The Earth has lived through 5 mass extinctions of animals and plants. The last one - 66 million years ago - took out the dinosaurs. That extinction is believed to have been caused by a giant asteroid smashing into the Earth. Now we are living through extinction number 6. And the cause is us. Human behavior. Elizabeth Kolbert is staff writer at the New Yorker, covering environmental issues. Her new book is ‘The Sixth Extinction: An Unnatural History.'