The "big five" are not Earth's only extinctions but they have been the most devastating.
Fossils of marine animals indicate that each event wiped out at least 17 percent of families.
(A family such as canids can have thousands of species.) Scientists are still puzzling over
what caused the big five-climate change,perhaps caused by cosmic impacts,is a leading
suspect-but experts agree that human kind has ignited the sixth.Mass extinction can last for
millennia,and it takes millions of years for new species to make up the loss.The animals
illustrated here represent what has already vanished and what could be next.
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