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woolly mammoths, Paleozoic Era, mastodons, island of Mauritius, trilobites

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Extinction (biology), the end of existence of a group of organisms, caused by their inability to adapt to changing environmental conditions. The history of life on Earth is influenced by both evolution, which allows organisms to adapt, and extinction. Extinction affects individual species—that is, groups of interbreeding organisms—as well as collections of related species, such as members of the same family, order, or class. The dodo, for example, a species of flightless pigeon formerly living on the island of Mauritius, became extinct in 1665. About 10,000 to 12,000 years ago, most of the woolly mammoths and the last of the mastodons, both members of the elephant family, died. And about 251 million years ago at the end of the Paleozoic Era, an entire class of primitive marine animals called trilobites disappeared forever.

Fossils, the remains of prehistoric plants and animals buried and preserved in sedimentary rock or trapped in amber or other deposits of ancient organic matter, provide a record of the history of life on Earth. Scientists who study this fossil record, called paleontologists, have learned that extinction is a natural and ongoing phenomenon. In fact, of the hundreds of millions of species that have lived on Earth over the past 3.8 billion years, more than 99 percent are already extinct. Some of this happens as the natural result of competition between species and is known as natural selection. According to natural selection, living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment. Those species incapable of adapting are faced with imminent extinction. This constant rate of extinction, sometimes called background extinction, is like a slowly ticking clock. First one species, then another becomes extinct, and new species appear almost at random as geological time goes by. Normal rates of background extinction are usually about five families of organisms lost per million years.

More recently, paleontologists have discovered that not all extinction is slow and gradual. At various times in the fossil record, many different, unrelated species became extinct at nearly the same time. The cause of these large-scale extinctions is always dramatic environmental change that produces conditions too severe for organisms to endure. Environmental changes of this caliber result from extreme climatic change, such as the global cooling observed during the ice ages, or from catastrophic events, such as asteroid or comet impacts or widespread volcanic activity. Possible causes even include bursts of radiation from exploding stars called supernovas. Whatever their causes, these events dramatically alter the composition of life on Earth, as entire groups of organisms disappear and entirely new groups rise to take their place.

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