Extinction Trends
In general, the diversity of life on Earth has been constant or increasing slightly over the past 200 million or 300 million years. The geologic record shows new species emerging and old ones dying away, resulting in broad range of forms over geologic time. On closer inspection,the geologic record also shows distinct short-term increases in the rate of extinction termed extinction events. One such event occurred 65 million years ago and marked the end of the Mesozoic Era, the age of reptiles. For years, the cause of this event puzzled scientists. However, in the980 s thin layer of unique sediment containing the element iridium was discovered in sedimentary bedrock whose age coincided with this event. Research showed that this sediment was created by an explosion on Earth’s surface, probably caused by an asteroid impact. The explosion threw a huge mass of debris into the atmosphere, which, scientists reason reduced incoming solar radiation and cooled Earth. Plants declined and the food sources for many herbivores (plant-eating animals) apparently disappeared. followed shortly thereafter by a sequence of extinctions among predators.
Many other extinction events are also identifiable in the geologic record. Some occur at 26-million-year intervals, and scientists speculate that they may be associated with extraterrestrial impacts similar to the one at the end of the Mesozoic Era. Between these events, many smaller episodes of extinction are also apparent. The influence of humans on extinction patterns. however. does not appear until about 10.000 to 15.000 years ago.
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