Did you ever thought about your family tree closely? Common sense says that we all have 2 parents, 4 grandparents, 8 great grandparents and so on following a geometric progression, doubling in each generation. But, as someone said, “common sense is what tells us that the world is flat“.

If we assume that there is a new generation each 25 years and we go back 600 years, that’s 2²⁴ which is already a respectable number of 1 million grandparents. But things get trickier when we go further. For instance, if go back another 600 years (to 800AD), that will get us 2⁴⁸. That’s 281.5 trillion grandparents and with this we get a problem: in 800AD there weren’t that much people on the world. Actually, today we are only 6.3 billions.

So where’s the catch? You probably guessed it already. Our ancestors along the way married their cousins without knowing. This is called pedigree collapse and the closer the cousin, the bigger is the percentage of the collapse. Of course the worst case is when two siblings got married (think royalty-wise for instance) when it’s a major 50% collapse. With a first cousin it’s a 25% cut-down but collapses are still relevant with more distant cousins which is the most common case.

Some geneticists believe that everybody on earth is at least 50^th cousin to everybody else and most of us are a lot more closer, which really puts the mankind family concept into perspective. So, please keep up in mind that if you get mad with some complete stranger, chances are that he/she is probably your 25^th cousin. :)