Most Recent Common Ancestor, revisited

I’ve been doing some more thinking after my post of last weekend.

1) Rates of inter-country migration: The results of my poll are really rather startling. OK, it’s not completely scientific – there are a few inconsistent answers about grandparents, and one case of siblings giving different answers about their own parents (cough, cough). But I think it’s interesting.

94 of you answered all three questions. Of your 188 parents, 32 (17%) were born in a different country to you; and of your 376 grandparents, 87 (23%) were born in a different country to you. This compares with the inter-country migration rate of one in a thousand (0.1%) assumed by Rohde in the most conservative of his models. I’m sure that those of you who read my LJ are more likely than the norm to to have an interest in travel, as I do, and to have had parents who, like mine, were also interested in travel, but even so I’m sure that one in six is close to the real rate of inter-country migration than one in a thousand.

Noteworthy also that the survey results do not show a random distribution. If one of your parents was born in a different country from you, it is more likely than not that the other was as well.

It’s only impressionistic, but it seems to me that migration from country to country has been pretty frequent for a very long time. As I look at my history shelf, the nearest book is F.E. Peters’ study of Jerusalem, which chronicles the waves of settlement over that city in the last 3000 years starting when David conquered it from the Jebusites. Next along is my abridged version of Gibbon’s Decline and Fall of the Roman Empire. On the same sheld I have Peter Balakian’s account of the Armenian genocide and diaspora, Keay’s History of India and a collection of essays on the life of the much-travelled twelfth-century queen Eleanor of Aquitaine (whose son Richard I, if you remember, was married to a Spanish princess shortly after conquering Cyprus). Probably it’s even easier now than it was, and probably more people can travel now than did a hundred years ago; but moving to different countries has probably been part of human life since there were different countries to move to.

2) Age between generations. I wrote before that [Rohde’s] model assumes that women have an equal probability of bearing children every year between the ages of 16 and 40, thus giving an average age difference between mothers and their children of 28. I reckon this flattens out the natural bump (!) at the lower end of that age range, and my suspicion (without any proof) for most of human history is that most children were born to women aged between 14 and 30. That too will decrease the time to our most recent common ancestor, as the time between generations will be shorter.

replied, Most children may have been born to women between 14 and 30 but the ones born to older mothers may be more likely to have survived (and their mothers to have died as a result or shortly after). A large number of children born to younger mothers succumbed to poor infant care (nursemaids dropping babies off battlements, bizarre infant-rearing theories not contradicted by inexperienced parents, malnutrition etc).

It’s an interesting point. Rather arbitrarily picking the year 1250, and the European monarchs then in power according to Wikipedia, and filtering down to those whose birthdates (and whose spouses’ and childrens’ birthdates) I could identify pretty quickly, I find that mothers gave birth between the ages of 13 and 39, average age 25; and fathers had children between the ages of 15 and 45, average age 32. So in fact Rohde’s average of 28 years between generations is not too bad, for monarchs in the mid-thirteenth century at least, though I suspect the comparatively younger age of mothers will shorten the time to the most recent common ancestor.

(data set: James I of Aragon and Yolande of Hungary; John I of Brittany and Blanche of Navarre; Ferdinand III of Castile and Jeanne of Ponthieu; Henry III of England and Eleanor of Provence; Margaret of Constantinople, countess of Flanders in her own right, and William of Dampierre; Béla IV of Hungary and Maria Laskarina; Håkon IV of Norway and Margrét Skúladóttir; and Henry I of Cyprus and Plaisance of Antioch – counting only children who survived to adulthood, ie older than 16, and only parents who were alive and married in 1250; which gave me 40 child/mother pairs and 47 child/father pairs).

3) The influence of people with lots of progeny. I have no research of my own to add to this, but I’m sure that most of you will have seen this week’s news item about Niall of the Nine Hostages (see full research paper here (PDF)). News reports that he may have 3 million descendants alive today are exaggerated. Under-exaggerated, that is. That’s three million male-line descendants – the real number of descendants must be much higher, indeed, Niall himself is at about the right time-frame to be Rohde’s Most Recent Common Ancestor (though not in the right place; difficult to see how his descendant line would have penetrated indigenous populations in the Americas or Australia).

I emailed Rohde about my previous post and he was good enough to send a brief reply. I guess my research stops here, but I hope his continues.

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