Of all the species that have ever lived on Earth, only about 1 in a 1000 of them are still around.

The vast majority of the extinct species died out within ten million years of their first appearance in the fossil record. This is a very short time. This is probably evidence of evolutions which were not successful -- species were produced that ultimately had evolutionary disadvantages.

Sometimes many, many species all seem to disappear from the fossil record at the same time. We call these disappearances mass extinctions.

http://www.carleton.ca/~tpatters/teaching/intro/extinction/extinction1.html

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