Mass extinctions of the earth's living species have happened at least five times over the past few hundred million years, due to things like volcanoes, asteroid strikes, and changing sea or oxygen levels.
Now humans are starring in what's being called Earth's sixth mass extinction event.
Since 1900, at least 500 species of fish, amphibians, birds, mammals and reptiles have become extinct, and this is a very conservative estimate. According to the World Wide Fund for Nature at least 200 species are becoming extinct every single year.
De-extinction offers us the tantalising prospect of restoring species that have long since died out.
So, armed with the latest genetic technologies,how can scientists resurrect extinct species from the dead? And when and if de-extinction is possible then what species should we bring back? The moa? Or the T-rex?!
Helen Pilcher reviews the latest science and some of the controversies surrounding de-extinction in her book Bring Back the King: The New Science of De-extinction.