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.
Can we say that 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?
I like to fancy that it’s one of Jack’s Time Agent adventures, mostly ‘cause it’s a time frame completely forgotten about as time goes on. Jack did a lot back then–remember how he got them laughing during Boom Town with one wild crazy story? He’s got dozens of them before he ever became the immortal man.
What if one of the Agency’s policies was to ensure certain facts happened? Like, say, that mammals survived one of those epic meteoroids. They caught wind of an artificial thing happening (the crashing of a spaceship) and went ‘Oh shit. Got to make sure those forebears survive.’ So they send Jack and maybe a couple of others. But the impact knocked Earth off a bit (just a tiny smidge fraction) enough to fuck with the manipulator calculations. No rifts? No easy way to access the vortex. Took the three of them six weeks to reconfigure their manipulators and get home.
Oh I know that’s much more likely the case. I’d watch a show about nothing but Jack’s younger adventures. Or a book series.