The First Timeline is the very first course of events in the history of the multiverse. This is never shown on Fringe, but must have existed at some point prior to the beginning of the series. Nothing is known about the First Timeline except that in 2167, Observers are created. After the Observers travel back in time, the timeline is overwritten by the Original Timeline, which is presumably identical to the First Timeline (save any Observer sightings) until 1985.