Floods, droughts and fires caused by climate change could cause as much damage as an asteroid or a massive volcanic eruption. Here's what the experts say. The most famous mass extinction in Earth's history occurred 66 million years ago, when a gigantic asteroid wiped out the dinosaurs and many other living species. And while it was a major event, wiping out about 76 percent of the world's species, there have been two more devastating apocalypses in the 4.5 billion years of our planet's history, for a total of five. MailOnline asked scientists what could cause the sixth mass extinction and when, if this scenario is already underway, it could happen. (More on passionastronomia.it)