Here is a puzzle for you to figure out: did Genghis Khan really have 16 million direct male descendants? Note the careful wording: direct male descendants. It is a factoid that has been around since 2003 when a now famous genetic study concluded that 16 milllion men in Central Asia share a remarkably similar Y-chromosome, meaning that at some time in the past they had the same forefather. The factoid that it was Genghis Khan has been doing the rounds in the international media ever since.
How can they know such things? Well, because they know how many copying mistakes there are in each generation in bits of the genome, so by looking at how much variation there is in a population they can work out how long ago they started with the same chromosomes. With a bit of statistical trickery you can also find clusters of men who will have a later common ancestor, essentially by first divvying up a large population in more homogenous ones that have similar Y-chromosomes.
Now, it is true of all men that if you go back far enough that they will have the same forefather, but the unusual things about the 16 million in Central Asia is that their common forefather appears to have arisen around the year 1000, give or take a century (and with Genghis Khan you have to give 1.5 centuries, which is your first main clue). The year 1000 is rather late for so many related people however.
The factoid is thus that Genghis Khan, who had many sons and whose many sons had a lot of sons themselves currently has 16 million male descendants. This is 8% of the Asian men studied in the 2003 study, but more than 8% in particular areas. With a roughly tenfold general population increase in that region, Genghis would have been 1.6 million times as successful as any other man living in that era.
I encourage you in the comment thread below to say if you believe this and if so, why. If not, tell me why this factoid, which to my knowledge has been unchallenged till 2012 would in fact not be true.
I will let you know on Monday what I think about it.
As a bit of background on those who believe that because something has been unchallenged for a while that it must be true: the world is full of factoids that survive in the popular media and even in the scientific community for a while, even if on closer inspection they are not all that plausible. An obvious example of a now ‘disproven factoid’ is whether the Great Wall be seen from outer space or the moon? It seemed plausible because it is so long. However on reflection you should already have your doubts: the Wall is less broad than your average house. It is basically as visible as a very long lane with large trees of a slightly different colour to the rest. Now, if you can’t see something as thin as the average house or a lane of trees from outer space, why would you be able to see a whole string of things smaller than the house or a large tree? So with a bit of common sense you could already have known it was unlikely that you’d be able to see the Great Wall from outer space (unless with special equipment, of course, and in closer orbit you might get a glimpse). It took a few decades but ultimately, indeed, we found out you can’t see the Great Wall from the moon and that what was bandied around for decades was a myth.
Perhaps the same is true of this factoid also?