I googled the amount of people that lived on earth throughout its entire history, it’s roughly 108 billions. If I take 1 person and multiply by 2 for each generation of ancestors, at the 37th generation it already outnumbers that 108 billions. (it’s 137 billions). If we take 20 years for 1 generation, it’s only 740 years by the 37th generation.
How??
(I suck at math, I recounted it like 20 times, got that 137 billions at 37th, 38th and 39th generation, so forgive me if it’s not actually at 37th, but it’s still no more than 800 years back in history)
In: 1391
You’re assuming that everyone alive right now has different ancestors.
This is wrong because you and your sibling have the same parents. Your parents and their siblings have the same parents which means you have the same grandparents as your cousins. Follow it back enough and every ancestral line joins up eventually.
If everyone in history was an only child then your calculations would be correct, except for the fact that if that was the case then the population would decrease over time.
You’re assuming that all 137 billion of those ancestors are different people.
If your parents happened to be brother and sister, then you’d only have 2 grandparents instead of 4, because your parents shared both parents. That’s an extreme example, but if you researched your family tree far enough you would eventually start seeing ancestors who appear on both sides of a particular branch…4th cousins getting married, that sort of thing.
>I googled the amount of people that lived on earth throughout its entire history, it’s roughly 108 billions. If I take 1 person and multiply by 2 for each generation of ancestors, at the 37th generation it already outnumbers that 108 billions. (it’s 137 billions). If we take 20 years for 1 generation, it’s only 740 years by the 37th generation.
>How??
There’s only ever a limited amount of possible mates for any person at any point in time, no matter their geographic mobility. The further you go down (…up?) the tree the more of all those people will simply be **the same person**. I.e. your great-great-great-great-great-great-great-grandfather on your great-great-great-great-great-great-grandmother’s side will also be one of your great-great-great-great-great-great-great-*great*-grandfathers on your great-great-great-grandfather’s side and so on.
You assume that every one of your ancestors is only related to you in one way, that is an incorrect assumption.
A simple example is if your parent was siblings. Then there are only two instead of 4 people two generations back. The rest of the ancestor tree now gets halved.
Siblings having children is not common, it is just a simple example to show that the ancestor tree can collapse in size. Globally it is not uncommon that cousins to get married. Worldwide 10% of all marriages are between first or second cousins.
The more generation you go back the more likely it is that two persons that have children have a common ancestor. Do you know who your ancestors are 5 generations back? If your parents are from families that have lived in the same relativity small location for a long time I would say it is likely you find common ancestors not that far back.
There exists, in mathematics, something known as the “pigeonhole theorum” which simply states “if you have n available slots and more than n objects then by necessity there must be at least one slot which contains more than one object.”
In other words if I roll a 6 sided die 7 times, I have to have rolled at least one number at least twice.
So back to your question: for every generation back you go in your family tree (g) you have a number of ancestors 2^g at that generation level. So if 2^g is greater than the number of viable adults in the population than the pigeon hole theorun states that at least one person is at least two of your ancestors.
The simple answer is: the family “tree” isn’t actually a tree. Many of the 138 billion you mention are actually the same persons, those “slots” overlap.
In reality, you have, say, a billion people on Earth. They intermingle, randomly, maybe not too incestuously and all a bit simplified, to create a new generation of a billion. Those again do the thing. On and on. But the population doesn’t increase in this scenario. How? Because quite soon, there will be someone who’s mother **and** father share a common ancestor; quite a lot of such people, actually.
Put differently: ever heard of statements such as “everyone is related to (or even descended from) [insert famous figure from 2000 years ago here]?” That in return also means that both your father and mother are related to that person, so somewhere long back their heritages meet.
The 50/500 rule says that a minimum of 50 individuals are required to repopulate the world after a near-extinction, and 500 are needed to combat genetic drift.
This means that no matter how far back you go in your family tree, a generation could have as few as 50 individuals in it, even if there wasn’t an extinction event. And that’s assuming there was no inbreeding at any point in it, which is highly unlikely.
Consider King Joffrey from Game of Thrones. If you go back two generations, you would expect four grandparents, but he only actually has two. His grandparents on his mother’s side and father’s side are the same two people because he’s an incest baby.
Everyone’s family tree is ultimately like that, except usually much, much less icky.
What boggles my mind is the sheer amount of time and evolution and the amount of reproduction that has taken place for me to be in this place at this time.
A paper in [Molecular Biology](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2427203/) which comes at the problem from a different direction (matrilineal DNA studies) suggests that for about 100,000 years there was a “long bottleneck” during which human populations dropped at times to as few as 2000 individuals.
That’s 2000 people who are the ancestors of everyone alive today.
Hello, cousins.
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