We interrupt the trilogy with a small digression - something that I've been thinking about for a day or so. Nothing scientific, I promise - just something silly to mull over.
Each of us has, or had, 2 (biological) parents, and thus 4 grandparents, 8 great-grandparents and so on. You have no choice in the matter - that's the way it works. As you go back to each previous generation, the number doubles, for obvious reasons. If we assume, as a rough-but-handy approximation, that 10 generations take about 300 years, then you should have 1024 great-great-great-great-great-great-great-great-grandparents who were alive somewhere around 1700. Go back a further 300 years and you now have over 1 million direct ancestors; go back yet another 300 years - around 1100 AD - and there are over a billion. That is mind boggling enough, but we have hardly started yet - mankind has been around for far, far longer than that.
That's OK - let's stick at the year 1100. 1 billion people required for the family. Now, as far as I can determine, our current best estimate for the total population of the Earth in 1100 AD is only 300 million. That's everyone - all races, all religions, all over the globe. Is the difference between these numbers explained entirely by various degrees of inbreeding?
My assumptions are consciously simplistic, but the principle of the thing stands up. If, as you go back through the generations, you find the odd individual who (accidentally?) occupies more than 1 slot in your upside-down family tree (and this must get more likely as the numbers increase), then a whole section of the tree above him/her disappears, since we only need him/her to be conceived once. That would get the numbers down a bit, and let us not, gentle readers, distress ourselves by dwelling on just how he/she occupies the two slots. And so on. I guess that must be it - there doesn't seem to be another explanation.
Discuss.
Depends on what you call in-breeding. If you are considering cousins 4 and 5 times removed to be inbreeding then that would be unavoidable and closer would have been common at a time when many if not most people lived and died within 20 or so miles of where they born in a rural society.
ReplyDeleteMaybe I used the wrong word - I don't think it matters who the partners are in my definition, it's whether they feature as more than one of your direct ancestors. In the case of the remote cousins who marry, the guy that cuts down the overall numbers is the common grandfather(?) who will appear up both lines. I think.
ReplyDeleteWhatever, we need those numbers cut down, or else there would have to be alien or non-human species present to make up the balance!
It wasn't a serious issue, just one of those "just a minute" moments. In remote, sparsely populated areas, there would very quickly be nobody but more-or-less-distant relatives to marry, as you say.
Well would you Adam & Eve it? Actually Tony that's a very interesting point but its making my brain hurt this early in the morning...
ReplyDeleteIan
A very interesting idea, for the numbers you've mentioned there must have been a lot of inter family breeding. If you look back through the British royal family, it's easy to trace back there lines, and there was a lot of inter marriage, cousins marrying cousins and so forth, so it must have gone on elsewhere too. Perhaps to keep the wealth of the family, in the family. You've certainly got my brain ticking overtime this morning!!
ReplyDeleteAbsolutely, not serious but interesting anyway, esp as an example of how a simple model can go quickly astray. Just take my part of my own family. My father's sister married my mother's brother. If you look at my generation, there are 5 of us (3+2). For all 5 of us in the 3rd generation, there are only 4 people in the 2nd generation and......4 people in the first generation where you would expect 8! Only 1/2 and that's only one minor quirk.
ReplyDelete(even worse, all 4 of the 1st generation died when I was young so that by age 7, the grand total of my living ancestors was 2 where it might have been 6).
Reminds me a bit of trying to figure out how much damage a volley does. "If there are X number of muskets ordered to be fired......."
-Ross
Ross - that is an impressive family example - a big reduction in numbers straight off, and I guess that must be a fairly common situation. Thanks for that - interesting.
ReplyDeleteVolley calculations - now, as it happens, that's exactly what I am working on at present. I hope to get a posting out tomorrow - I doubt if it will influence anyone's thinking but, as ever, I find it useful to be forced to sort my thoughts into something coherent enough to write (well, maybe...).
Cheers
Tony