Extinction, which can have a variety of causes, results in the permanent elimination of one or more species. According to most scientists, 99.9 percent of all species that have ever lived are now extinct.

All organisms have to contend with the pressures of predation, competition, and environmental change. Throughout geological history, we see a "background" pattern of extinction of some species. The background extinction rate removes a family of organisms about every million years. By contrast, mass extinctions appear as abrupt discontinuities in the fossil record, affect many species at once, and have a much greater impact on biodiversity.

Most scientists agree that five extinction events stand out from the rest. Each time, at least 50 to 90 percent of all species living on Earth disappeared. Different causes that have been proposed to explain these mass extinction events include meteor impact, volcanic activity, sea-level change, oxygen depletion, global cooling, changes in ocean chemistry, and glaciation. Scientists generally agree that the reason for any mass extinction can be traced to a variety of environmental factors that may have resulted from a triggering event.

The extinction that occurred at the end of the Permian period, 250 million years ago, was probably the most severe in Earth's history. For even one family of organisms to be wiped out at once is extraordinary. Yet the death toll at the end of the Permian included 400 marine families—more than half of all marine families living at the time. It is estimated that this total included 90 to 96 percent of all marine species. On land, three-quarters of all reptiles and two-thirds of the amphibians disappeared from the fossil record. Most plant life, as well as one-third of the orders of insects, also disappeared, marking the only time insects have ever suffered mass extinction.

The video presents an emerging theory to explain the cause of the end-Permian extinction. It suggests that there was simply too much heat and too little oxygen in the environment as a result of prolonged volcanic eruptions. Studies of the chemical composition of rocks formed at this time indicate warm, oxygen-poor oceans. Such an environment would favor population explosions of lethal bacteria, whose metabolism generated toxic sulfur gas.

As compelling as this explanation is, it's not the only one. In fact, a hypothesis first presented in 2001 suggested that the impact of an asteroid as large as seven miles across could have been the triggering event, releasing at least a million times more energy than the strongest recorded earthquake. However, as with any hypothesis, this one will remain unfounded until a large body of physical evidence is collected that supports it.

Although mass extinctions are deadly events, they open up the planet for new life forms to emerge. For example, during the Triassic period that followed the Permian, dinosaurs became the dominant land animals. And following their extinction 65 million years ago at the end of the Cretaceous period, dinosaurs made room for mammals to rapidly diversify and evolve.

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