Okay, I had a random thought:

Jack mentions in Fragments that he was on Earth sometime around the extinction of the dinosaurs (”Dinosaurs? Had them for breakfast. Had to. Only source of pre-killed food protein after the asteroid crashed. Long story.”)

Of course we know that it wasn’t actually a meteor, it was a spaceship. That had Adric on board when it crashed. (Earthshock)

So.

What if Jack Harkness was there to save Adric as a favor to the Doctor without him knowing? And then dropped him off somewhere he wouldn’t cross the Doctor’s path again?