According to PNAS, the Proceedings of the National Academy of Sciences on January 19, 2010, a volcanic eruption in Indonesia 70 thousand years ago killed all but 15 thousand humans on Earth. That means, if we trace our ancestry back to the aftermath of that disaster, we have only 15,000 ancestors at most for the entire 7+ billion of us now.

I am sure you have noticed, looking at your family tree, that you have two parents, four grandparents, eight great-grandparents and so forth, each preceding generation having double the number of ancestors as the one after. Assuming the average age at which my ancestors begat my ancestors was twenty years (a conservative estimate, I believe) that means 3500 generations have passed since that terrible event in Indonesia nearly pre-empted my existence.

To calculate how many ancestors I have in a particular past generation, I need only raise 2 to the power of the number of generations back from me. That is, my parents are one generation before me, so I have 21, or 2, parents; 22, or 4, grandparents, etc. Since 70 thousand divided by 20 equals 3,500 (generations) the number of direct ancestors I would expect to have in the generation that survived the Indonesian catastrophe is 23500, or more than 4X101053 (four billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion, billion; that’s nine zeros for each billion) ancestors in that one generation alone. Clearly, in the words of Desi Arnaz, we have some ‘splainin’ to do.

There are four explanations that come to my mind:

- Some of my male ancestors had multiple wives/mates (polygyny).
- Some of my female ancestors had multiple husbands/mates (polyandry).
- There was quite a bit of mate swapping among the two groups above (polyamory).
- Some of my male and female ancestors were close relatives (incest).

The first should come as no surprise to anyone familiar with the Old Testament or Tales from the Arabian Nights, the second to no one who has taken a class in sociology, but the second two go a bit farther than what we were told in Sunday school or at bedtime.

Could it be that we owe our existence, individually and as a species, to both free and compulsory love, rather than the one-man-one-woman family values some would tell us is the divinely mandated norm of all but this wicked and perverse generation?

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