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. Credit: Wikimedia Commons