extinctions are still under study and open to debate.

The first global mass extinction occurred about 440 million years ago in the Ordovician Period. At this time, all animals and plants on Earth still lived in the ocean. More than 85 percent of the species became extinct, including many families of invertebrate marine animals belonging to groups such as brachiopods, echinoderms, and trilobites. Climate change may have caused many animals that lived in warm tropical reefs to die out as ice sheets formed over a continent at the South Pole and sea levels dropped. Some researchers have proposed a different cause and think that a supernova explosion at the time may have bathed Earth in deadly gamma-ray radiation that stripped away Earth’s ozone layer, exposing the planet to high levels of ultraviolet light from the Sun. Such radiation would have been harmful to things that lived near the surface of the oceans.

The second mass extinction took place about 360 million years ago, near the end of the Devonian Period, when 82 percent of all species were lost. Animals and plants now lived on land as well as in the sea. The greatest extinctions affected marine animals, including cephalopods and armored fish. Climate change associated with the development of glaciers on a giant southern continent may have been the main cause.

The third and greatest mass extinction to date occurred 251 million years ago at the end of the Permian Period. In this mass extinction, as many as 96 percent of all species in the oceans and 70 percent of all species on land were lost. The devastation was so great that paleontologists use this event to mark the end of the Paleozoic Era, and the beginning of the Mesozoic Era, when many new groups of animals evolved. Current theories suggest that the Permian extinction was caused by a series of events that began with massive volcanic eruptions in Siberia. The gases released included the greenhouse gas carbon dioxide, which caused the atmosphere to heat up. The oceans eventually warmed. Methane trapped as icy hydrates on the seafloor may have been released into the atmosphere—methane is an even more powerful greenhouse gas than carbon dioxide. Low oxygen conditions deep in the oceans may have caused bacteria to release huge amounts of hydrogen sulfide gas.

The chemistry of the air was affected and oxygen levels in the atmosphere were reduced to about 13 percent compared to 21 percent today. The low oxygen helped kill off many animals in the oceans and on land. Plants also died, and soil eroded. Chemical and geological studies of sediments offer some support for parts of these scenarios. More controversial alternative theories have proposed that a comet or asteroid impact was involved in the Permian extinctions.

About 200 million years ago at the end of the Triassic Period, the fourth mass extinction claimed 76 percent of the species alive at the time. Primitive fishlike animals called conodonts disappeared from the seas, as did many types of marine reptiles. Many species of amphibians and reptiles on land also died out, including a number of groups of archosaurs, advanced reptiles that included dinosaurs. However, dinosaurs themselves managed to survive the mass extinction and went on to become the dominant animals on land for another 140 million years. The cause of the Triassic extinction is debated. The giant supercontinent Pangaea began to break up at the time. Volcanoes and climate change may have contributed to the extinctions. Attempts to link the extinctions to a comet or asteroid impact are based on weak evidence for now.

The fifth and most recent mass extinction occurred about 65 million years ago at the end of the Cretaceous Period and resulted in the loss of 76 percent of all species, most notably the dinosaurs. Many geologists and paleontologists speculate that this fifth mass extinction occurred when an asteroid struck Earth. They believe the impact created a dust cloud that blocked much of the sunlight—seriously altering global temperatures and disrupting photosynthesis, the process by which plants derive energy. As plants died, organisms that relied on them for food also disappeared. Supporting evidence for this theory comes from a buried impact crater in the Yucatán Peninsula of Mexico. Measured at 200 km (124 mi) in diameter, this huge crater is thought to be the result of a large asteroid striking Earth. A layer of the element iridium in the geologic sediment from this time provides additional evidence. Unusual in such quantities on Earth, iridium is common in extraterrestrial bodies, and theory supporters suggest iridium traveled to Earth on the asteroid.

Other scientists suspect that widespread volcanic activity in what is now India and the Indian Ocean may have been the source of the atmospheric gases and dust that blocked sunlight. Ancient volcanoes could have been the source of the unusually high levels of iridium, and advocates of this theory point out that iridium is still being released today by at least one volcano in the Indian Ocean. No matter what the cause, the extinction at the end of the Cretaceous Period was so great that scientists use this point in time to divide the Mesozoic Era (also called the Age of Reptiles) from the Cenozoic Era (otherwise known as the Age of Mammals).