If you actually go back and look at who those people are, though, you likely don't have anything like as many as calculated, because your family isn't so much a tree as a DAG—there's plenty of duplication.
For example, my wife and I recently discovered that we're sixth cousins. This means that when our kids trace back their ancestry, there's an entire subtree on my side that is duplicated on her side, which dramatically shrinks the number of ancestors down from the theoretical maximum. And this is far from an uncommon occurrence—we most likely have many other subtrees in common that we simply don't know about.
My parents are related through one set of grandparents four or five generations back.
My wife and I are related through the same grandparents.
These are weird, but it makes sense - we're from smaller towns, which were settled not that long ago in the grand scheme of things. If you trace enough of us back, we're all probably related.
But the seriously trippy part is - my younger sibling and their spouse are related through the same great(x4 or so) grandparents. The sibling is from a different country that isn't even close to where we're from. Like literally the other side of the planet.
If one of you is from one corner of the former British empire and the other from another corner, it becomes a lot less surprising. The British showed out everywhere the past ~400 years and often forcibly relocated certain subjects (e.g. the Irish).
My old man, bless him, found one instance of inbreeding going back to 1800 for each one and going back to 1700 on a few.
My son's mother's ancestry is very different from mine.
If anything I think we might live in period of the greatest number of long distance gene flow events in history. And I think this has been true for hundreds of years at this point.
EDIT: That's not to say inbreeding doesn't hugely reduce your number of ancestors, just that the modern world for many social groups has much less inbreeding than earlier societies.
No, because fully random would still cause duplication, because the population of Earth is finite not infinite. With matings specifically selected to create no common ancestors back to X generations, then you theoretically have 2^X ancestors.
It absolutely does not. Going backwards in time like this, most people will have many generations where the number of their ancestors in that generation go down, not up.
For the majority of the past four thousand years, almost everyone lived in farming villages that had ~50-100 families living in them, and mostly married inside those communities. Even just going to a neighboring village for a bride was something that only happened occasionally, but even when that happened, it was almost always the same few neighboring villages. This means that when you trace ancestry backwards, you always end up related to everyone living in a few villages, and then every few generations this extends to a new village. Then if you are related to a successful trader or soldier or something, it will suddenly double the amount of ancestors in a generation, but I really need to stress that this is rare.
The introduction of the bicycle did a lot for reducing inbreeding in farming communities, because it was something that a normal people could own, and allowed day visits to neighboring communities. Before that, it was entirely ordinary to live and die without ever traveling more than 10 miles from the place where you were born.
I guess it doesn't, or you could go back to the time where the early human lived expecting an astonishingly high number too, or even before the big bang if you wanted.
As long as you're comparing two people from the same country and ethnic group (which tend to be the most common pairings for childbearing), it's pretty close to true. It doesn't work so well if you're comparing people on different continents.
> With fully random matings the number of your ancestors doubles every generation. Ten generations ago (250-300 years) we might expect about 1,000 ancestors each contributing 0.1 percent of their total variants to your genome... when the number of ancestors exceeds 2,000-3,000 (12 generations would be about 4,000), it is likely that some ancestral sections of DNA have been lost altogether... by about 15 generations back many of the ancestors contribute no DNA variants at all but they are still ancestors on the lineage.
So cool, I never thought about a generation limit to meaningful genetic contribution.
I guess from a genetic standpoint, it doesn't really matter. Since many of those near-relations (as other comments have mentioned) who mated and had children would have somewhat similar genetic profiles, one particular SNP from a great-x10 grandparent might still propagate to us, simply because other mates would also have that gene.
Multiple people intuitively pointed out that the OP is actually quite wrong. The shocking fact, to me at least, is how far this goes: all of humanity share a single male ancestor [1] (and no, it was not the first man to ever live), and a single female ancestor [2], who likely lived thousands of years apart.
The genes book I mention in the post talks about Y-Chromosomal Adam and Mitochondrial Eve. Super cool stuff. The directed acyclic graph nature of lineage totally makes sense why the number of ancestors is smaller in reality than the theoretical numbers. I don't follow how Adam and Eve relate, though. I know this example is crude but bear with me; hopefully it's valid enough to have discussion around. If we follow the lineages back to the point where we have 1000 ancestors each, it seems like Adam just represents the 1 we all have in common, and by extension the other 999 are the ones not in common.
From the Wikipedia article:
> Although the informal name "Y-chromosomal Adam" is a reference to the biblical Adam, this should not be misconstrued as implying that the bearer of the chromosome was the only human male alive during his time.[7] His other male contemporaries may also have descendants alive today, but not, by definition, through solely patrilineal descent; in other words, none of them have an unbroken male line of descendants (son's son's son's … son) connecting them to currently living people.
(Reading the Eve article left me with even more questions than answers, but I'm still getting the sense that Adam and Eve are orthogonal to the "number of ancestors in 1600" topic.)
This makes some crazy assumptions that not only do people not have kids with their first cousins (they do!) but they don't with there Nth cousins for N going into the dozens, with the family tree organizing itself into a perfect inverted pyramid. Also a generation can be 15-50 years, and is unique to each parent-child pair in the DAG (to use the term from the sister comment).
I'm kinda wrapping my head around the DAG thing (but still have no clue how large of an effect it has on the overall calculation) but don't get how 25 years per generation is off. I know that in the last few centuries people have been having children at older and older ages but you're saying that an average like that doesn't hold for the long-term? Or maybe it's just very complicated? E.g. some 60 year-old man gets a 20 year-old woman pregnant 700 years ago and it has a dramatic change on the calculation?
It’s very strange to look at a long genealogy of oneself and be reminded that you only know the last names of your grandparents and maybe great grandparents, but that 300 years ago, almost all your descendants would’ve had totally different names. Everyone wants to know who their father^N was, just because we’re attached to a silly little name.
Because of cousin marriages the number of ancestors is reduced, it's called Pedigree Collapse. In certain populations it's very evident if anyone tries to build a genealogical tree with the help of a dna test.
Thanks for the lead. Someone in another comment described it as an inverted pyramid. The graph of Cleopatra VII's lineage seems to be the best example of that: https://en.m.wikipedia.org/wiki/Pedigree_collapse
> This paper is the third and final one in a sequence of three. All three papers emphasize that a proposition can be justified by an infinite regress, on condition that epistemic justification is interpreted probabilistically. The first two papers showed this for one-dimensional chains and for one-dimensional loops of propositions, each proposition being justified probabilistically by its precursor. In the present paper we consider the more complicated case of two-dimensional nets, where each "child" proposition is probabilistically justified by two "parent" propositions. Surprisingly, it turns out that probabilistic justification in two dimensions takes on the form of Mandelbrot's iteration. Like so many patterns in nature, probabilistic reasoning might in the end be fractal in character.
FWIW, a free site for finding and preserving info about your ancestors, near and distant, is at https://www.familysearch.org . I used to work there; they have put a lot of resources into making this available and preserving the data long-term, permitting collaboration with others who have overlapping lines, etc.
I believe it also has a feature somewhere for determining how closely related you are to someone. My wife and I discovered we are 13th cousins (or so).
And it has a really broad wiki to help you do accurate genealogy, with many country-specific tips and other pointers to other resources.
It will not disclose info on living persons, in the tree, except to that person (and maybe their children? I forget).
Lately I have not been active on the site and get less than one email per month from them, I think.
Readit News
For example, my wife and I recently discovered that we're sixth cousins. This means that when our kids trace back their ancestry, there's an entire subtree on my side that is duplicated on her side, which dramatically shrinks the number of ancestors down from the theoretical maximum. And this is far from an uncommon occurrence—we most likely have many other subtrees in common that we simply don't know about.
My wife and I are related through the same grandparents.
These are weird, but it makes sense - we're from smaller towns, which were settled not that long ago in the grand scheme of things. If you trace enough of us back, we're all probably related.
But the seriously trippy part is - my younger sibling and their spouse are related through the same great(x4 or so) grandparents. The sibling is from a different country that isn't even close to where we're from. Like literally the other side of the planet.
It blew my mind.
Dead Comment
My son's mother's ancestry is very different from mine.
If anything I think we might live in period of the greatest number of long distance gene flow events in history. And I think this has been true for hundreds of years at this point.
EDIT: That's not to say inbreeding doesn't hugely reduce your number of ancestors, just that the modern world for many social groups has much less inbreeding than earlier societies.
> With fully random matings the number of your ancestors doubles every generation.
For the majority of the past four thousand years, almost everyone lived in farming villages that had ~50-100 families living in them, and mostly married inside those communities. Even just going to a neighboring village for a bride was something that only happened occasionally, but even when that happened, it was almost always the same few neighboring villages. This means that when you trace ancestry backwards, you always end up related to everyone living in a few villages, and then every few generations this extends to a new village. Then if you are related to a successful trader or soldier or something, it will suddenly double the amount of ancestors in a generation, but I really need to stress that this is rare.
The introduction of the bicycle did a lot for reducing inbreeding in farming communities, because it was something that a normal people could own, and allowed day visits to neighboring communities. Before that, it was entirely ordinary to live and die without ever traveling more than 10 miles from the place where you were born.
So cool, I never thought about a generation limit to meaningful genetic contribution.
[1] https://en.wikipedia.org/wiki/Y-chromosomal_Adam [2] https://en.wikipedia.org/wiki/Mitochondrial_Eve
PS: see also references fromthe linked wikipedia pages
From the Wikipedia article:
> Although the informal name "Y-chromosomal Adam" is a reference to the biblical Adam, this should not be misconstrued as implying that the bearer of the chromosome was the only human male alive during his time.[7] His other male contemporaries may also have descendants alive today, but not, by definition, through solely patrilineal descent; in other words, none of them have an unbroken male line of descendants (son's son's son's … son) connecting them to currently living people.
(Reading the Eve article left me with even more questions than answers, but I'm still getting the sense that Adam and Eve are orthogonal to the "number of ancestors in 1600" topic.)
> fathers consistently older (30.7 years) than mothers (23.2 years)
https://www.science.org/doi/10.1126/sciadv.abm7047
https://projecteuclid.org/journals/notre-dame-journal-of-for...
> This paper is the third and final one in a sequence of three. All three papers emphasize that a proposition can be justified by an infinite regress, on condition that epistemic justification is interpreted probabilistically. The first two papers showed this for one-dimensional chains and for one-dimensional loops of propositions, each proposition being justified probabilistically by its precursor. In the present paper we consider the more complicated case of two-dimensional nets, where each "child" proposition is probabilistically justified by two "parent" propositions. Surprisingly, it turns out that probabilistic justification in two dimensions takes on the form of Mandelbrot's iteration. Like so many patterns in nature, probabilistic reasoning might in the end be fractal in character.
I believe it also has a feature somewhere for determining how closely related you are to someone. My wife and I discovered we are 13th cousins (or so).
And it has a really broad wiki to help you do accurate genealogy, with many country-specific tips and other pointers to other resources.
It will not disclose info on living persons, in the tree, except to that person (and maybe their children? I forget).
Lately I have not been active on the site and get less than one email per month from them, I think.