 In Piper's Paratime universe, there are an infinite number of timelines, but in each timeline, events occurred differently.They are grouped into five Levels, based on the probabilities of success of an attempt by Martians to colonize Earth 75,000 to 100,000 years ago; humans, on timelines where they are present, are the descendants of the Martian colonists.Timelines with points of divergence before the Martian colonization attempt are not discussed.