Meet the Family- a dumbed-down science article
Ladies and gentlemen, find below my humble (read: not very good) offering to the Oxford Times science writing competition. You're welcome. I personally don't like the ending much- it's truncated as if (hypothetically) I realised I was running up to the word limit and had to finish it in as mawky a way as possible. Oh well....
Have you ever gone to a distant town, on holiday say, only to walk down the high street and meet a relative going the other way? Perhaps it happens more than you think.
Every one has family, at least in the biological sense. Everyone has two parents, four grandparents, eight great-grandparents and so on. The majority of people either have or plan to have children, and like the thought that whatever may happen to them, a thousand years hence their descendants will carry on the family. The kin instinct is very strong in all of us.
Of course, you only have to watch any soap opera to know how complicated family relationships can be. So, let’s imagine a way it might be simpler. Imagine that some people set up a colony on a distant island. Being civilised folk, they record all marriages and deaths. Naturally, they outlaw incest, and their lawmakers phrase it like this: A couple may not marry if they have a common ancestor in our records. Let’s follow the fortunes of a particular colonist, say, the descendants of Mr Smith. Mr Smith himself may marry anyone he pleases, since there are no records. His son (Mr Smith the 2nd) is forbidden to marry his immediate siblings as they share a parent, but this is no great hardship. Mr Smith the 3rd is forbidden to marry his cousins, since they share his grandfather, and so on. You can see how the list of potential brides shrinks, as the ties of common ancestry spread ever wider.
Let’s look at this a different way. Every person on the island knows who all their ancestors are. For each generation you go back the number of ancestors doubles (because every ancestor must have had two parents). Let’s say Mr Jones lives in the third generation. 8 of the original settlers (2 X 2 X 2) are his direct ancestors. When he marries, his wife will also have 8 settler ancestors. By law, none of her 8 ancestors can be the same as Mr Jones’s, so their children will be descended from 16 of the original settlers. If there were, say, 512 original settlers, then children born in the 9th generation will be the direct descendants of all of those settlers. In other words, everyone shares the same ancestors and everyone is family.
Mathematically, it’s easy to use the rules above to say how long a population can last before people are forced to marry relatives by the simple fact that everyone is family. It happens when the original population (e.g. 512) is smaller than the number of ancestors people should have alive at that time. If we apply this rule to our world instead of a fictional island, we can go back to about 1250 before our ancestors top 1000 million. That is to say, dear reader, everyone alive in 1250 is my ancestor, and yours, and the ancestor of everyone alive today. Actually, we have to except those poor unfortunates who have no living descendants today. We know this does not apply to King Henry III (r. 1216-1272), so we conclude that his royal offspring includes every single one of us.
Now you’re saying “Hold on. My family came to this country in 1947. How can I be descended from a King of England?” I’m afraid to say you’re quite right to object. The answer is that our ancestors, indeed we ourselves, don’t follow the ultra-strict laws that our island colonists do. Indeed, the existence of different races is testament to the fact that our family tree is a long history of incest. Incest, at least between very distant cousins, is both ubiquitous and inevitable.
So how is it that we are more or less in good health, when we know that inbred populations are prone to all sorts of disease and disorders? Simply enough, it is because I share half (on average) of my genes with my sister. If our family carries a lot of defective genes there is a high chance that our children would inherit a lot of them. I share only a quarter of my genes with my cousin, but that’s still too high for comfort. By the time you get to someone who shared my great-great-great-grandparents, the genes we share are about one part in thirty-two, and that’s a bet nature doesn’t mind taking. The genetic material that I, or Queen Elizabeth II, shares with William the Conqueror (assuming no incest) is about one part in a million million.
The moral of this story is a simple one: That in a surprisingly literal way, humanity is a family. We are all related- maybe that’s something to reflect on when we pass in the street.
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