In "The End of All Things", September claims that the Observers are a team of scientists from the far future, several generations removed from contemporary humans, having traveled to the past to observe the events that led to their creation.
In "The End of All Things", it is revealed that the group of Observers seen in the first four seasons are a team of scientists from the far future, or at least from one of humanity's many possible futures. This group of Observers traveled to their past to observe the events that led to their creation.