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?

Random theory about Mummy and Sherlock Holmes:

So mum was a brilliant mathematician who gave it up to raise children, as was expected at the time.

What if she planned on going back to her career when Mycroft was in school, only to suddenly find herself pregnant with Sherlock.

Might she resent that, just a bit?