
The fact that the oldest TV sci-fi hero can change their appearance has proven handy for longevity. Once Doctor Who introduced the concept of regeneration in 1966, any actor, of any background could, technically be the Doctor. But, within the Who canon, a Time Lord was always detectable. If Daleks and Cybermen didn’t recognize them, then the two hearts would give the Doctor away. That is, until 16 years ago. On May 26, 2007, Doctor Who dropped the episode “Human Nature,” and changed the rules of the Time Lord game forever.