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Maybe I'm stupid but I'm not sure I understand the difference between most recent common ancestor and the identical ancestors point. Can someone explain differe

Maybe I'm stupid but I'm not sure I understand the difference between most recent common ancestor and the identical ancestors point. Can someone explain differently?

tzs 5y ago
Another way to think of it is at any given time in the past, you can put any given person into one of three classes: 1. Nobody alive today is descended from them, 2. Some people alive today are descended from them, and some people alive today are not descended from them, 3. Everybody alive today is descended from them. Go back a short time, and everybody is in groups #1 or #2. Go back farther and you reach a time where there is someone in group #3. Keep going back and you reach a point where nobody is left in group #2. The most recent common ancestor point is when group #3 going back gets its first member. The identical ancestor point is when group #2 going back loses its last member.
setr 5y ago
From my reading of the above, the “most recent common ancestor” is a single individual who acts as a common ancestor to all alive today Whereas “identical ancestors” is a time period where everyone alive is an ancestor to those living today. So a group of ancestors are being selected — rather than just the one. So this should generally always be closer to now than the “single recent ancestor”, simply by virtue of allowing the ancestor tree to be more splayed out. However, my understanding of the latter must be wrong, because yesterday would always be the most recent time period holding true to that definition — or if it only counts if none of the people alive today are alive in the identical ancestor time period, then it’s always going to be today - current oldest human’s age - 1 day.
- ml_basics 5y ago
  My reading was that the “identical ancestors point" is the point at which every person is either: 1. an ancestor of all currently living people; or 2. an ancestor of no one currently living (ie their line died out). Alternatively, it’s the time when all currently living humans have exactly the same set of ancestors. Whereas “most recent common ancestor” means that the intersection of ancestries of all living people is non-empty. (That intersection being the common ancestor)
- zuminator 5y ago
  During the "most recent common ancestor" period, as you said, there's a single individual who acts as a common ancestor to all those alive today. Let's for sake of argument say that person is Genghis Khan, that Genghis Khan is somewhere in every current living person's family tree. How about Genghis's parents? Mr. and Ms. Khan (actually Yesugei and Hoelun) are surely BOTH common ancestors to everyone alive today. And surely Grandpappy Khan and the other three grandparents, and so on, going back. That ancestral tree keeps growing and eventually, you'll get to the point in the past that everyone alive at that period is an ancestor to Genghis Khan. Which means they are all also in our family tree. Therefore we all have an identical looking family tree from that point backwards.
lordnacho 5y ago
MRCA: Take every current person's family tree, who is the most recent person who is in all of them? This doesn't mean there was only one ancestor around at the time, though if all life came from the same cell that is of course true if you go back that far. IAP: For any given time X, every person at time X is either an ancestor of all current people, or none. Take everyone's family tree, and since it can't be size 2^x generations back, the overlaps eventually mean that all those people at time X who have a current descendent actually have everyone alive currently as their descendant, and everyone else at time X is the ancestor of nobody currently alive.
deleted 5y ago
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quietbritishjim 5y ago
At any given moment in time, let n be the number of people who are an ancestor of everyone alive in 2021 (or 2022!). Right now, n = 0 because there is nobody on earth that is the anscestor of everybody else. If you go back far enough, you'll get n = 1. That's a single person that is an anscestor for everyone alive today. That's the most recent common anscestor. Other people then were anscestors to only some people alive today. If you go back further, you'll get n = total number of people on the planet at the time. Everyone on earth is an anscestor of people alive today. Except, wait, that's not true! There will always be some people alive who aren't anscestors to anyone alive today. Maybe they simply didn't get lucky at all in their lifetime, or were infertile. Maybe a collection of people only bred within a community (tribe, city, etc) which then got wiped out in a war. Maybe something more complex happened. So n=1 is possible, but n=everyone never happened. So, totally ignore all the people who aren't anscestors to anyone in 2021, leaving only people that still have at least some anscestors today. Then n = 1 is (still) the most recent common anscestor. And the earlier date, n = everyone (that we care about) is the identical anscestors point.
- fastball 5y ago
  Ah makes sense, I think the use of present tense in the original explanation was throwing me for a loop.

Maybe I'm stupid but I'm not sure I understand the difference between most recent common ancestor and the identical ancestors point. Can someone explain differently?

tzs 5y ago

Another way to think of it is at any given time in the past, you can put any given person into one of three classes: 1. Nobody alive today is descended from them, 2. Some people alive today are descended from them, and some people alive today are not descended from them, 3. Everybody alive today is descended from them. Go back a short time, and everybody is in groups #1 or #2. Go back farther and you reach a time where there is someone in group #3. Keep going back and you reach a point where nobody is left in group #2. The most recent common ancestor point is when group #3 going back gets its first member. The identical ancestor point is when group #2 going back loses its last member.

setr 5y ago

From my reading of the above, the “most recent common ancestor” is a single individual who acts as a common ancestor to all alive today Whereas “identical ancestors” is a time period where everyone alive is an ancestor to those living today. So a group of ancestors are being selected — rather than just the one. So this should generally always be closer to now than the “single recent ancestor”, simply by virtue of allowing the ancestor tree to be more splayed out. However, my understanding of the latter must be wrong, because yesterday would always be the most recent time period holding true to that definition — or if it only counts if none of the people alive today are alive in the identical ancestor time period, then it’s always going to be today - current oldest human’s age - 1 day.

ml_basics 5y ago
My reading was that the “identical ancestors point" is the point at which every person is either: 1. an ancestor of all currently living people; or 2. an ancestor of no one currently living (ie their line died out). Alternatively, it’s the time when all currently living humans have exactly the same set of ancestors. Whereas “most recent common ancestor” means that the intersection of ancestries of all living people is non-empty. (That intersection being the common ancestor)
zuminator 5y ago
During the "most recent common ancestor" period, as you said, there's a single individual who acts as a common ancestor to all those alive today. Let's for sake of argument say that person is Genghis Khan, that Genghis Khan is somewhere in every current living person's family tree. How about Genghis's parents? Mr. and Ms. Khan (actually Yesugei and Hoelun) are surely BOTH common ancestors to everyone alive today. And surely Grandpappy Khan and the other three grandparents, and so on, going back. That ancestral tree keeps growing and eventually, you'll get to the point in the past that everyone alive at that period is an ancestor to Genghis Khan. Which means they are all also in our family tree. Therefore we all have an identical looking family tree from that point backwards.

lordnacho 5y ago

MRCA: Take every current person's family tree, who is the most recent person who is in all of them? This doesn't mean there was only one ancestor around at the time, though if all life came from the same cell that is of course true if you go back that far. IAP: For any given time X, every person at time X is either an ancestor of all current people, or none. Take everyone's family tree, and since it can't be size 2^x generations back, the overlaps eventually mean that all those people at time X who have a current descendent actually have everyone alive currently as their descendant, and everyone else at time X is the ancestor of nobody currently alive.

deleted 5y ago

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quietbritishjim 5y ago

At any given moment in time, let n be the number of people who are an ancestor of everyone alive in 2021 (or 2022!). Right now, n = 0 because there is nobody on earth that is the anscestor of everybody else. If you go back far enough, you'll get n = 1. That's a single person that is an anscestor for everyone alive today. That's the most recent common anscestor. Other people then were anscestors to only some people alive today. If you go back further, you'll get n = total number of people on the planet at the time. Everyone on earth is an anscestor of people alive today. Except, wait, that's not true! There will always be some people alive who aren't anscestors to anyone alive today. Maybe they simply didn't get lucky at all in their lifetime, or were infertile. Maybe a collection of people only bred within a community (tribe, city, etc) which then got wiped out in a war. Maybe something more complex happened. So n=1 is possible, but n=everyone never happened. So, totally ignore all the people who aren't anscestors to anyone in 2021, leaving only people that still have at least some anscestors today. Then n = 1 is (still) the most recent common anscestor. And the earlier date, n = everyone (that we care about) is the identical anscestors point.

fastball 5y ago
Ah makes sense, I think the use of present tense in the original explanation was throwing me for a loop.