One of the earliest character outlines of the Doctor (or Doctor Who as he was being called at this point) runs as follows -
DR. WHO A frail old man lost in space and time. They give him this name because they don't know who he is. He seems not to remember where he has come from; he is suspicious and capable of sudden malignance; he seems to have some undefined enemy; he is searching for something as well as fleeing from something. He has a "machine" which enables them to travel together through time, through space, and through matter.
By May 1963 this had been expanded somewhat with the an indication that the Doctor may be more than human -
DR. WHO
A name given to him by his three earthly friends because neither he nor they know who he is. Dr. Who is about 650 years old. Frail looking but wiry and tough like an old turkey - is amply demonstrated whenever he is forced to run from danger. His watery blue eyes are continually looking around in bewilderment and occasionally a look of utter malevolence clouds his face as he suspects his earthly friends of being part of some conspiracy. He seems not to remember where he comes from but he has flashes of garbled memory which indicate that he was involved in a galactic war and still fears pursuit by some undefined enemy. Because he is somewhat pathetic his three friends continually try to help him find his way 'home', but they are never sure of his motives.
THE SHIP
Dr. Who has a 'ship' which enables them to travel together through space, through time, and through matter. When first seen, this ship has the appearance of a police telephone box standing in the street, but anyone entering it finds himself inside an extensive electronic contrivance. Though it looks impressive, it is an old beat-up model which Dr. Who stole when he escaped from his own galaxy in the year 5733; it is uncertain in performance; moreover, Dr. Who isn't quite sure how to work it, so they have to learn by trial and error.
Other comments in the preparatory documents indicated that he might possess some great secret, though Sydney Newman didn't like this idea. It was always indicated that he was from the future, but was somehow cut off from, or possibly exiled from, this future civilization. In some documents his home is a future civilization on a distant planet, but the interesting point though is his suggested age - 650 years old. This however is the only indication that he may not be simply a future human. And, what I find most significant, as soon as script editor David Whitaker came on board in July 1963 the Doctor's age was modified to a far more obviously human age of 'over sixty', thus removing the only real indication that the Doctor wasn't human.
The original pilot has Susan stating she was born in the 49th century. In the transmitted An Unearthly Child this is changed to the more vague 'another time, another world' which, although this may hint at alien origins, still doesn't definitely prove they were not simply future humans from another planet. This is certainly what I believe was David Whitaker's intention.