walkitout (walkitout) wrote,

50th cousin assertion update

The "some geneticists believe" assertion from Cecil Adams "Straight Dope" quoted in the wikipedia article on pedigree collapse was a cite to Alex Shoumatoff's _The Mountain of Names: A History of the Human Family_, which is an interesting and probably worth reading book that I'll be poking at over the next few days. In the interim, I did find the 50th cousin quote in it on page 244 of the Kodansha Globe paperback. While my copy dates from later, the original was published 1985 and excerpted earlier in the New Yorker and therefore written at some unknown earlier date. Here is the relevant bit:

"Most geneticists are in agreement that, as the science writer Guy Murchie explains, "no human can be less closely related to any other human than approximately fiftieth cousin, and most of us are a lot closer... [ that, in other words ] the family trees of all of us, of whatever origin or trait, must meet and merge into one genetic tree of all humanity by the time they have spread into our ancestors for about fifty generations."

That's bad enough (and obviously now known to be false on multiple levels). However, Shoumatoff exacerbates Murchie's understandable errors from the mid- to late 1970s and expands upon them.

"The main point is, rather, that each of us contains genetic contributions from practically everybody who ever lived. All it takes for widely divergent populations to merge genealogically is migration by one person. "A single indirect genetic contact between Africa and Asia in a thousand years can make every African closer than fiftieth cousin to every Chinese," Murchie writes. "Surprisingly, this may happen without any natives of either continent doing any particular travelling at all, but simply in consequence of the wanderings of nomads in intermediate territory." And Lewontin remarks that "a very small amount of migration -- as little as one migrant individual exchanged between groups in each generation -- is quite sufficient to prevent differentiation between groups by genetic drift."

Guy Murchie is _Seven Mysteries of Life_, 1978
Richard Lewontin is _Human Diversity_, 1982

I've ordered the Murchie (because it has unbelievably good reviews on Amazon); I'll look at the Lewontin next but I doubt I'll buy that one as it sounds textbook-y.

I'm mildly curious as to the math underlying the "single contact in a thousand years" and the one guy per generation idea. I suspect what happpened here is a bad interface between the people-who-do-math and the people-who-understand-human-behavior. There are a lot of indications that Shoumatoff is one of the latter, rather than the former. It was clear that Rohde et al were members of the former, rather than the latter. I want to find the person who both understands math and understands human behavior AND thinks this comes down to fiftieth cousins. I don't think they existed even in the 1980s. They _definitely_ don't exist now. The error about DNA-in-us-from-everyone is just wildly fantastic now (and, arguably, was then as well, altho I'm less certain about that).

This feels like an echo chamber "fact", sort of like that one from the bicycling community and people in America and how far they live from where they work. Obviously, on the face of it, cannot be true, yet repeated hither and yon for reasons that make excellent psychological sense but no sense whatsoever in terms of reality (and boil down to, "we _want_ it to be true").
Tags: genealogy
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