History gets more distant
An unexpected genetic revolution started about 1850.
We’ve already noted two instances in which aspects of human life, familiar for thousands of years, changed in a generation: the retirement of horses as work (and war) animals, and the replacement of solid-fuel fires as the main tool for heating and cooking. A recent scientific paper brings up another historic change, this time involving genetics. None of our consultants has any special expertise in this field, but we find it fascinating nonetheless.
The researchers analyzed a vast amount of genealogical data with new computer techniques. Among the statistics we find that, from 1650 (the beginning of useful data) to about 1850, the average distance between the birthplaces of married couples stayed roughly stable at 10 kilometers. That’s not far. It’s fair to say that most people married within the same village. Then, over the next century, that distance exploded to 100km. People were looking for partners not only in the next village, but in the next country, or farther away. The reason involves the railroad, though the picture is a bit more complicated than people simply dispersing along the rail lines.
Over the same earlier period, 1650-1850, the average relatedness of couples at marriage was stable at about fourth-cousin. This is not dangerously close. While you can almost certainly name all your first cousins and second cousins, it’s probably rare today to be able to identify someone as your fourth cousin. But it’s still sharing a set of great-great-grandparents, and fits the picture of people marrying within the village. Starting around 1850 the relatedness of couples drops off precipitously, to (if we’re reading the graph right) well beyond sixth-cousin level by 1950. (We’re not sure what the relatedness of two people taken at random in a given country would be. One of our consultants remembers it as seventh-cousin, but can’t find the source for that.) Again, that century broke out of the village.
The data are not evenly distributed around the world, being concentrated in Europe and North America, and of course different families and different places will see different results. (On a trip to Ireland not too long ago, our navigator heard one of his fourth-cousins refer to a neighbor as a “foreigner,” because she came from another county in Ireland. The usage was not ironic or metaphorical, but matter-of-fact.) But the enormous changes are real. The world in which you would look for your spouse in your own or the next village, riding your horse to visit and warming yourself by the fire when you arrived, is gone.
What have the consequences been? We don’t think it’s possible to say. So much else changed during that century, improvements in health and diet and so on, that disentangling any genetic effects is problematic. Maybe real geneticists can tell.
And we wonder what happens next. The data cannot really be followed past 1950, since many people born then are still in the process of living their lives. But the mixing of people from ever more distant places must have continued, even increased in pace with the advent of airliners and the world economy. Our tutor looks at his diverse group of students and can only speculate.
