Riddle: A certain large animal lives happily and thrives here on Earth. One day, every single one of these critters is wiped out by a mysterious disease which affects only this particular animal.
There are none left anywhere on earth -- they are all gone. About a year or so later, they begin to reappear on Earth again. How can this be?
Answer: The animal is the Mule. Since all Mules are born sterile, you can only get a Mule by crossing a donkey with a horse. That is how the species is able to repopulate itself.