Nick Ryder wrote:
Why should Hartnell be the '1st' Doctor that we just happened to be following for the past 50 years.
We've been told multiple times over the years that he was the first. The only story to imply that there were earlier Doctors was "The Brain of Morbius," but that was years after "The Three Doctors" had established him as the earliest incarnation of the Doctor. In "The Five Doctors" he said he was "the original, you might say." Various later Doctors, especially the past three, have been referred to by the corresponding numbers -- for instance, Dorium referring to "the Fall of the Eleventh" at Trenzalore.
Although, of course, the Doctor said that the Hurt incarnation was the one who broke the promise of the name "Doctor" and thus is not considered one of the Doctors, resolving the inconsistency. Still, in order for him to have broken the promise and been unworthy of the name, it seems he would've had to come after the Doctor had adopted the name and the promise that went with it; thus it seems unlikely that he preceded the First Doctor.
And really when you think about it - the Doctor has been doing this only for about 50 years - if we're to take all his other adventures as happening in real time sorta.
Except that in the '70s the Doctor generally said that he was in his 400s, but by the late '80s he was saying he was 900, a number the new series reused. And Eleven explicitly aged another 200 years before the end of last season; he's said to be 1100 now.
Personally I prefer not to take the age references too literally, since there are far too few gaps in the saga where the Doctor's without human companions long enough for decades or centuries to pass between aired episodes. But that last one is pretty unambiguous. And even though the Doctor claimed to be 900 from his sixth lifetime through his eleventh, a great deal of time certainly seems to have passed between the Seventh and Ninth Doctors.