Irish Roots

Some deep genealogy

Mon, Nov 11, 2013, 01:00

There is only one thing certain about absolutely every ancestor you have: all of them had at least one child. This is obvious – otherwise you wouldn’t exist.

Does this mean that we’re all the winners of some sort of evolutionary competition to reproduce? With a world population of 7 billion, humanity can seem spectacularly successful, but self-congratulation is a bit premature. Before we all start clapping each other on the back and thinking of ourselves as champions bred of the loins of champions, it’s worth examining some details.

First, the genes of even the most fecund of our ancestors eventually cease to matter. A child receives exactly half of their genetic makeup from each parent, meaning that the original genome is diluted further and further with each generation. So it doesn’t matter if Niall of the Nine Hostages was your 35 times great-grandfather. There’s none of him left in you.

And what about all those who have no living descendants? Were they all spinster aunts and bachelor uncles? Not at all. Entire multiple-generation dynasties of the rich and powerful, spawning dozens of rich and powerful children who had dozens of children in their turn, have simply vanished from the face of the earth. Burke’s Extinct Peerages provides plenty of object lessons (and is always good for a little schadenfreude).

In fact, the iron laws of statistics show that there was a point, somewhere between 5 and 15 millennia ago, where each individual then alive was either the ancestor of every individual alive today, or has no living descendants at all. Genetic genealogy calls it the “Identical ancestors point”, because, logically, earlier than this point everyone now alive shares precisely the same set of ancestors.

Like a lot of genealogical musing, this stuff can seem very trivial or very profound, but it’s hard to stop thinking about it once you start. And yes, I do suffer from insomnia.