Biological species emerge and disappear in the natural course of evolution. There have been times in the Earth’s past when mass extinctions have occurred, causing a large number of species to disappear in a short time. This is generally believed to have happened due to external pressures on ecosystems, or sudden shocks and catastrophic events. We are currently going through such a period due primarily to the impact humans have had on the environment, and on plant and animal habitats. Some scientists believe that 50% of existing species may become extinct in the next 100 years.
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