That's not how ancestry works.
First, generally speaking, exponentiation of ancestry breaks down rapidly (pedigree collapse), otherwise we arrive at the absurd conclusion that any living person has a trillion great-to-the-thirtieth-grandparents. In reality, go back far enough and ancestors start occupying many positions in the tree.
Second, obvious counterexample: suppose Salazar Slytherin marries and has one child, who marries and has one child, etc...and fifty generations later, there is still only one descendant of Salazar Slytherin per generation. This counterexample can be broadened; suppose the descendants of Salazar Slytherin's second child all died in the Black Plague. In short, it's not the case that a distant ancestor is either everyone's ancestor or no one's...not until you get to mitochondrial Eve and y-chromosomal Adam, anyway, but that's another story (and much older than 1000 years ago).
Actually, the exponentiation of ancestry proves that Salazar is the ancestor of everyone in Great Britain by the pigeonhole principle, except in your case where each of his descendants only has one child. That is an extreme outlier and not likely to happen, all things considered.
A recent paper shows that everyone who lived in Europe 1000 years ago is an ancestor of everyone living in Europe today (barring immigration of course):
...We can furthermore conclude that pairs of individuals across Europe are reasonably likely to share common genetic ancestors with
Two new short chapters! Since the next one is coming tomorrow and we know it'll be short, let's use one thread for both.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 105 (and chapter 106, once it comes out tomorrow). EDIT: based on Alsadius' comment about thread creation for MOR chapters, let's also use this thread for chapter 107 (and future chapters until this nears 500 comments) unless someone objects to doing so. Given that this is the final arc we're talking about, thread titles should be updated to indicate chapters covered.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: