I actually just discussed this one in the course of a very long answer I happened to post a couple minutes before you posted this question. We have no direct onscreen evidence, but in the answer to question 1.19 from the Official BTTF FAQ, Gale and Zemeckis discuss the idea that in the BTTF universe there is a "self-preservation instinct of the spacetime continuum" effect that tries to nudge history back on track when it changes. And in my answer I talked about my speculations about what might happen if history was changed too drastically for this to work, I'll just quote the relevant section below:
What happens if the timeline is too badly altered to ensure that a certain time trip can happen "naturally"? For example, in the "1985A" version of history where Biff is rich and powerful (timeline 4 and 5 on the above chart), Doc has been put in a mental asylum, so unless he escapes and gets the money for equipment, he presumably can't build a time machine. Also, Biff says at one point to Marty "You're supposed to be in Switzerland you little son of a bitch!", indicating that the Marty who grew up in that timeline--let's call him Marty-A--was supposed to be abroad on that date. So it seems unlikely that the self-preservation effect could have ensured that Marty-A would meet up with Doc-A at the location of the Lone Pine Mall (if it even still existed in this timeline) and travel back to 1955 in a DeLorean time machine provided by Doc-A. In general we know the self-preservation effect can't be perfect since if it was there would be no danger of a time traveler erasing themselves from existence by changing the past.
One possibility here is that the "laws" of the BTTF universe would allow two alternate versions of Marty at the same biological age to coexist in a single timeline (with Marty-A still in Switzerland while the Marty from timeline 1 confronts Biff). Another is that Marty-A would simply be forcibly pulled out of time on Oct. 26 1985 of this altered timeline (perhaps along with a DeLorean sitting in a garage somewhere) and would spontaneously jump to 1955 with all memories and physical appearance reset to those of the Marty of timeline 1 upon arrival. This would be a sort of brute-force method the universe could ensure there is only a single version of Marty at each biological age (and biological age seems to be important in the BTTF universe, since the answer to question 1.8 in the official FAQ mentions they had wanted to have old Biff disappearing upon returning to 2015, because in the new altered history he had died at a younger age than the old Biff we saw).
If the brute-force method seems weird, note that there actually is precedent for it in this early BTTF script draft written by Gale and Zemeckis. In this version of the script, the changes Marty made in the past (1952 in this version) had more drastic effects on his present--when he returned to his own time he found a world filled with Jetsons-style retro-futuristic technologies, some labeled as being products of "E. Brown Enterprises", since Doc's experiences with Marty in 1952 led him to abandon time travel as too dangerous and focus his inventive skills on other things. So if the Doc of this timeline never built a time machine, what happened to the Marty who grew up in this timeline? Doc explains that he just disappeared on the appropriate date:
But if you didn't rebuild the Time Machine, how did I go back in time in the first place?
According to your girl friend, Suzy Parker, you and she were at the movies. You went to the restroom, and you never came out. Obviously, you stepped through an inter-dimensional time warp, created by the original operation of the time machine.