“AMY: Now I get why Riddell’s here. Bit of brute force.
DOCTOR: He saved my life. Long time ago. This is the least I could do.
AMY: Why?
DOCTOR: In his timeline, he dies tomorrow. On the African plains. This is his last night.
AMY: This is your gift to him.“
–
The Doctor Who Companion: Editing Dinosaurs On A Spaceship
(Safe to say, that would’ve changed the tone of the ending.)