system
Given the following post, respond as if you're commenting on Reddit:
human
[Red Dwarf] What happened to humanity?

We know that Lister is the last human left alive three million years in the future.  Humanity had spread out among the stars.  So why was Lister the last human left alive in that distant future?  What happened to cause the extinction of the race?

Edit: three not one.
gpt
We never actually find out whether he really is the last human alive, Holly just assumes it. There are plenty of human creations still around - GELFs, the Holoship, the Justice World, simulants, wax droids, Legion, the polymorph, and Kryten was still functional when they found his wrecked ship. All still alive and functional. Its quite possible humans do still exist, and Red Dwarf is just nowhere near them... maybe in a "no mans land" region of space because of all the dangerous shit that's there. 

And, after the events of series VIII, we know for sure other humans do exist. The resurrected crew abandons the ship and probably are still alive somewhere.