ancestral interdependence

this simple mathematical thought-experiment has blown me away a few years ago when it first came to me while reading “Das Kartengeheimnis” from Jostein Gardeer. It expanded my mind so much – now i want to share it :)



If you are an average person born in the year 1980 your parents were born in 1955 (average generation = 25 years) and your four grandparents were born in 1930. Going further back your 8 direct ancestors (parents of parents of parents…) were born in 1905.  yes, it’s *2 every 25 years ;)

Now – if there would be no ancestral interdependence that makes it 256 direct ancestors in 1805 and 8192 in 1680. As we know from the story with the rice-corns on the chessboard this growths exponentially. That means if there would be no ancestral interdependence that you have over one million direct ancestors in the beginning of the 16th century (and probably more since the average generation was lower then 25 years in earlier times).

As we go further back and take a look at the curve of the world population we can notice that if there would be no ancestral interdependence that in the early 14th century over half of humanity were your direct ancestors (full was 511 million at that time).

Ok – but if we go one generation further back, into the year 1280, our math tells us that if there would be no ancestral interdependence that there were more direct ancestors of you alive at that time than people on earth at that time!?

interesting, interesting…

well, maybe you can ask your 2.2 trillion direct ancestors in the year 1000 – or maybe your 604 sextillion (a 6 with 23 zeros) around the year zero what they think about ancestral interdependence of human beings on this planet :)

and then it might be interesting to notice that there is ONE straight line of men going into your past behind you – from your father to your grandfather, your grandfather’s father and so on. And at the same time there is ONE straight line of women going into your past behind you… and… where do this lines go? And isn’t it funny to think that your mum has the same straight line of women behind her than you but has a different ONE-liner of men behind her than you? do they ever intersect as they go into the past? what are the weavings between them and the pathways back and forth in the humongous tree of life from there?…. how would we know? we don’t know… we have it in our biology and it’s impossible to reconstruct it on paper :)

Think about what would have happened if just one of this millions and trillions of your ancestors would have died because of war, a disease or a natural distaster before they passed on their genetical code? Only one and the whole branch going forth from them would never have come into existence – you would never have come into existence! well, maybe a weird perspective – however, it makes me humble and grateful to be a leaf on the tree of life at this time on earth


Published by

Benjamin Aaron Degenhart

Currently pursuing a Masters in Computational Science and Engineering at TU Munich.

4 thoughts on “ancestral interdependence”

  1. Nice thought Benji but there’s something I don’t fully understand. Haven’t there only been 12 or so billion homo sapiens sapiens or so in the past 200.000 years? I might be unclear on my evolutionary history but either way, I’m thinking that it stops being exponential because of limitations in geography, ancestors that repeat and similar factors. In fact I believe there’s a study that shows that 80 percent of all humans trace back to the same woman somewhere around the Mediterranean about 100.000 years ago.

    Okay – my question: How does this factor into your graph?

    // M


    1. Hi Mark, thanks for putting your teeth into this, appreciate it. And yes, you are right, the exponential growth of the ancestral curve is only mathematical – it’s far away from being real. I mean for sure every parent of your parents had two parents and so on but it has surely not grown like that. And that’s also the point of this graph, to show that there HAS to be MASSIVE ancestral interdependence, because it’s not possible otherwise!
      Nice, i didn’t know about that woman… is there a picture of her somewhere? can i add her on facebook? ;)


  2. What a ZUFALL that we exist, hugs my friend, for your special view


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