Saturday, July 2, 2011

an anthropologist explains descent to fiction writers

I've been reading a lot of young-adult fiction this summer, looking for things to read to Bunny at bedtime, and also scoping out her options for books with positive social messages (gender roles, diversity, etc.) when she is old enough to read herself. Ancestry, and the talents/privileges/burdens that come with ancestry, are frequently a theme in these books. Fantasy is particular prone to this - all those poor orphan boys with their hidden destiny to take their "rightful" place on their great-great-great-great-great-great-great-great grandfather's throne.

The last two books I've read happen to fall into this category. Rick Riordan's The Kane Chronicles, follows two young siblings who are the most powerful magicians born in millenia, because they are born from two separate lines of Egyptian pharaohs; their father was a descendant of Narmur, while their mother was a descendant of Ramses the Great. They've inherited their predecessors' magical powers, and hey! the boy even looks like the portrait of Narmur on the famous palette!

Now, let's see, Narmur lived about 5,000 years ago, which is about 250 human generations. If none of their ancestors married other descendants of Narmur (more on that later), then Narmur contributed about 1/1.8x10^77 of the child's genome. Wow, them's some powerful genes!*

My second case in point can also be attributed to Mr. Riordan. Scholastic printed ten books in the 39 Clues series, all written by different authors, but Riordan was the author of the first book, and created the overall story arc. In this series, two young siblings are members of a powerful family that have contributed most of the important historical personages of the last 500 years. There are four branches of the family, from four original siblings, and the branches have distinct talents and personalities. Descendants of Jane, for example, are artistic geniuses (Mozart, Charlie Chaplin, etc.), while descendants of Luke are brilliant strategists (Napoleon, etc.).

Ignoring the improbability of a deep, yet meaningful, family connection between such diverse people as Benjamin Franklin and Toyotomi Hideyoshi, let's think about the logic of four separate branches of the family lasting 500 years. Around 25 human generations have passed in those 500 years. A person living today would have around 2^25 ancestors that were alive 500 years ago, or 33,554,432 people. The current world population is estimated at 6.9 billion. So, if everyone alive today had separate ancestors, there would have had to have been 2.3x10^25 people 500 years ago. Obviously, that's not the case. Estimates put world population 500 years ago at only 500 million. So any two people likely share a large percentage of their ancestry, whether they know it or not. This is particularly true for people of European descent, as is (of course!) the fictional family. Today, the population of Latin America, North America, Europe, and Australia is largely European, but Europe had a population of only 50 million or so in 1500. In other words, even if you accept the idea that each sibling of a certain family had personality traits and talents that they passed on to all of their descendants, those descendants would be so heavily mixed by now that every descendant of one sibling is probably a descendant of them all.

You know how every U.S. president is eventually found to have a European king in his background? That's not because kings' distant offspring are more likely to be presidents (although they might, for various reproductive and economic reasons), but rather because everyone likely has a king in their background. Almost all Europeans are probably related to Charlemagne. There just aren't enough medieval Europeans to account for our ancestry, otherwise.
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*That number is not realistic for a variety of reasons, but you get my point.

10 comments:

  1. So does that mean I'm related to almost everyone 400 or 500 years back?

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  2. I think it means any two people alive today are related through some common ancestor{s} within 500 years. Is that what this means?

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  3. It's a little more complicated than that, because people's choices of partners are not random. That is, we tend to have children with other people with whom we share a religion, or class background, or ethnicity. Plus, 500 years ago, fewer people were able to travel as far as most of us can today. So, people from far eastern Asia, far southern Africa, and far western Europe were unlikely to meet, unless all three went to the same American colony. However, if two people share a background (both European, say, or both from West Africa), then the likelihood that they share at least one ancestor in the last 500 years would be incredibly high.

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  4. From what I've got from my own research, people of African, Asian or European descent DO likely share common ancestors 300-500 years ago in the past. This only strictly applies to these populations, not totally isolated ones like Native Americans, Australian natives, etc. Mating is random over a certain period of time and the age of exploration helped.

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  5. Yes, I'm sure you're right that people of African, Asian, and European descent do share common ancestors. That would be particularly true in the Americas, but elsewhere, too. But, it seems to me, the closer two peoples ancestors were geographically to each other 100 years ago (or 200), the more likely they are to share ancestors over the last 500 years. I assume that I share a number of ancestors with almost anybody who is also largely of Bavarian or Irish decent. I may share an ancestor in the last 500 years with someone from Japan or Mozambique, but it seems likely that we would have fewer overlaps, unless that person, too, was largely of Bavarian and Irish decent. In the context of the particular Riordan book I was discussing, the likelihood that an individual person descended from two, three or all four of the original siblings would be much higher if that person was of European decent, since the original family was from Dublin.

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  6. Yes, some people are more related than others but we still share common ancestors to some degree.

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  7. Wait. When you said any two people likely share a percentage of their ancestry in common, did you mean any two people or just European-descended ones?

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  8. Any two people likely share a percentage of their ancestry, if you go back far enough. But, the likelihood is much higher for people whose ancestors were more likely to encounter each other. For example, all Europeans are more likely to be related to any other European than they are to someone whose whole ancestry is from east Asia. But, there's been a lot of movement around the world, so, for example, people in the Americas are mostly a mixture of lots of different ethnic groups. Much of Asia and eastern Europe can trace at least part of their ancestry to Ghengis Khan, etc.

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  9. When you say "If you go back far enough" to share common ancestry, would you say, for people from non-isolated populations, 400-500 years? Yes, I know people of more similar ancestry share more ancestors than people of other ancestries but they do share common ancestors to a lesser degree in that approximate timeframe mentioned above.

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  10. I read an article by Ned Kock that states most of us {assuming we came from non-isolated populations, as stated above} share at least some common ancestors 640 years ago and that most of us are much closer. So would 400-500 years be too much of a stretch?

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