An historical thought experiment

A 'family tree', Hardwick Hall, Derbyshire

I’ve been asking my new History classes (they happen to be younger teenagers this year) to take part in a thought experiment. But you don’t have to be young to try it.

It’s hard for anyone to feel that people who lived and incidents that happened hundreds or thousands of years ago have any relevance or importance to us today. It’s especially hard for young people to do this. When you’re (say) 13 you find it very difficult to feel an affinity with life 130 years ago, let alone 1300. The past seems impossibly distant and shapeless, peopled by alien beings. Why should we take any interest in them?

This thought experiment is intended to help create a new perspective on the past, to show that the people who lived then were not utterly unlike us, and to shrink the years down to more tangible proportions. 

Imagine you’re sitting in a medium sized room. (If you’re actually in a medium sized room, such as a classroom, so much the better.) Let’s say that there are thirty chairs in this room, and that you are sitting in one corner of the room. Now imagine that one of your parents is sitting next to you (either your father or mother, it doesn’t matter which, but only one of them). Next, imagine that one of your four grandparents (again, it doesn’t matter which) is sitting next to them. This is very easy; almost everyone knows their parents and at least some of their grandparents, and even if you don’t know them you can still imagine their presence.

You can see what comes next: now imagine that one of your eight great-grandparents is sitting in the fourth chair. This is a bit harder, because a lot of people, perhaps most, never knew their great-grandparents, but they might have seen photos of them or heard relations talk about them. Again, even if you have no knowledge of them, you can still imagine their presence without too much difficulty. The fifth chair is occupied by one of your sixteen great-great-grandparents, and so on. A lot of imagination is needed from this point, because we’re unlikely to know anything about our more distant ancestors, but almost everyone, surely, can conjure up in their minds some people, ancestors, to sit in those chairs.

Let’s assume that the average age at which people have children is 33 1/3. How many chairs do we have to ‘fill’ in order to get back to an ancestor who lived at the time of, say, the Gunpowder Plot in 1605, or the Battle of Hastings in 1066? (These are the events my classes are studying first.) The answer is really surprisingly small. If there are three generations per century, then to reach about 400 years ago only twelve chairs need to be occupied – less than half the room. You don’t even need to fill the whole room to reach the Norman Conquest a little less than a thousand years ago (28 or 29 chairs will do it). 

So imagine yourself sitting in this room inhabited by your ancestors, (your bloodline, if you like), all of whom have of course contributed something to what and who you are. They lived through the events that we now think of as history, events that might seem to us remote, irrelevant and uninteresting. We can’t talk with them, and we probably can’t know much about them as individuals, but we can imagine ourselves into their lives. By doing so we can make history come alive.

There are many variations you can play on this theme. Instead of a classroom, imagine a small theatre with 200 seats. Filling this with ancestors takes us back 67 centuries, that is to c.4700 BCE, on the cusp between the Mesolithic and Neolithic in Britain, when agriculture was first introduced to these islands, and centuries before the first stones were erected at Stonehenge. Two hundred is not that big a number; imagine, a mere 200 people in a room, but stretching back to the Stone Age. It's a thrilling thought, all those men and women sitting side by side, an unbroken chain of forebears linking us to the distant past.

I have to admit that the further you go back in time, the more inaccurate the numbers become, because people must have had children younger in the distant past. Many, if not most, people were dead before they were 33. On the other hand, it's possible to have children at a greater age. There are people alive now whose paternal grandfather was born in the 18th century. This might sound ludicrous. But say you're 100 years old and your father had you when he was 60, that means he was born in 1855. If his father also had his son at 60, he (your grandfather) was born in 1795. In this case, just three chairs in our imaginary room take us back to the Napoleonic Wars, Beethoven and Blake.

Having asserted that the numbers involved are very much smaller than expected, looked at another way they're astonishingly large. We can calculate how many ancestors we had a millennium ago (i.e. 30 generations): 2 to the power of 30 is 1,073,741,820. This is an astoundingly big number. The population of the world is estimated to have been at most 310,000,000 in 1000 CE, a substantially smaller number. How can we possibly have had more ancestors than there were people on the whole earth? The answer to this conundrum is, of course, that many of those ancestors are related to us in multiple ways. Someone say five generations ago is quite possibly my great-great-great-grandparent several times over, because their grandchildren and later generations could have inter-married. (If you marry your first cousin, your children will have six rather than eight great-grandparents.)

This leads to another initially mind-boggling thought: we are all cousins. Not in some abstract, brotherly love, fraternal feeling way, but absolutely literally. If you go back enough generations you're mathematically certain* to be linked genetically with every single other person in the world. We're all related.

Athelhampton House, Dorset

* Or do I mean mathematically almost certain? Someone with a better grasp of statistics than me can answer this.