Extinction is the complete demise of a species. It takes place when all individuals of a species die out. Extinction has occurred throughout the history of life on Earth. It is the ultimate fate of all species. In fact, it has been estimated that 99.9% of all species that have ever lived on Earth are now extinct. The causes of extinction are many and highly variable. They vary from environmental changes brought about by events such as floods, volcanic eruptions, or meteorite impacts, to fluctuations in sea level or climate, to competition between species. In the past 25,000 years humans have become a significant additional cause of extinction for many species.

Species are constantly going extinct, often for reasons that are not particularly obvious, and at other times for reasons that are peculiar to the species in question. The processes that lead to this pattern of constant background extinction occur continuously, so that at any given time, while some species are going extinct, others are making their appearance for the first time. Over time, this process of continual turnover produces great changes in the species composition of the Earth’s ecosystems. However, there have been times in the past when rates of extinction have been significantly higher than normal background rates. These are referred to as mass extinctions. During such events, vast numbers of species disappear in a relatively short period of time.

In the history of Earth, there have been five mass extinctions. The best known and most recent occurred at the end of the Cretaceous period, 65 million years ago. At that time, many kinds of organisms, plants and animals, marine and terrestrial, went extinct, the best known being the dinosaurs. A total of 85% of all species on Earth disappeared. Most scientists believe that this mass extinction was caused by the impact of an asteroid, which threw vast amounts of dust into the atmosphere, blocking sunlight and lowering global temperatures for many months. Study of this extinction continues.
Four other mass extinctions are known to have occurred:

1. Late Triassic (210 million years ago): extinction of 76% of species including many sponges, gastropods, bivalves, cephalopods, brachiopods, insects, and vertebrates; possibly due to glaciation or an extraterrestrial bolide impact.
   
2. Permian (250 million years ago): extinction of 96% of marine species, including all trilobites and many terrestrial animals—Earth’s biggest extinction. Among the explanations are global cooling, glaciation, lowered sea levels, and ash clouds produced by volcanic eruptions.
   
3. Late Devonian (365 million years ago): 85% of species went extinct, including many corals, brachiopods, and single-celled foraminiferans, probably because of global cooling and glaciation.
   
4. Ordovician (440 million years ago): 85% of species disappeared, including trilobites, graptolites, and conodonts, as a result of global cooling, glaciation, and lowering of sea levels.

During the last ice age, about 15,000 to 25,000 years ago, many large mammals became extinct in North America. These included mammoths, mastodons, glyptodonts, horses, and ground sloths. These extinctions may have been caused by overhunting on the part of humans, who had reached North America from Asia at about this time. Climate change may also have played a role in the demise of these animals. Similar extinctions occurred in Australia about 18,000 years ago, also timed to the arrival of humans on that continent.

Since the ice age, humans have become a dominating factor in the extinctions of many species through hunting, habitat destruction, or competition caused by the introduction of non-native species. One of the most famous examples of recent extinction is the dodo, a flightless pigeon that lived on the island of Mauritius, in the Indian Ocean. After Europeans reached Mauritius they introduced non-native species such as pigs and cats, which eradicated the defenseless dodo. Human overhunting may have doomed other flightless birds, such as the giant moas of New Zealand and the elephant birds of Madagascar. Today, several species of whales are near extinction from overhunting as well. Habitat destruction threatens countless plants and insects of the Amazon, while the introduction of the brown tree snake into Guam has exterminated most of that island’s bird species.