An­swer by Dr Richard Dawkins (evo­lu­tion­ary bi­ol­o­gist)

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Yes, we are all re­lated. You are a (prob­a­bly dis­tant) cousin of the Queen, and of the pres­i­dent of the United States, and of me. You are I are cousins of each other. You can prove it to your­self.

Every­body has two par­ents. That means, since each par­ent had two par­ents of their own that we all have four grand­par­ents. Then, since each grand­par­ent has to have two par­ents, every­one has eight great-grand­par­ents, and 16 great-great-grand­par­ents and 32 great-great-great­grand­par­ents and so on.

You can go back any num­ber of gen­er­a­tions and work out the num­ber of an­ces­tors you must have had that same num­ber of gen­er­a­tions ago. All you have to do is mul­ti­ply two by it­self that num­ber of times.

Sup­pose we go back ten cen­turies, that is to An­glo-Saxon times in Eng­land, just be­fore the

Nor­man Con­quest, and work out how many an­ces­tors you must have had alive at that time.

If we al­low four gen­er­a­tions per cen­tury, that’s about 40 gen­er­a­tions ago.

Two mul­ti­plied by it­self 40 times comes to more than a thou­sand tril­lion. Yet the to­tal pop­u­la­tion of the world at that time was only around three hun­dred mil­lion. Even to­day the pop­u­la­tion is seven bil­lion, yet we have just worked out that a thou­sand years ago your an­ces­tors alone were more than 150 times as nu­mer­ous. And we’ve so far dealt only with your an­ces­tors.

What about my an­ces­tors, and the Queen’s and the pres­i­dent’s? What about the an­ces­tors of ev­ery one of those seven bil­lion peo­ple alive to­day? Does each one of those seven bil­lion peo­ple have their own thou­sand tril­lion an­ces­tors?

To make mat­ters worse, we’ve so far only gone back ten cen­turies. Sup­pose we go back to the time of Julius Cae­sar: that’s about 80,000 gen­er­a­tions. Two mul­ti­plied by it­self 80 times come to more than a thou­sand tril­lion tril­lion. That’s more than a bil­lion peo­ple packed into ev­ery square yard of the Earth’s land area. They’d be stand­ing on top of each other, hun­dreds of mil­lions deep!

Ob­vi­ously we must have done our sums wrong. Were we wrong to say that every­body has two par­ents? No, that is def­i­nitely right. So, does it fol­low that every­one has four grand­par­ents? Well, sort of yes, but not four sep­a­rate grand­par­ents. And that is ex­actly the point. First cousins some­times marry. Their chil­dren have four grand­par­ents, but in­stead of eight great-grand­par­ents they only have six (be­cause two great-grand­par­ents are shared).

Cousin mar­riage cuts down the num­ber of an­ces­tors in our cal­cu­la­tion. First-cousin mar­riages are not par­tic­u­larly com­mon. But the same idea of cut­ting down the num­ber of an­ces­tors ap­plies to mar­riages be­tween dis­tant cousins. And that is the an­swer to the rid­dle of the very big num­bers that we cal­cu­lated: we are all cousins. The real pop­u­la­tion of the world at the time of Julius Cae­sar was only a few mil­lion, and all of us, all seven bil­lion of us, are de­scended from them. We are in­deed all re­lated. Ev­ery mar­riage is be­tween more or less dis­tant cousins, who al­ready share lots and lots of an­ces­tors be­fore they have chil­dren of their own.

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