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             Evolution is a process of gradual change. The word evolution may refer to various types of change, but most commonly refers to the formation and development of life on earth. The idea that living things evolved from non-living matter and changed through the ages is called the theory of evolution. According to this theory, the first single-celled organisms appeared about 3 or 4 billion years ago, soon after the earth's crust had formed and cooled. As time passed, more complex organisms gradually developed specialized characteristics that helped them adapt to their environment. The evolutionary process eventually produced all the species that exist on the earth today.
             Most evolutionary changes occur too slowly to be observed directly. Scientists, however, have found much evidence to support the theory of evolution from various sources like fossils, adaptations in organisms, comparative studies of species and embryology. Fossils provide the most direct evidence of evolution. Scientists are able to determine the age of fossils by means of radioactive dating. The fossil record has many gaps because only relatively few species were preserved in fossils. However, scientists have found enough fossils to document much of the history of life on the earth. Many fossils reveal different stages in the evolution of life from simple organisms to progressively more complex species. Some of the oldest fossils are imprints of one celled organisms. More recent fossils disclose a variety of species with increasingly complex adaptations. Horses are among the best-documented examples of evolutionary development. Fossils show many stages in the evolution of the horse. .
             According to the theory of evolution, changes in an environment may cause organisms to develop various adaptations. During most of the earth's history, environmental changes have occurred in gradual stages that have lasted hundreds of years. Since the 1800's, however, the environment has changed at a much faster rate.

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