3 ms·

One of my favorite papers highlighting the counter-intuitiveness of the world on a truly long term is Rohde et al, 2004, "Modelling the recent common ancestry o

by xamuel 6y ago

One of my favorite papers highlighting the counter-intuitiveness of the world on a truly long term is Rohde et al, 2004, "Modelling the recent common ancestry of all living humans". Wherein they show using computer simulations, with conservative assumptions, that it's almost certainly the case that the so-called "identical ancestor point" was as recent as a few thousand years ago (a blink of an eye in geological terms). What is this "identical ancestor point"? The IA point is the most recent time t such that if you choose any human alive at time t, then there are only two possibilities: (1) everyone alive in 2020 is their descendant, or (2) they have no descendant alive in 2020. Or in other words: it's the most recent time t such that for every two people in the world today (say, me and you), the set of {my ancestors alive at time t} and the set of {your ancestors alive at time t} are identical. And, with high probability, that time is within the past few thousand years. One consequence: if you were alive a few thousand years ago, it would be easier for you to literally become an ancestor of everyone alive in all of 2020, than it would be for you to write a book and have that book survive to 2020.

thaumasiotes 6y ago
> Or in other words: it's the most recent time t such that for every two people in the world today (say, me and you), the set of {my ancestors alive at time t} and the set of {your ancestors alive at time t} are identical. This isn't of much interest, though; the multiset of your ancestors will still look very different from most everybody else's. If I have a hundred thousand lines of direct descent from someone, and you have three, in what sense do we have identical ancestry?
- xamuel 6y ago
  Well, it's going a little beyond Rohde et al (and could be an interesting future paper if you're into writing papers), but I would imagine the same sort of modelling would probably show that it's unlikely for anyone to have exactly one line of direct descent to an IA point ancestor. Anyway, on the fun side of things, if you think of it in terms of ancient prophecies (the ancient prophecy says the chosen hero shall be a descendant of X), such prophecies usually don't distinguish between 1 line vs. many :)
PaulDavisThe1st 6y ago
I don't understand how (or why) you believe you can project the IA concept forward in time. There may be many reasons why a given population has an IA at some point in the past. Those reasons might, or might not, recur in the future.
- xamuel 6y ago
  Well, if you're an evolutionary extremist, it actually becomes tautological. Imagine half of mankind flies off to colonize space and the two halves of mankind never inter-breed ever again (the prototypical example of a future with no "reverse IA point"). Well, the evolutionary extremist would say that under those circumstances, inevitably the two branches would evolve separately, so that eventually, they would no longer be the same species. So at most one of them would be "human", and so the split would not prevent a "reverse IA point" like it initially seems (because of the word "human" in the definition of the IA point). This is formalized in the internodal species concept of Willi Hennig. According to Hennig's species notion, speciation occurs exactly when a permanent split occurs. If you could somehow contrive that mankind split in half, whether by space colonization, a gigantic wall along the equator, or by some segregationist law (but good luck enforcing that law in perpetuity!), then it would no longer be "mankind" at all, but two new species.
  - PaulDavisThe1st 6y ago
    But if you're not an evolutionary extremist ... merely someone who notes that our previous IA was around when the human population was a tiny fraction of the current level, and that there is no reason to think that it had been anywhere close to our current level before the IA. Essentially, for there to be a future IAP, there would need to be a massive near-extinction level event for humans. Obviously, there's no reason to insist that this cannot happen, but if it doesn't happen, I don't see there will ever be an IAP in the future.
    - xamuel 6y ago
      Why would there need to be a massive near-extinction event? Maybe we're talking past each-other. What I mean by "reverse IA point" is a point Y in the future such that every human in 2020 is either an ancestor of every human in Y or of no human in Y. As an over-simplified example, imagine starting this generation, everyone produces exactly 2 kids, so the total population remains static forever. In 1 generation, you'll have 2 kids, in 2 generations, you'll have 4 grandkids, ..., in 33 generations, you'll have over 7 billion grand^32kids, i.e., everyone on earth will be your descendant. Of course that's inaccurate because the growth will slow when your descendants start marrying your other descendants. But the point is, when you think of it this way, it would actually be a miracle for a reverse IA point not to exist (if we don't go extinct or colonize space). In fact, the main innovation in Rohde et al wasn't to come up with a simplistic model like the above example, but rather to factor in real things like the isolation of different continents/tiny islands for long stretches of time. With modern air travel, populations should tend to mix more, not less, and that should encourage a reverse IA point.
hinkley 6y ago
That is either a very Eurocentric calculation or there has been a lot more congress with sailors than anyone wants to admit. How could the people in the Andes trace a common ancestor a few thousand years ago with people in Tibet? Their ancestors probably last saw each other 12-14,000 years ago.
- deleted 6y ago
  [deleted]
- xamuel 6y ago
  Bear in mind, you don't need regular trade routes from Andes to Tibet, you only need the tiniest "leakage" (e.g., one outcast per 250 years leaving/entering the Andes, one outcast per 250 years leaving/entering Tibet, one explorer per 25 years starting after 1400AD traversing the Atlantic, that would probably be more than enough). In their paper, Rohde et al take all this into account, they do their homework, see p. 564 for the data they use about most recent establishment of routes. I'll quote this interesting tidbit from the paper: "Several factors could cause the time to the true MRCA or IA point to depart from the predictions of our model. If a group of humans were completely isolated, then no mixing could occur between that group and others, and the MRCA would have to have lived before the start of the isolation. A more recent MRCA would not arise until the groups were once again well integrated. In the case of Tasmania, which may have been completely isolated from mainland Australia between the flooding of the Bass Strait, 9,000–12,000 years ago, and the European colonization of the island, starting in 1803 (ref. 13), the IA date for all living humans must fall before the start of isolation. However, the MRCA date would be unaffected, because today there are no remaining native Tasmanians without some European or mainland Australian ancestry." To force the IA older than 9000-12000 years ago would require Tasmania be totally isolated, without even a single sexually productive immigrant/emigrant per 1000 years, for all that time. http://www.stat.yale.edu/~jtc5/papers/CommonAncestors/NatureCommonAncestors-Article.pdf http://www.stat.yale.edu/~jtc5/papers/CommonAncestors/Nature...
  - lazyasciiart 6y ago
    The authors express the problems with their paper very well within it: "the model contains several obvious sources of error, as it was motivated more by considerations of theoretical insight and tractability than by realism."

by xamuel 6y ago

thaumasiotes 6y ago

> Or in other words: it's the most recent time t such that for every two people in the world today (say, me and you), the set of {my ancestors alive at time t} and the set of {your ancestors alive at time t} are identical. This isn't of much interest, though; the multiset of your ancestors will still look very different from most everybody else's. If I have a hundred thousand lines of direct descent from someone, and you have three, in what sense do we have identical ancestry?

xamuel 6y ago
Well, it's going a little beyond Rohde et al (and could be an interesting future paper if you're into writing papers), but I would imagine the same sort of modelling would probably show that it's unlikely for anyone to have exactly one line of direct descent to an IA point ancestor. Anyway, on the fun side of things, if you think of it in terms of ancient prophecies (the ancient prophecy says the chosen hero shall be a descendant of X), such prophecies usually don't distinguish between 1 line vs. many :)

PaulDavisThe1st 6y ago

I don't understand how (or why) you believe you can project the IA concept forward in time. There may be many reasons why a given population has an IA at some point in the past. Those reasons might, or might not, recur in the future.

xamuel 6y ago
Well, if you're an evolutionary extremist, it actually becomes tautological. Imagine half of mankind flies off to colonize space and the two halves of mankind never inter-breed ever again (the prototypical example of a future with no "reverse IA point"). Well, the evolutionary extremist would say that under those circumstances, inevitably the two branches would evolve separately, so that eventually, they would no longer be the same species. So at most one of them would be "human", and so the split would not prevent a "reverse IA point" like it initially seems (because of the word "human" in the definition of the IA point). This is formalized in the internodal species concept of Willi Hennig. According to Hennig's species notion, speciation occurs exactly when a permanent split occurs. If you could somehow contrive that mankind split in half, whether by space colonization, a gigantic wall along the equator, or by some segregationist law (but good luck enforcing that law in perpetuity!), then it would no longer be "mankind" at all, but two new species.
- PaulDavisThe1st 6y ago
  But if you're not an evolutionary extremist ... merely someone who notes that our previous IA was around when the human population was a tiny fraction of the current level, and that there is no reason to think that it had been anywhere close to our current level before the IA. Essentially, for there to be a future IAP, there would need to be a massive near-extinction level event for humans. Obviously, there's no reason to insist that this cannot happen, but if it doesn't happen, I don't see there will ever be an IAP in the future.
  - xamuel 6y ago
    Why would there need to be a massive near-extinction event? Maybe we're talking past each-other. What I mean by "reverse IA point" is a point Y in the future such that every human in 2020 is either an ancestor of every human in Y or of no human in Y. As an over-simplified example, imagine starting this generation, everyone produces exactly 2 kids, so the total population remains static forever. In 1 generation, you'll have 2 kids, in 2 generations, you'll have 4 grandkids, ..., in 33 generations, you'll have over 7 billion grand^32kids, i.e., everyone on earth will be your descendant. Of course that's inaccurate because the growth will slow when your descendants start marrying your other descendants. But the point is, when you think of it this way, it would actually be a miracle for a reverse IA point not to exist (if we don't go extinct or colonize space). In fact, the main innovation in Rohde et al wasn't to come up with a simplistic model like the above example, but rather to factor in real things like the isolation of different continents/tiny islands for long stretches of time. With modern air travel, populations should tend to mix more, not less, and that should encourage a reverse IA point.

hinkley 6y ago

That is either a very Eurocentric calculation or there has been a lot more congress with sailors than anyone wants to admit. How could the people in the Andes trace a common ancestor a few thousand years ago with people in Tibet? Their ancestors probably last saw each other 12-14,000 years ago.

deleted 6y ago
[deleted]
xamuel 6y ago
Bear in mind, you don't need regular trade routes from Andes to Tibet, you only need the tiniest "leakage" (e.g., one outcast per 250 years leaving/entering the Andes, one outcast per 250 years leaving/entering Tibet, one explorer per 25 years starting after 1400AD traversing the Atlantic, that would probably be more than enough). In their paper, Rohde et al take all this into account, they do their homework, see p. 564 for the data they use about most recent establishment of routes. I'll quote this interesting tidbit from the paper: "Several factors could cause the time to the true MRCA or IA point to depart from the predictions of our model. If a group of humans were completely isolated, then no mixing could occur between that group and others, and the MRCA would have to have lived before the start of the isolation. A more recent MRCA would not arise until the groups were once again well integrated. In the case of Tasmania, which may have been completely isolated from mainland Australia between the flooding of the Bass Strait, 9,000–12,000 years ago, and the European colonization of the island, starting in 1803 (ref. 13), the IA date for all living humans must fall before the start of isolation. However, the MRCA date would be unaffected, because today there are no remaining native Tasmanians without some European or mainland Australian ancestry." To force the IA older than 9000-12000 years ago would require Tasmania be totally isolated, without even a single sexually productive immigrant/emigrant per 1000 years, for all that time. http://www.stat.yale.edu/~jtc5/papers/CommonAncestors/NatureCommonAncestors-Article.pdf http://www.stat.yale.edu/~jtc5/papers/CommonAncestors/Nature...
- lazyasciiart 6y ago
  The authors express the problems with their paper very well within it: "the model contains several obvious sources of error, as it was motivated more by considerations of theoretical insight and tractability than by realism."