He is referring to the inevitable extinction event that will cause humans to be wiped out. The history of the planet suggests that it periodically wipes the slate nearly clean. And while there are human reasons we might get wiped out (war, warming, techno catastrophe) even being ideal stewards will eventually result in the loss of the human race. The only survival option is to not be on planet when the 'bad thing' (what ever it happens to be) happens.