The surprising connectedness of human genealogies over centuries

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A new article by Adam Rutherford in Nautilus may be a good one for students in my genetics course this upcoming semester: “You’re Descended from Royalty and So Is Everybody Else”. The article is an excerpt of Rutherford’s book, A Brief History of Everyone Who Ever Lived: The Human Story Retold Through Our Genes.

Chang’s calculations get even weirder if you go back a few more centuries. A thousand years in the past, the numbers say something very clear, and a bit disorienting. One-fifth of people alive a millennium ago in Europe are the ancestors of no one alive today. Their lines of descent petered out at some point, when they or one of their progeny did not leave any of their own. Conversely, the remaining 80 percent are the ancestor of everyone living today. All lines of ancestry coalesce on every individual in the 10th century.

When you get up to around a trillion potential lines of genealogy, you have room for some surprisingly long-distance connections. Still, what people tend to ignore about this kind of logic is that incredibly tiny fractions of the genealogical tree are not different from zero when it comes to DNA ancestry.

The argument is based purely on number logic, and is not falsifiable by any empirical observations, short of impossibly complete genealogical knowledge.

In my opinion, we should be a bit more conservative (as Rutherford also reflects in this excerpt). But still, it’s very likely that common ancestors of all living humans have lived within the last 2000 years.

By the same logic, of course, every living human is a descendant of Neandertals. And every Neandertal that was an ancestor of the last Neandertal-modern hybrid was also an ancestor of all of us.