During the course of life on Earth, which began 3.6 billion years ago, the diversity of species on our planet has been determined by a myriad of environmental factors. Any given factor favored certain species over others (essential to the evolutionary process) while some events have had widespread deleterious effects on life in general. The latter are known as mass extinction events and have been triggered by a variety of catastrophic alterations in the planet's atmosphere, climate, oceans and terrestrial ecosystems. Proposed causes have included disruptions in atmospheric and marine chemistry, catastrophic volcanism, massive basalt flows, methane release from sub-marine or subterranean pockets, altered ocean currents due to continental drift and impacts from asteroids or comets.
There is evidence that several mass extinctions occurred during the Precambrian Era and that five major extinction events have occurred since the onset of the Paleozoic Era (542 million years ago); the latter extinctions developed in the late Ordovician Period, the late Devonian Period, the late Permian Period, the late Triassic Period and at the end of the Cretaceous Period, each lasting millions of years. Based on fossil records, the Permian Extinction was the most severe, eliminating more than 95% of species on Earth, while geologic evidence suggests that the late Cretaceous Extinction, which ended the reign of the dinosaurs, was primarily triggered by an asteroid impact over the Yucatan of Mexico. Lesser waves of extinction have also occurred between these major events.
Many scientists believe that we have entered another period of mass extinction, triggered by the activity of humans; natural habitat destruction, environmental pollution, overhunting, overfishing and global warming from the industrial revolution are all playing a role. Unlike past, natural extinctions, human induced extinction can be slowed (if not eliminated) should we garner the collective will to minimize our impact. Of course, a failure to do so will threaten the welfare of our own species as much as (if not more than) the others that inhabit this planet.
