Hello, cousin! (re: Recent common ancestors to all living humans)

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Okay. Right at the start I have problems with the paper.

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Just past the abstract it says:

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In investigations of the common ancestors of all living humans, much attention has focused on descent through either exclusively maternal or exclusively paternal lines, as occurs with mitochondrial DNA and most of the Y chromosome4,5. But according to the more common genealogical usage of the term ”ancestor’, ancestry encompasses all lines of descent through both males and females, so that the ancestors of an individual include all of that person’s parents, grandparents, and so on.

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For a population of size n, assuming random mating (and so ignoring population substructure), probabilistic analysis2 has proved that the number of generations back to the MRCA, Tn, has a distribution that is sharply concentrated around log2n. We express this using the notation Tn ~ log2n, meaning that the quotient Tn/llog2n converges in probability to 1 as n approaches infinity. In contrast, the mean time to the MRCA along exclusively matrilineal or patrilineal lines is approximately n generations6, and the distribution is not sharply concentrated. For example, in a panmictic population of one million people, the genealogical MRCA would have lived about 20 generations ago, or around the year AD 1400, assuming a generation time of 30 years. The MRCA along exclusively maternal lines would have lived something like 50,000 times earlier”in the order of one million generations ago.

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(Source: NATURE | VOL 431 | 30 SEPTEMBER 2004 |http://www.nature.com/nature, "Modelling the recent common ancestry of all living humans" by Douglas L. T. Rohde, Steve Olson & Joseph T. Chang - pdf document)

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First off lets take the age of that "MRCA along exclusively maternal lines" and see what we get:

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2004 (time of article) - 1400 AD = 600 years in round numbers.

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600 x 50,000 = 30,000,000 ... 30 million years ago?

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That's several times the age of the earliest hominids to say nothing of the age of Homo sapiens (~160,000 years), so there is obviously something wrong with this calculation.

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No, there is nothing wrong with your calculation. If humans had had a panmictic population of size 1 million for their entire history, the most recent common ancestor in the maternal line would have occurred something like 1 million generations ago(*). In the real world, however, the effective population size of humans was about 10,000, so the most recent common maternal ancestor actually lived about 100 times more recently.(*) There is one mistake, in fact, but it's not yours: the expected time to the MRCA for purely maternal inheritance is N/2 generations, not N generation. (Also, 20-25 years is a more realistic generation time for most of this period than 30 years.)On your other points, read nwr's responses. He or she understands the paper, and so far you don't.

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Get it? Genealogical ancestry is not equivalent to genetic ancestry.

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One way of thinking about it is that your genealogical ancestors are the set of people from whom it is possible for you to have inherited genetic material.

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In other words the letter throws out figures without adjusting them for population nor mentioning that such should be done, meanwhile claiming to have solid proof of a theoretical result?

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The paper is here using the example of a simple model to explain the conceptual difference between Mitochondrial Eve and the MRCA they're talking about. The simple model has nothing to do with their theoretical results.

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Thanks for the kind comment. So far I don't see anything WORTH understanding, it's a flawed model.

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My comment was not intended to be unkind. In science there is no point in pretending to understand something when one doesn't, and it is important to know what one understands and what one doesn't. (If you want a list of things I don't understand, I could supply one, but it will be lengthy, and only a small sample.) Since you don't understand it (or didn't when you were writing), you aren't able to judge the flaws in the paper.I don't think the paper has obvious major flaws, since its main result is likely to be quite robust under a wide range of models. I just don't think it's very interesting. Genealogical relationships without consideration of genetics have little biological relevance, although I suppose they might be useful for encouraging everyone to join hands and sing Kum Bah Ya or something.

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To me, looking at it as a mathematician, it doesn't much matter if the MRCA is 1000 years earlier than they suggest. The interesting thing is that it is far earlier than the time of mitochondrial eve. Note that this is interesting in the sense that it is counter-intuitive. I still agree that the result is totally insignificant.

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When you think about it, it's actually a pretty trivial result (that the common ancestor was very recent, I mean). The number of ancestors increases exponentially as you go back in time, so the time to the MRCA has to be logarithmic in the population size, for a single population. Since your ancestors quickly take over your own population, any migration rate at all means that one of them had to have come from a different population. In the source population, the migrated ancestor will also take logarithmic time to have an MRCA there. So for a widely dispersed island model with local migration only, I'd expect a logarithmic dependence on population size with a multiplicative factor for the number of populations. The details of their demographic and mating models will modulate this a little, but not much.I don't think this is a really deep result, since the logarithmic dependence was already known. Mind you, I'd never thought about how recent the MRCA must be until I heard about this paper -- but I still can't think of a reason why I should have thought about it. So I find the idea moderately interesting, but not worth publication in a top journal.

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Now, if we get a paper on "mitochondial moms" for not only the females but the males, and this was used to generate a genetic possible recent "universal mom(s)" we can expect this result to be between "mtDNA eve" and "handshake jack" -- any bets on which it will be closer to?

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Sorry, but I don't understand the question. What are mitochondrial moms for males, and what is "handshake jack"?

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Correct. And because this is universal we can use it to determine a most likely genetic ancestor time for both male and female halves of the population. There is nothing in the male genes that compares, so a male genetic ancestor cannot be determined from evidence, it can only be speculated.

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The question is whether such a common mom time be longer or shorter than the mtDNA mom for just the female population -- I suspect about the same elapsed time -- the males of the last generation have their mom's mtDNA eh?

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I'm afraid I still can't figure out what you're talking about, or what you're asking, if indeed you're asking a question.

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I repeat, if a model has not been ground-truthed {tested against data to see if the prediction rates simulate real patterns} it is not worth considering. It is not the work of other scientists to do this for them. If they want to move into this area of modeling then they need to form a cooperative effort.

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Your rules for how scientists should conduct science will come as a big shock to all the theoretical physicists out there who routinely construct models without doing the empirical testing themselves.

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you should ask cavediver or son goku what I think of such physicists ...

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Or perhaps you should ask physicists, whether experimental or theoretical, what they think of your rule.

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And why should it be any different?

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Because the scientists who do the science are more likely to know what division of labor works than you are.