How do we know that snowman is the survivor of an apocalypse and is the last genetically normal human?

It opens with Snowman, who as far as he knows is the last fully human survivor of an apocalyptic event. Snowman is the sort of caretaker and lawgiver to a group of sentients he calls the Crakers. As the story unfolds , largely in flashback, we understand that all "natural" humans have perished from an apocalyptic virus save for Snowman (Jimmy) who was protected in a special place in the paradise complex.