And shozbot! There are a lot of them!
Not all of these would be candidates for the human encountered by the First Doctor. For example, that last one is of Mortimer Tregennis, a character from "The Devil's Foot". It's the account of one of Sherlock Holmes' cases while he was vacationing in Cornwall (not far from Portwenn), which was recounted by Dr. Watson and presented by his agent, Sir Arthur Conan Doyle.
While this murder mystery did take place, this particular version of it occurred in an alternate TV dimension, where Holmes looked like Douglas Wilmer and not Jeremy Brett.
As far as a likely candidate for a Troughton character, I figured it best to focus on one who might have a better chance at meeting the First Doctor.
And that's why I settled on Nasca, the Aztec high priest from 'The Feathered Serpent'.
From Wikipedia:'The Feathered Serpent' is a British children's television series. Set in Aztec Mexico and starring former 'Doctor Who' Patrick Troughton as the scheming High Priest Nasca. Two series were made for ITV by Thames Television and transmitted in 1976 and 1978.
In my readings about this series, I can't find anything definite as to the exact point in History when it took place. So I don't know whether it happened before the Doctor's arrival in 1450 AD. (The date is conjecture from "The Doctor Who Chronology", but the site-master's reasoning is sound for that choice.)
However, in the series the Toltecs are basically wiped out by the end of Season One, and that would have happened about two to three hundred years before the events of "The Aztecs".
But, as we all should know by now, Life continues off screen. Just because we in the Trueniverse don't see it happening that doesn't mean it didn't happen. (Known as The Khan-Chekov Encounter.)
When the Doctor and his Companions materialized in an Aztec temple. the Doctor met and inadvertantly married a woman named Cameca. By the end of the adventure, Cameca knew that she could not go with the man she had come to love, but hoped that he would always remember her.
It's assumed that he would, since he decided to take her token of love with him when the TARDIS departed that time.
So what if, in some untelevised adventure, the Doctor decided to go back and visit her? We know the Doctor had no compunction about loving humans in a carnal way in his later incarnations, so why couldn't the original bad boy get a bit of Terran nookie as well?
But as we all know, the TARDIS is very unreliable when it comes to delivering the Doctor to the time and place he wanted. For example, the Tenth Incarnation of the Doctor thought he and Rose were going to see Elvis at the Ed Sullivan theater in New York. Instead, they ended up in London in 1953, in time for the Queen's coronation. (And thankfully so, since they were able to defeat The Wire.)
So the TARDIS takes the Doctor to where he needs to be, not where he wants to be. And that's probably what happened in this case. Whereas the Doctor wanted to return to his "wife" Cameca at some point after his first visit in 1450, it's likely the TARDIS brought him to an earlier point in the Aztec Empire when the Totlecs were in decline.
And that's how the First Doctor could have met Nasca, the murderous high priest, and absorbed his DNA for future use.
Nasca has to be one of the most evil characters played by Patrick Troughton, and that's saying something since he played some right rotters in his time.
But the qualities of a man's character would not be a factor in the absorption of his DNA by a Time Lord's biological imperative. It was the physical aspects that were of concern. And somehow, the Doctor's bio-system knew that the strength exuded by Nasca would be of importance.
And it proved to be true, since the Second Incarnation of the Doctor had personality quirks that made him a bit timid and trepidatious. He would need all the strength he could summon.......
And perhaps some of Nasca's tastes rubbed off on the Doctor. He certainly had a flair with that disguise he wore in "The Underwater Menace"!
This is all conjecture, of course.....
(My thanks to the "Oh, My Giddy Aunt" Tumblr blog for the animated gifs.)