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In the beginning of the 19th century, people didn’t even believe living species could become extinct. Thus, when Lewis and Clark explored the unknown American Northwest, Thomas Jefferson was surprised no live mastodon was found, even though their bones had been found in New York. In the following two centuries, scientists have uncovered countless fossil records that provide evidence of five major mass-extinction events in the last 540 million years. Moreover, scientists have estimated that more than 90% of all living species of plants and animals that ever lived are now extinct.
The world environment has not been constant. When a living species could not adapt to the changes in the environment, they died. New species have evolved to cope with a different environment. Throughout the history of earth, mass extinction events have occurred to various extent, but in a major mass-extinction event, at least half of all living species have died simultaneously or over a short time. Permian-Triassic, the largest extinction event, occurred 250 million years ago when only 5% of living species survived. Life on earth recovered with many new species. Dinosaurs, having lasted over 160 million years, disappeared in the fifth event that occurred 65 million years ago. The void left by the extinction of 76% of living organisms has been replaced mainly by mammals.
What were the possible causes for these mass extinctions? Fossil records in stones and deep ice have been helpful as evidence of past events. From growth rings on plant fossils, chemicals found in rocks and ice and gases trapped in different depths of ice, scientists have theorized the conditions of ancient times. Earth’s climate has varied from ice age to tropical heat, and from steamy jungles to searing deserts. When such climate change occurred abruptly – either in the form of a global warming or cooling – animals and plants had no time to adapt, resulting in mass extinctions.
What were the causes for environmental changes? Volcanic super-eruption and asteroid collision could have created tons of debris into the atmosphere to block sunlight for many months, so plants and plant-eating creatures would quickly die. Such large disruptions could have unleashed toxic and heat-trapping gases that resulted in rapid global warming. Also, they could have led to tsunami and land movements which endangered living things. Other causes for extinction could have been the imbalance of quantities of gases – oxygen, CO2 and methane. Aerobic species needed oxygen to live. Excess CO2 and methane caused global warming, but their deficiency led to excess global cooling. Excess CO2 acidified sea water so that marine lives died off. The fall of sea level in glaciation and its rise in ice melt harmed living things, too.
The drastic changes in the environment in the earth’s past 540 million years have led to five major mass extinctions. By understanding the survival of living species requires a delicate balance of conditions in nature, we can evaluate, in the next installment, if world conditions forebode survival problems ahead for us.