If You have 2 parents, 4 grandparents, 8 great parents, so if you go back 40 generations you’d have a supposed 1 Trillion ancestors, But only 100B humans have ever existed. Explain.


If You have 2 parents, 4 grandparents, 8 great parents, so if you go back 40 generations you’d have a supposed 1 Trillion ancestors, But only 100B humans have ever existed. Explain.

In: Biology

One word – interbreeding.

The further you go back the more common ancestors you’ll find between any two random people.

Simple: a lot of those 1 trillion people happen to be the same people. There is unavoidably a lot of n-th degree cousin fucking all through history, the important thing is to not to fuck too close relatives too often or you get interesting congenital defects. A couple villages with a population above a thousand or two can get by fine marrying among themselves.

People have shared ancestors. They might not be mating with their close relatives, like shared grandfathers or even great-grandfathers, but if you look far enough back everyone is going to be related.

This seems obvious from a practical standpoint, how are you going to be able to know that a prospective mate shares no ancestors with you *for all time*? There is no way of knowing, so such relationships occur. In fact how far removed one must be to mate varies by culture, with some middle eastern cultures viewing first cousins as ideal mate prospects.

There is not a requirement for all those ancestors to be different people.

The extreme case is if your partner was siblings then you only have 2 grandparents, not 4. I am not saying that siblings having children is or where common just that is the most extreme example to show that your ancestors in different paths can be the same person.

If you go back to your ancestral tree the different branches will merge and different points. So you a great great great grandparent on your mother side might be in your farther ancestral tree at that level.

There might be someone that live 2000 years ago you can find is you ancestor along 100 diffrent paths,

So all humans are realted to all othe human is a complex mesh.

if your great great great great grandfather had 12 children and 3 of them moved across the country and themselves had 12 children each and a few decades later their children lost touch and eventually met again they’d have no idea they were related and there’d be hundreds more descendants by that time, eventually leading to a giant country of incest but not too detrimental to the species type incest because the genes are mixed enough across different families

You are looking at it upside down. Instead of thinking about your family tree think about “Adam and Eve”. Someone was the first human and they had sex with an almost human, those genes spread to make more humans.

After the genes spread enough then there is enough diversity for cousins (4th, 5th, 6th) to never know each other and their kids can date without the gross out factor.

This means instead of being a real pyramid shape of “Adam and Eve” it’s shaped more like the Washington Monument. The lower down you are the more brinks above you(ancestors) that you have in common with other bricks at that level.

Well you must read about Ghengis Khan. He apparantly has about 16 million descendants alive today. Source – [https://www.nationalgeographic.com/culture/article/mongolia-genghis-khan-dna](https://www.nationalgeographic.com/culture/article/mongolia-genghis-khan-dna)

Ghengis Khan ancestry was studied further because it was documented in history that he had many many “official” wives. There would be hundreds – if not thousands of such cases spread over history – across ages / countries. So 100 Billion humans – ever existed – could possibly be correct.

at certain points in the far history it is suspected (backed by genetic diversity) that the human population went through bottlenecks.

its suggested the pre-1942 Americas population is derived from 70 individuals who crossed the land bridge from asia. Also the sub-sarahan population was as low as 2000 individuals for many thousands of years before spreading out of africa.


that would be true if every one would have unique ancerstors, but we dont, for example if you go far back enough you are related to most of the world people.

I would have assumed that no matter how many generations we go back, they would all be deceased. That being said, I understand the 100B number, but I could never understand how that has been proven and have always been skeptical. I still assume that whatever realistic number makes sense is what I believe.

I think that you have to consider how many years ago 40 generations would really be. There is a possibility that going back that far may not be possible as it may predate humans. Just for an example, I was born in 1980, my mom was born in 1961, my grandfather was born in 1933, my Great Grandfather was born in 1887, my Great Great grandfather was born in 1853, and my Great Great Great Grandfather was born in 1822. So 6 generations from a single line takes us back 200 years. Depending on age at the time of giving birth you could go back further with less generations. I’ve got genealogy that traces one line of my family back to the early 1300’s, and that’s only about 10-15 generations from me. It would be interesting to see if someone could trace back 40 generations and see what years those ancestors were born.

everyone is related to some degree. people have lots of kids with lots of different people, so there’s more overlapping than you would think in family trees all over the world.

Basically, the same person represents multiple branches in the ancestral tree. This is called pedigree collapse. That also tells us that people who were alive a few thousand years ago are either the ancestor of almost everyone alive today, or no one.