Extinction
Extinction is the loss of a species, family, or order of living things from the Earth. Species become extinct if they lose their natural habitat, are relentlessly preyed upon by a competitor, or unable to adapt to a changing environment. In the course of evolution, one species becomes extinct while another replaces it. Crises in geological history are factors in extinction. During the Earth's history, four or five episodes of mass extinction interrupted long periods of diversity. Scientists use the term punctuated equilibrium to describe extended times of balance interrupted by relatively rapid changes.
An episode mass extinction occurred 250 million years ago at the end of the Permian period. At that time, the continents were merging to form Pangea, and tremendous volcanic eruptions in Siberia were spewing lava and ash into the atmosphere. Fossil records reveal the extinction of more than 90% of the species of marine animals alive at the time. Fifty percent of species did not survive into the next geological period. Another episode of mass extinction occurred sixty-five million years ago at the end of the Cretaceous period when the dinosaurs went extinct along with one third of the plant and animal species on the Earth.
To explain mass extinction, Charles Lyell in 1830, presented a theory of uniformitarianism. The theory states that biological and geological changes occur gradually over long periods. Slow processes cause species to replace one another or mountains to wear away. In 1973, Walter Alvarez, and other researchers from the University of California, Berkeley, exploring a limestone gorge in Italy, discovered a thin layer of clay that contained iridium. In 1980, the team interpreted the iridium layer as evidence that sixty-five million years ago a huge comet or meteor crashed into the Earth. Known as catastrophism, this theory states that a sudden and devastating event brought about mass extinction. It explains that the Cretaceous impact sent up clouds of dust that blocked the sunlight and cooled the planet. Not adapted to the new climate, the dinosaurs and other forms of life died out. Many craters exist on Earth as scars from the impact of extraterrestrial objects. In the early 1990s, scientists discovered a huge crater beneath the Yucatan Peninsula that they felt clearly supported the impact theory. Presently, scientists are finding other large craters and are trying to tie them to mass extinctions. Researchers, divided on the theories of uniformitarianism and catastrophism, remain puzzled as to the cause of mass extinction.
Human cultural development and increased population are responsible for over-hunting, habitat destruction, and environmental pollution. The six billion people on Earth require food, clothing and homes, and desire technological advances. They change ecosystems all over the Earth to fulfill their needs. They are cutting down tropical rain forests that are host to some of the planet's richest habitats. Ecologists report that the rate of extinction in the twentieth century is fifty times greater than the average for the past 100,000 years. They describe it as mass extinction with a devastating reduction in biodiversity.
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