A pivotal episode 1.5 of The Crossing tonight, in which a big piece of the back story is revealed, and the stakes are dramatically raised for the near future.
The group that figures in both is the first migration from 200 years in the future, which we now learn got back here on a road late one night in 2008. Unlike the water arrivals, these folks apparently all survived, and hit the ground running with a plan to stop Apex from ever happening, by taking out the small number of humans identified as crucial to Apex’s later emergence and ascendance.
In this, The Crossing is revealed as partaking of a classic time travel gambit - prevent something bad from happening in your present by going back in time and eradicating its progenitors. Of course this plan will fail - if it had succeeded, there would have been no need to go back in time to prevent something which never happened - but the First Appearers are not quite that astute in their understanding of time travel and its paradoxes (at least not yet) and they come up with more drastic plan which will call for far many more people to die, after The Firsters realize their first plan didn’t work.
Interestingly, they see the evidence of their first plan’s failure in the Second Migration in the water, which until tonight had been the main focus of the series. And this means the First and Second Appearers are on a bigger collision course than we first supposed. The Firsters now want more than keeping the Seconders secret so as not to draw any attention to themselves - they want the Second never to have happened in the first place.
Never mind that if Apex is prevented from happening there will be no migrations from the future at all. This conflict between First and Second Migrants makes for a strong upcoming story.