I’m my own Grandpa.

An in-law once told me, in the course of a political argument, that for such a smart person I had no common sense.  This struck me as a strange thing to say, not least because it was taken pretty much verbatim from a campaign slogan used against Lyndon Johnson’s father in, like, 1918.  But it is true that I have always been fascinated by the places where, if we want to get things right, we have to abandon our everyday heuristics and actually do the math.

I have a lot of respect for the accomplished cook who rarely measures and the carpenter who can eyeball the correct proportions.  But these skills are fine-tuned to a narrow range of experience—they wouldn’t get you far  in weighing a molecule or a planet.  And one thing that is almost guaranteed to take you out of common-sense range is a geometric progression.

For example, think about ancestry.  People with a decent amount of luck will be familiar with both of their parents and all 4 of their grandparents.  But can you name all 8 of your great-grandparents?  I certainly can’t.  It gets worse from there, and if we know anything, it is probably about a very pruned-down version of our ancestral tree.

My oldest brother is working on a family history, and has restricted it to the paternal line of descent, a questionable decision on both ideological and historical grounds (my siblings were mostly  raised as urban Irish-American Catholics like my mother, not quasi-Southern farm boys like my father).  Still, he had to use some kind of filter in going back 200 years, since at that time we probably had several hundred living ancestors.  If we estimate 25 years per generation, then going back 200 years gets us to 256 great^6-grandparents.  Not all of them would be alive at the same time, but there would be many from the neighboring generations.  That would be a rather daunting research project.

While patriarchal bias is undoubtedly the most common way to distort and simplify our inheritance, there is a complementary modern alternative.  I believe that heritage is sometimes traced using mitochondrial DNA, which is inherited only from the mother’s side, and that people think of this as embodying half their background.  But of course it is only one-half for the first generation, and narrows to a tiny fraction of the whole picture just as fast as the patrilineal model.

Taking another step back, if you had an ancestor on the Mayflower, that person might have contributed one 65,536th of your DNA…hardly seems worth making a fuss over.  At this point we begin to encounter another factor that most people probably don’t think about: some of your ancestors must be each other’s cousins.  Look at it this way—it is simply not possible that you had 1 billion living ancestors in the year 1000, since there were nowhere near a billion humans alive at the time.  My guess is that you didn’t have a million living ancestors in 1500 either  Just how close to the present the significant duplication of ancestors comes will depend on your background.  If you are a Bourbon or Romanov, it will come very close indeed; damage due to inbreeding may be added to the other repellent features of aristocracy.

On the other hand, if your background is aboriginal Australian, your ancestors probably prevented short-term duplication through some of the world’s most hypertrophied incest taboos, often making one’s entire village off-limits for marriage.  There is a certain irony in this, since aboriginal Australians were a small population largely isolated for thousands of years, making their ancestral tress  more like an ancestral topiary maze.

Going even further back, the genetic homogeneity of modern humans suggests that there have been bottlenecks in our history, times when the ancestral population was reduced to a few thousand individuals.  This is especially true for those whose ancestors left Africa, but not only for them; everyone alive today shares common ancestors who lived something like 130,000 or 140,000 years ago.  Consider such a bottleneck, and consider that, statistical distributions being what they are, some of those few thousand people of reproductive age will have had more descendants than others,–it seems very likely that at least one of them will be connected to you by so many lines of descent that she contributed one 500th, say, of your DNA.  This means that someone in the Danube valley 40,000 years ago or in North Africa 70,000 years ago, living in a cave decorated with red ochre and speaking an unknowably alien language,  contributed more your genetic makeup than that dude on the Mayflower, or his equivalent in Ireland or Korea or wherever.  Kinda defies common sense.

 

(This post was partly inspired by Chris Stringer’s Lone Survivors.  Stringer is one of the main advocates of the Recent African Origin theory of human evolution.)

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