Bring back the woolly mammoth?
Scientists estimate that throughout the course of Earth's history, more than a billion different species of animals and insects have million species left, according to the World Animal Foundation. That means that for every species living on Earth today, 20 more are already extinct. Scientists study extinct species to get a better understanding of the past. In addition to studying what these species might have been like when they were alive, some scientists are interested in the possibility of seeing some of them walk the Earth again.