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deep ancestry

2 september 2008

One generation back, you had one male ancestor. Two generations back, you had two (your grandfathers), and three generations back you had four (your great-grandfathers). Four generations back, you had eight great-great-grandfathers. Two thousand generations back, how many male ancestors did you have?

If you answered 2 to the 1,999th power, a figure with roughly 600 zeroes, you probably figured it wasn't correct; it assumes that there were 21999 men alive 60,000 years ago, not to mention their mates.

In Deep Ancestry, Spencer Wells calculates that each of us had the same number of male ancestors 2,000 generations ago as we did one generation ago: one. One guy. One special guy, as the song in Bye, Bye, Birdie has it.

That's the way it should be. In fact, this is probably pretty elementary stuff in any population-genetics course. As you go back further and further in time, more and more of those greats-grandfather turn out to be the same individual. The more cousins that figure in the mix of each previous generation, the fewer the relative number of grandparents.

That there should be a bottleneck of one in the male line as recently as 60,000 years ago, though, is both startling and sobering. It is not that long since many anthropologists argued that the current human species developed slowly out of relatively-isolated populations spread around the globe for hundreds of thousands, even millions, of years. The most recent genetic research, however, suggests that we are all descendants of one male clan – still rather distantly in the past, of course, but hardly at an unimaginable distance. Our species is quite a bit older, and our single female line is about three times as old as the male line (because that 60K-ago Adam had kids by a number of wives). But homo sapiens is much more a single family than anyone would have dared to speculate until very recently.

It's a comforting meme, though one that is unlikely to lead to immediate world peace. The story of Adam, after all, tells us that the human family has a single father; but the story of Cain and Abel tells us that it takes about one generation for that family to fall together by the ears.

Spencer Wells works with the Genographic Project to gather data on DNA from around the world, with a view to seeing if prehistoric trends in human migration can be inferred from the DNA both of indigenous peoples and their more mobile relatives. Deep Ancestry suffers a bit from an attempt to tell us, in just over 200 pages, everything about that project, the broad-brush picture of how it has reconstructed the spread of the human clan, the human-interest stories of individuals who contributed DNA to the research, and parallel research from many other sciences, plus just about every analogy or metaphor that Wells has ever thought of.

It's not a confusing book, though. It points to an interesting trend in research on human origins, which has changed as dramatically as any body of knowledge during my lifetime. Every year or so we get a rewritten story for the general public, confidently confuting the previous stories, and every year the previous story is proven deeply wrong. It's not the fault of scientists, who by and large are attentive to the factual implications of their research. It's more the fault of rhetoric itself. People will not buy books that say "we dunno much yet" (even though Wells himself is careful to note that much, much more needs to be learned). I suspect that the story Wells narrates here will change so much in the next ten years as to be largely unrecognizable then. But it's the provisional master narrative that we have now. And what a nice one to have arrived at: alle Menschen werden Brüder, at least provisionally.

Wells, Spencer. Deep Ancestry: Inside the Genographic Project. Washington: National Geographic, 2006.