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The more that I think about this the weirder it gets. What fundamental error am I making here? How can I have 1K ancestors 10 generations ago, 32K 15 generation

The more that I think about this the weirder it gets. What fundamental error am I making here? How can I have 1K ancestors 10 generations ago, 32K 15 generations ago, 1M 20 generations ago?? But when I start with my parents, and think about how they have two parents each, and then go from there, the numbers seem to check out...

xamuel 3y ago
You do not necessarily have 4 grandparents. Try to think of ways in which you can have fewer than 4 (hint: it's a little bit impolite to talk about). Neither do you necessarily have 8 great-grandparents, for the same reason.
- kaycebasques 3y ago
  We're talking incest? And the theory (or fact) is that it was a very widespread practice throughout human history? Trying to wrap my mind around how that changes the calculation... it would drop the number from 2x every generation down to what?
  - lolinder 3y ago
    We only call it incest when the nearest common ancestor is a parent or (depending on the jurisdiction) a grandparent. If your nearest common ancestor is a great-great-great-great-grandparent, that's not incest (it would most likely not even be known at the time of marriage), but it still leads to large amounts of duplication in your family tree. Every family tree will have many instances of nth-cousin marriages simply because people tend to marry people who live close to them. It's very hard to calculate what the actual number would be because it depends a lot on where your ancestors lived and how much they migrated over time.
    - kaycebasques 3y ago
      "Inbreeding" was the word I was looking for, methinks
  - thaumasiotes 3y ago
    > We're talking incest? And the theory (or fact) is that it was a very widespread practice throughout human history? That really depends what you're imagining when you say "incest". In literal terms, yes, we're talking about mating between two people who share common ancestors. Those do not need to be particularly recent. In genetics there is the concept of the "coefficient of inbreeding" which tracks how much of an individual's genome is identical-by-descent across both chromosomes. (That is, if you inherit a particular stretch of DNA from your mother, and you inherit the same stretch of DNA from your father, and the reason they are the same is that your father and your mother both inherited that DNA from the same person, then that stretch of DNA contributes to your coefficient of inbreeding.) If you're imagining brother-sister marriages, that is a much higher level of inbreeding than would be common in human history. (Expected coefficient of inbreeding in the offspring: 25% assuming zero inbreeding in the parents, perhaps because they are themselves the offspring of a cross between an African Pygmy and an Australian Aborigine.) But it is true that different populations have more or less inbreeding than others; we can say that Arabs tend to be more inbred than the French. This is not necessarily because they tolerate incest at closer degrees of kinship than the French. It is mostly because they have a greater quantity of still-relatively-remote incest. (Not the greatest example; Western Europe hugely stigmatized cousin marriage under the influence of the Catholic Church, so in reality Arabs do tolerate closer degrees of kinship in a marriage than the French do. But the concept is sound - inbreeding will "build up" in a population over time.)
  - chromatin 3y ago
    the word is "Consanguinity" https://en.wikipedia.org/wiki/Consanguinity https://en.wikipedia.org/wiki/Consanguinity
thaumasiotes 3y ago
> What fundamental error am I making here? How can I have 1K ancestors 10 generations ago, 32K 15 generations ago, 1M 20 generations ago?? You can't. The population of the world gets smaller as you go back in time. There are that many open spots in your family tree, but, as you go back in time, you will see that each person in the tree simultaneously fills a large number of different spots. A related but not identical observation: the set of all your ancestors is roughly 1/3 male and 2/3 female, despite the fact that there are an equal number of male and female spots in your genealogy. Males are doing more double duty than females.
- kaycebasques 3y ago
  > the set of all your ancestors is roughly 1/3 male and 2/3 female, despite the fact that there are an equal number of male and female spots in your genealogy. Males are doing more double duty than females. As in one man getting multiple women pregnant?
  - thaumasiotes 3y ago
    Yes. The estimate I'm familiar with also says that historically about 80% of women, compared to 40% of men, successfully reproduce.
    - Ekaros 3y ago
      That 80% figure feels very high. I suppose it depends on how a women is counted first of all(for example capable of reproduction). But then how many of them actually died in child birthing process without producing any off-spring that survived.
behringer 3y ago
It's more or less correct because you must remember people don't live forever. Every time you go back more than a couple generations you're starting back at 0. It's why evolution is such a no brainer. Think of a creature with just a few years of life. They have countless ancestors every century, every millenia. Countless millions and billions of parents came before us.

xamuel 3y ago

You do not necessarily have 4 grandparents. Try to think of ways in which you can have fewer than 4 (hint: it's a little bit impolite to talk about). Neither do you necessarily have 8 great-grandparents, for the same reason.

kaycebasques 3y ago
We're talking incest? And the theory (or fact) is that it was a very widespread practice throughout human history? Trying to wrap my mind around how that changes the calculation... it would drop the number from 2x every generation down to what?
- lolinder 3y ago
  We only call it incest when the nearest common ancestor is a parent or (depending on the jurisdiction) a grandparent. If your nearest common ancestor is a great-great-great-great-grandparent, that's not incest (it would most likely not even be known at the time of marriage), but it still leads to large amounts of duplication in your family tree. Every family tree will have many instances of nth-cousin marriages simply because people tend to marry people who live close to them. It's very hard to calculate what the actual number would be because it depends a lot on where your ancestors lived and how much they migrated over time.
  - kaycebasques 3y ago
    "Inbreeding" was the word I was looking for, methinks
- thaumasiotes 3y ago
  > We're talking incest? And the theory (or fact) is that it was a very widespread practice throughout human history? That really depends what you're imagining when you say "incest". In literal terms, yes, we're talking about mating between two people who share common ancestors. Those do not need to be particularly recent. In genetics there is the concept of the "coefficient of inbreeding" which tracks how much of an individual's genome is identical-by-descent across both chromosomes. (That is, if you inherit a particular stretch of DNA from your mother, and you inherit the same stretch of DNA from your father, and the reason they are the same is that your father and your mother both inherited that DNA from the same person, then that stretch of DNA contributes to your coefficient of inbreeding.) If you're imagining brother-sister marriages, that is a much higher level of inbreeding than would be common in human history. (Expected coefficient of inbreeding in the offspring: 25% assuming zero inbreeding in the parents, perhaps because they are themselves the offspring of a cross between an African Pygmy and an Australian Aborigine.) But it is true that different populations have more or less inbreeding than others; we can say that Arabs tend to be more inbred than the French. This is not necessarily because they tolerate incest at closer degrees of kinship than the French. It is mostly because they have a greater quantity of still-relatively-remote incest. (Not the greatest example; Western Europe hugely stigmatized cousin marriage under the influence of the Catholic Church, so in reality Arabs do tolerate closer degrees of kinship in a marriage than the French do. But the concept is sound - inbreeding will "build up" in a population over time.)
- chromatin 3y ago
  the word is "Consanguinity" https://en.wikipedia.org/wiki/Consanguinity https://en.wikipedia.org/wiki/Consanguinity

thaumasiotes 3y ago

> What fundamental error am I making here? How can I have 1K ancestors 10 generations ago, 32K 15 generations ago, 1M 20 generations ago?? You can't. The population of the world gets smaller as you go back in time. There are that many open spots in your family tree, but, as you go back in time, you will see that each person in the tree simultaneously fills a large number of different spots. A related but not identical observation: the set of all your ancestors is roughly 1/3 male and 2/3 female, despite the fact that there are an equal number of male and female spots in your genealogy. Males are doing more double duty than females.

kaycebasques 3y ago
> the set of all your ancestors is roughly 1/3 male and 2/3 female, despite the fact that there are an equal number of male and female spots in your genealogy. Males are doing more double duty than females. As in one man getting multiple women pregnant?
- thaumasiotes 3y ago
  Yes. The estimate I'm familiar with also says that historically about 80% of women, compared to 40% of men, successfully reproduce.
  - Ekaros 3y ago
    That 80% figure feels very high. I suppose it depends on how a women is counted first of all(for example capable of reproduction). But then how many of them actually died in child birthing process without producing any off-spring that survived.

behringer 3y ago

It's more or less correct because you must remember people don't live forever. Every time you go back more than a couple generations you're starting back at 0. It's why evolution is such a no brainer. Think of a creature with just a few years of life. They have countless ancestors every century, every millenia. Countless millions and billions of parents came before us.