Everything is circular. What once was here and is now gone will be back again. Take evolution for example: suppose there was a nuclear holocaust that wiped out everything on earth (on a lot of peoples minds in the 50/60s); the theory was that cockroaches would probably have a higher chance of survival than any other species, and could grow unchecked into "super-sized" cockroaches. Take a leap and say that the natural predator of the cockroach survived, some kind of lizard. The reptile grows in size along with it's prey, and pretty soon they evolve into dinosaurs on our renewed, globally-warmed, tropical planet. Then a meteor hits and and there's a massive ice age, which then evolves climatically into a more seasonal model. The giant reptiles go away, and fish, mammals, and finally primates evolve. Warfare-prone hairless monkeys settle down and make tools and plant crops, build cities and monuments to their Gods, conquer each other and build empires, then finally have their industrial/technological revolution and blow each other up in a mushroom cloud as they compete for the few resources left on the planet. Then a cockroach emerges from the fallout...