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Prior to the 1980s, the causes of the breaks were unknown...

In the late 1970s, a team of geochemists headed by Luis Alvarez and his son Walter Alvarez, both with connections to the University of California at Berkeley, was studying chemical changes in soil layers corresponding to breaks in the fossil record.


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Mammals were not common until after 65 My ago, and humanlike creatures appeared only in the last 4 My.

[Side comment: The above description has been challenged for two centuries by religious fundamentalists.

Early calculations in the 1980s (using in part ideas worked out by Carl Sagan and his colleagues) showed that so much dust entered the high atmosphere that the Earth was shrouded in a dust layer that blocked sunlight for several weeks or months.

This would have killed some plants, disrupting the food chain.

Indeed, these breaks were the basis for dividing geologic time into different eras with different names.

For example, the Paleozoic era gave way to the Mesozoic era of middle life forms, which in turn gave way to the present Cenozoic era of recent life.Many asteroids of this type are now known; their orbits pass through the inner solar system and cross Earth's orbit.Some of these could potentially hit Earth in the future.Most, but not all are smaller than the one that hit us 65 million years ago.Fossils found in soil layers of different ages show a record of slow, gradual changes in species, with simple organisms gradually being replaced by more complex organisms, apparently by evolutionary processes driven by natural selection.This method of learning about nature continues to be under challenge in America from fundamentalist groups, for example in school boards and state legislatures.

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