Fossil-fuel burning, deforestation, overhunting and other human activities are driving more and more animals, birds and plants to extinction, scientists say. Since 1970, the number of vertebrates — mammals, reptiles, amphibians, birds and fish — has dropped by more than half, and almost 200 species have become extinct. The loss of so many species in such a short time signals that a mass extinction, in which at least 75 percent of all species disappear, is occurring, many researchers say. A mass extinction would take place over thousands of years, endangering the global food supply and perhaps even human survival. But other scientists deny such a catastrophe has begun. The losses are part of the planet's evolutionary history, they say, noting that as species die new ...

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