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My main question is this - I heard Richard Dawkins say in a video that after 1000s of years, any given individual alive today will be either an ancestor to ALL of the humans (in that future time) or none of them. What is the rationale behind this? I'm unable to find the link to that video. I'll keep searching and add it once I find it.

But my fundamental question is perhaps this - is it necessary that a given species must have at least one common ancestor? I understand that there are multiple common ancestors to humans, the most recent one being MCRA. But could it have been otherwise? I think there's some underlying logic that I'm missing.

I'd appreciate it if someone could answer in a non-technical manner since I know very little about genetics and how all this works.

Update:

Consider the following example. Here's there's heavy inbreeding and as you can see no matter how many generations pass, the population are never going to share a common ancestor from current generation or later. (They could still have a common ancestor from before). But this case violates Dawkins' statement.

enter image description here

But this is an unrealistic example with no mixing and cross breeding. Let's consider another example as shown below. I want to see how the individual A from current generation can be related to all the population in future. As shown in the diagram, although A is related to everyone in some future generation, he is not a common ancestor to all of them. The actual common ancestor belongs to some earlier generation.

enter image description here

For any given no. of generations, theoretically, I could conceive similar lineages by which someone from current generation need not necessarily be a common ancestor to all of the future population (or none). So I don't understand how Dawkins' statement is inevitable.

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  • $\begingroup$ Your examples don't have sexual reproduction: everyone has just 1 parent. Why? $\endgroup$ – Bryan Krause May 14 at 0:47
  • $\begingroup$ @BryanKrause first example does show two parents each. Avoided that in second example because the diagram was getting cluttered. The point of my examples is still conveyed correctly, I hope. $\endgroup$ – yathish May 14 at 3:01
  • $\begingroup$ It isn't at all, because you are missing that for each prior generation, the number of ancestors approximately doubles. This is only not true in your extreme example of inbreeding. $\endgroup$ – Bryan Krause May 14 at 5:38
  • $\begingroup$ @BryanKrause that's a good point. So the exponential increase in number of ancestors makes it extremely likely for the case of having a common ancestor. But it still doesn't prove that there has to be a common ancestor. $\endgroup$ – yathish May 14 at 6:24
  • $\begingroup$ Actually it does. Try drawing it out. $\endgroup$ – Bryan Krause May 14 at 12:55
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any giving individual alive today will be either an ancestor to ALL of the humans (in that future time) or none of them. What is the rationale behind this?

It's a simple mathematical observation nothing more than that.

One that doesn't actually require doing any math to understand.

Take a few billion people, let them mix & breed freely within the confines of a limited environment (the world) & eventually (after enough generations) every single one of the current population will be be related & therefore descended from every single one of the original population that have any surviving descendants.

  • Those whose line doesn't die out will inevitably be an ancestor of every living person at a future generation, eventually.
  • Those who die with no offspring or have last-descendants do it for them, have no descendants.

It's just a simple statement of something that's obvious when you think on it.

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  • $\begingroup$ Why is it necessary that after enough generations everyone will be related? Is it an assumption based on enough cross breeding? $\endgroup$ – yathish May 7 at 14:02
  • $\begingroup$ @yathish It's not "necessary", it is inevitable. $\endgroup$ – Pelinore May 7 at 16:40
  • $\begingroup$ but why is it inevitable? For example, if two populations of humans are separated in two different islands, it won't hold true. I feel like I'm missing something very obvious. $\endgroup$ – yathish May 7 at 16:49
  • $\begingroup$ @yathish: But the populations on those islands must have come from some common source (if you look back far enough), e.g. the populations of say Hawai'i, Tahiti, and New Zealand prior to European contact. Or the population of North America, effectively isolated from other humans for ~10,000 years. $\endgroup$ – jamesqf May 7 at 17:27
  • $\begingroup$ @yathish Your not thinking in big enough numbers when it comes to generations & years, if people got there once they can get there again so in the time scale of hundreds of thousands of years & more your islands aren't isolated, at all ~ besides that, where did these two populations come from & how are they the same specie if they didn't ever share a common ancestor? $\endgroup$ – Pelinore May 7 at 18:03
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is it necessary that a given species must have at least one common ancestor?

You could imagine a species divided into groups, each group having a single common ancestor. In that case, you might ask, do the common ancestors themselves have a common ancestor, somewhere further back? Our current hypothesis is that, if you go back far enough you can find a creature that was the common ancestor of any arbitrary group of individuals.

The alternative hypothesis is that life evolved independently several times, and there were originally more than one 'tree of life', and that these independent instances of life somehow evolved in parallel, maintaining compatibility with each other, enough so that there could be cross-breeding between the two trees at a later date. In that way it might be possible that some species may not have a single common ancestor. This seems so unlikely to be true though that it it not generally even considered, for current species.

The common ancestor of all humans might not necessarily look exactly like a human (Homo sapiens) though, it may have existed at a time before Homo sapiens evolved, because it is possible that branches on the tree of life can diverge a little and then cross again, as human and neanderthal branches did.

The wikipedia page on the human MRCA (https://en.wikipedia.org/wiki/Most_recent_common_ancestor) says the time since the MRCA is unknown, but has been estimated between 100,000 to 200,000 years ago. This is around the same time Homo sapiens first appeared in the fossil record, that we have found so far.

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