Fill in the Blanks FILL IN THE BLANKSOnline version Survival - Say how things could be different by Royner Villalobos Rojas 1 discovery species alive One scientific study suggests that up to two billion different may be now . That ? s a big number . However , since life began on Earth , scientists think at least five billion kinds of plants and animals have gone extinct . They know this from the of many unknown kinds of fossils : dead plants or animals that have turned to stone . 2 disaster survive Why did so many species not ? Some species died out slowly because conditions changed . Others were killed in a huge natural that happened about 66 million years ago . A big rock from space , called a comet , hit Earth . It killed the dinosaurs and caused terrible conditions . If humans had been alive at that time , they would have felt terror . 3 hero relationship rescue Many species are dying out these days , so some scientists think another mass extinction is happening now . They see a between human activity and these deaths . For example , they think the problem is happening because humans are causing climate change . Is there any way to species that are in danger ? Maybe . If a scientist found a way to stop climate change , he or she would be a to people who care about the planet .
New game