Most recent common ancestor between strangers?

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Most recent common ancestor between strangers?

Postby SunAvatar » Sun Feb 19, 2012 8:15 pm UTC

It's common knowledge that "we're all related if you go back far enough," but it's less commonly known exactly how far back is far enough. I'm guessing it's not actually that far. Obviously the degree will vary from place to place, based on cultural norms, rate of immigration/emigration, etc., but overall I suspect most people are more related to the people around them than they think. In particular, as an Anglo born and raised in the Boston area, I wouldn't be surprised to learn that I'm at least a fifth cousin to the majority of other Anglos born and raised in the Boston area.

But I really don't know for sure, and some cursory googling hasn't turned much up. So I ask the geneticists among us: do you know of any data on this subject? How closely related are two strangers, on average, in terms of distance to most recent common ancestor? And has this number gone up or down over time?
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Re: Most recent common ancestor between strangers?

Postby Diadem » Sun Feb 19, 2012 11:13 pm UTC

I do know the most recent common ancestor of all humans lived about two to four thousand years ago.

The most recent common ancestor of two random humans will of course be much closer. And if you look within regions, it'll be much closer still.

I read somewhere once that Obama and Bush are something like 12th cousins. Wouldn't surprise me if that was true for pretty much all Americans. With Europeans probably not far behind.
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Re: Most recent common ancestor between strangers?

Postby Interactive Civilian » Mon Feb 20, 2012 12:51 am UTC

Diadem wrote:I do know the most recent common ancestor of all humans lived about two to four thousand years ago.

Unless you are a creationist who flat out rejects evidence, this is absolutely wrong. You are off by an order of magnitude. To explain by way of example from Dawkins's "The Ancestor's Tale", there was an aboriginal population on Tasmania that was isolated for ~13,000 years until the 18th and 19th century, when they were exterminated by settlers (the last died in 1876). They were human, and thus your statement is immediately refuted.

The concestor of all current humans existed anywhere between a few to several tens of thousands of years ago (call 20,000 - 80,000 a reasonable window), and it is probably impossible to pin it down any closer than that. And, this is only talking about descent, not of genes. Currently the best molecular estimates for "mitochondrial Eve" is about 140,000 years ago, and "y-chromosome Adam" about 60,000 years ago.

The chapter titled Rendezvous 0 in "The Ancestor's Tale" covers this very well, and I highly recommend reading it.

Now, back to the original idea, yes the MRCA between any two random strangers will very much depend on your location. For immigrants living in the States, especially those who have not left the areas settled by their ancestors, it may very well be only a century or two (perhaps less). For many others, maybe only a few centuries or maybe a millenium or two back into Europe. For me, I live in Thailand, and as an American ex-pat here, I wouldn't be surprised if my MRCA with any random Thai person is anywhere between 1000 and 10,000 years.
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Re: Most recent common ancestor between strangers?

Postby Diadem » Mon Feb 20, 2012 1:27 am UTC

Interactive Civilian wrote:
Diadem wrote:I do know the most recent common ancestor of all humans lived about two to four thousand years ago.

Unless you are a creationist who flat out rejects evidence, this is absolutely wrong. You are off by an order of magnitude. To explain by way of example from Dawkins's "The Ancestor's Tale", there was an aboriginal population on Tasmania that was isolated for ~13,000 years until the 18th and 19th century, when they were exterminated by settlers (the last died in 1876). They were human, and thus your statement is immediately refuted.

Very few isolated tribes are truly isolated. It only requires one visitor somewhere in the last 2000 years to share a common ancestor. As for your example, if the last one died in 1876 then it doesn't matter if we share ancestors. When you look at the most recent common ancestor (MRCA) you only count living humans. If you look at all humans that ever lived, there is no common ancestor at all.

The concestor of all current humans existed anywhere between a few to several tens of thousands of years ago (call 20,000 - 80,000 a reasonable window), and it is probably impossible to pin it down any closer than that. And, this is only talking about descent, not of genes. Currently the best molecular estimates for "mitochondrial Eve" is about 140,000 years ago, and "y-chromosome Adam" about 60,000 years ago.

Mitochondrial Eve and y-chromosomal Adam are the most common recent ancestors in the direct female and direct male line. A hundred generations ago, I have 2^100 = 1,267,650,600,228,229,401,496,703,205,376 ancestors, not counting inbreeding. Only 1 of those though is a direct female line, and only one of those is in a direct male line. So obviously mitochondrial Eve and y-chromosome Adam represent only the minutest faction of our common ancestors.

In fact both mitochondrial Eve and y-chromosome Adam lived far longer ago than the so-called identical ancestors point. The point where every single human living today shares the exact same ancestors, which is placed, from memory, at about 10K years ago.

I'll look up some data for you.
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Re: Most recent common ancestor between strangers?

Postby SunAvatar » Mon Feb 20, 2012 1:30 am UTC

Interactive Civilian, what do you make of the 2004 Nature article by a Yale mathematician (PDF) suggesting that the MRCA of all or nearly all living humans ought to be about 3,000 years old? Particularly since ethnic "purity" is a lot less common than most people believe---we nearly all have ancestors scattered across several continents, regardless of what our parents told us---it doesn't seem that implausible to me that the descendants of at least one individual a few thousand years ago have managed to combine their genes with everyone else still around, with the exception of a few uncontacted peoples. (And total genetic isolation is probably rarer than it might seem---even in a population considered to be "uncontacted," a few individuals may have had some, ah, contact with strangers.)

[EDIT: Replaced the link with a link to the actual article.]
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Re: Most recent common ancestor between strangers?

Postby Diadem » Mon Feb 20, 2012 1:35 am UTC

Here, a source, though you need a login to read more than the abstract: http://www.nature.com/nature/journal/v4 ... 02842.html

edit: Ah, ninja'd by someone who does have the full article. Thanks SunAvatar!
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Re: Most recent common ancestor between strangers?

Postby Soralin » Mon Feb 20, 2012 1:45 am UTC

Diadem wrote:If you look at all humans that ever lived, there is no common ancestor at all.

There still always has to be a common ancestor. The only other alternative to that would be separate parallel lines of ancestry running all the way back to separate beginnings of life on this planet.

(Which is even crazier when you look at what that implies going forward in time, since it requires two separate beginnings of life, which then proceed to not interact with each other for the next 4 billion years, before eventually both, on their own, producing humans that are essentially identical, at the same time.)
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Re: Most recent common ancestor between strangers?

Postby mfb » Mon Feb 20, 2012 1:48 am UTC

To look at some easy things first:

Take a group of fixed size N>>1, 50% male 50% female, group them at random as pairs of (malei,femalei), let each pair have exactly two children at the same time.
A person within this generation has one brother or sister, 2 cousins of first grade, 4 of second grade, ... - this pattern changes significantly near the group size, as it becomes usual that two persons within a generation share multiple independent ancestors.
However, a real population of constant size will always have members which have less than two children, and some with more than 2 childen. In a wild guess, I make up that these two effects cancel.
This gives an expectation value of ~ld(N) generations (+-1, too lazy to calculate) for the common ancestor of two randomly chosen members of this group within the same generation.

However, reality is a bit more complicated. People tend to choose partners which have a similar location, a similar cultural background, ... each with some chances to deviate from the "usual" choice.

In Europe, there are some small villages where the model given above should be a useful approximation. Take one of these with ~500 members, and you get ~9 generations or ~200 years. Another interesting result is that these villages tend to have few but very common last names.

What about a town with 10^5 inhabitants? The same rules gives ~17 generations or ~350 years. Not so much more than the small village, but a town of this size has much more contact with its neighborhood, especially within this time scale (and even more if you live in America (all of it)).

Let's take a county: N=10^8, ld(N)=27 -> ~550 years. Certainly this needs some adjustment towards the higher side, as 550 years are a lot of time for citizens to move into the country or to leave it, which reduces the relationships and therefore increases the time to the last common ancestor. In addition, the assumption of random pairs is now wrong - people prefer to stay at the same place, which also increases the average time. And they tend to stay within the same cultural region. These effects are even more important as the time-scale reaches into the Middle Ages now.

We have ~2^33 humans on earth. Again using ~20 years per generation, I give a lower bound of ~700 years. Distance on earth will certainly increase this value, but I would expect that a loose connections between regions is actually sufficient to keep this effect small: Imagine one chinese which may have come to Europe at the time of Marco Polo (~1300) and got children in europe. Using the average of 2 children per generation, his descendents are likely to be spread across Europe (and all other regions of the world) now. Something in the region of ~2000 years does not look so implausible, at least if you exclude America.


I did not take the exponential population growth of the recent time into account here. However, I do not expect too large effects from it.


It should be possible to determine upper bounds: Using only mitochondrial DNA, it is possible to track lines of mothers. The deviation in the genome then gives an estimation about the meeting point of the lines of mothers. The same is true for the Y-genome for males and their lines of fathers. However, some Wikipedia searches showed me that there is an article about First-cousin marriages (sic!), but I did not find interesting numbers. However, the article shows that the assumption of random partners can be very wrong: "but marriages between first and second cousins nevertheless account for over 10% of marriages worldwide. They are particularly common in the Middle East, where in some nations they account for over half of all marriages".
Genetic history of Europe has a lot of stuff, bit it mainly focuses on the pool of genes, not on single individuals which have contributed at some time.

Well... very high, but quite solid upper bounds can be obtained from the big migration waves: Out of Africa ~10^5 years ago (should be enough time for the whole world to have a lot of common ancestor of all), or into America ~1.5*10^4 years ago (only for Native Americans and their descendants of course)


>> If you look at all humans that ever lived, there is no common ancestor at all.
Of course there is. It might be hard to define "human" at the border, but certainly there was a living species which is a common ancestor of all humans. Or do you really think the whole life on earth has two independent lines which both evolved Homo Sapiens independently?
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Re: Most recent common ancestor between strangers?

Postby Diadem » Mon Feb 20, 2012 4:06 am UTC

Soralin wrote:
Diadem wrote:If you look at all humans that ever lived, there is no common ancestor at all.

There still always has to be a common ancestor. The only other alternative to that would be separate parallel lines of ancestry running all the way back to separate beginnings of life on this planet.

Fair enough. I meant 'no human common ancestor'.
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Re: Most recent common ancestor between strangers?

Postby Interactive Civilian » Mon Feb 20, 2012 5:55 am UTC

Ouch. It appears I was completely wrong and using ideas from an outdated model. My apologies, and I retract my previous.

I would like to point out that both my earlier estimate (which would quite possibly have been accurate if they hadn't slaughtered all of the Tazmanians :p ) and the estimates in the 2004 Nature paper linked above (thanks for that, by the way; though some of the math is beyond me, it's still a fascinating read) are models which may or may not be based on realistic migration patterns and interbreeding estimates.

However, I do concede that within the last 3 millennia is a reasonable lowerbound on an estimate for MRCA.

My apologies for being vehemently wrong with my outdated information. :oops: :(
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Re: Most recent common ancestor between strangers?

Postby Ulc » Mon Feb 20, 2012 8:07 am UTC

Diadem wrote:Fair enough. I meant 'no human common ancestor'.


And you'd still be wrong

The Mitocondrial Eve lived between 152,000 - 234,000 BP, and the Y-crhomosonal Adam lived from 60.000 to 142.000 years ago.

For both of those all humans of that gender descends directly from them in a unbroken line of [appropriate gender] - to posit that the lines didn't mix entirely at least at some point a bit earlier, if we accept broken lines are a bit absurd. By necessity all men alive is descended from a woman, meaning that mitocondrial Eve is a common ancestor for all humans by a line of women finalized with a single man at the end.
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Re: Most recent common ancestor between strangers?

Postby yurell » Mon Feb 20, 2012 8:24 am UTC

Ulc wrote:
Diadem wrote:Fair enough. I meant 'no human common ancestor'.


And you'd still be wrong. The Mitocondrial Eve lived between 152,000 - 234,000 BP, and the Y-crhomosonal Adam lived from 60.000 to 142.000 years ago.
For both of those all humans of that gender descends directly from them in a unbroken line of [appropriate gender] - to posit that the lines didn't mix entirely at least at some point a bit earlier, if we accept broken lines are a bit absurd. By necessity all men alive is descended from a woman, meaning that mitocondrial Eve is a common ancestor for all humans by a line of women finalized with a single man at the end.


Edited formatting to save vertical space. They're the most common recent ancestor of all humans alive now; Diadem is saying (afaict) that there is no common ancestor for all humans that ever lived combined, although I think this eminently unlikely — the common ancestor may not have been human, in which case its contemporaries didn't need to be related to it, and when humans did evolve they were all already related to it through the inbreeding of the tribes of apes from which we descended.
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Re: Most recent common ancestor between strangers?

Postby gmalivuk » Mon Feb 20, 2012 8:25 am UTC

Right. For Diadem's point, simply note that Mitochondrial Eve is not a common ancestor of anyone who lived before she was born.
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Re: Most recent common ancestor between strangers?

Postby Diadem » Mon Feb 20, 2012 8:26 am UTC

Ulc wrote:
Diadem wrote:Fair enough. I meant 'no human common ancestor'.


And you'd still be wrong

The Mitocondrial Eve lived between 152,000 - 234,000 BP, and the Y-crhomosonal Adam lived from 60.000 to 142.000 years ago.

For both of those all humans of that gender descends directly from them in a unbroken line of [appropriate gender] - to posit that the lines didn't mix entirely at least at some point a bit earlier, if we accept broken lines are a bit absurd. By necessity all men alive is descended from a woman, meaning that mitocondrial Eve is a common ancestor for all humans by a line of women finalized with a single man at the end.

Right, true. But we were talking about a common ancestor of all humans who ever lived. Mitochondrial Eve isn't, because she obviously isn't an ancestor of her contemporaries. The same is true for any human, from which it immediately follows that the MRCA of all humans that ever lived wasn't human. This is actually kind of an important point. There was no singular mutation that caused the first human to be born from which all humans are descended. Evolution doesn't work like that. Humans evolved as a group.

Of course there is no singular point at which non-humans became humans, so it's not a very meaningful question anyway: If the MRCA of all humans living today lived only a few thousand years ago then the MRCA of all humans living X years ago lived a few thousand years before X. The longer back you go the shorter this period probably gets, because the population is smaller and not yet spread across the entire world. Over such short periods there's not really a meaningful distinction between 'human' and 'not human'. Speciation goes quite fast, but not that fast.


edit: Ninja'd, damn. Well in that case let me muse a bit more. We don't really know much about the origin of life. It's certainly not unimaginable that abiogenesis independently occurred several times. So I don't think we can be certain that every single living being that is alive now has a common ancestor. And we certainly can't be certain that all living beings that ever lived share a common ancestor.
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Re: Most recent common ancestor between strangers?

Postby Ulc » Mon Feb 20, 2012 10:23 am UTC

Diadem wrote:Right, true. But we were talking about a common ancestor of all humans who ever lived.


See, that would be me posting before coffee in the morning. But as you say the idea of "all humans that ever lived doesn't have a common human ancestor" runs into the problem of being a circular argument and doesn't make sense because it requires defining a sharp line between "Human" and "Non-human that have human offspring".
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Re: Most recent common ancestor between strangers?

Postby Diadem » Mon Feb 20, 2012 10:42 am UTC

But it's still an important point that there is no human from which we all descent in the sense that that particular person had no contemporaries. This is a common misconception about evolution. That you had apes, and one day a mutation occurred and one of those apes' offspring was a human, and every human ever is descended from that person. But that's not how it works. We evolved as a group. Mutations spread through genetic recombination, not because the offspring of everybody who doesn't have that mutation dies out.
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Re: Most recent common ancestor between strangers?

Postby Karantalsis » Mon Feb 20, 2012 3:13 pm UTC

Diadem wrote:Ninja'd, damn. Well in that case let me muse a bit more. We don't really know much about the origin of life. It's certainly not unimaginable that abiogenesis independently occurred several times. So I don't think we can be certain that every single living being that is alive now has a common ancestor. And we certainly can't be certain that all living beings that ever lived share a common ancestor.


Given that there are plenty of variant nucleic and amino acids, which are interelated through codon usage, not to mention that nucleic acid transmission of information is not the only plausible model for life, then separate abiogenesis events that intercombined, especially that intercombined without in some way leaving a universal common ancestor is so vanishingly unlikely as to be comlpetely negligible.

There is a mountain of evidence for this. The most obvious being that every living thing has the same genetic code. The same amino acids are used. And we all have demonstrably related genes (note that doesn't mean every gene is shared across everything).

http://www.ncbi.nlm.nih.gov/pubmed/21079939
http://www.ncbi.nlm.nih.gov/pubmed/21034819

Some papers on the subject.

In order for life as it unfolds before us to make sense, somewhere in the mists of time, there must be LUCA. And before that numerous UCAs.
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Re: Most recent common ancestor between strangers?

Postby goodscoop » Wed Mar 21, 2012 3:11 am UTC

Hi all,
I found this blog about MRCA and I had to jump in. I am dating someone and we can trace our MRCA to 1655/1660. Would you have any concerns marrying this person given this information? It sounds like I would be marrying the average stranger from a few of the posts on here. I'd love to hear your thoughts or potentially an idea on where I might research this further.
Any help is appreciated and thanks.
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Re: Most recent common ancestor between strangers?

Postby Diadem » Wed Mar 21, 2012 3:36 am UTC

I don't think that is any problem at all.

Don't forget that throughout most of history people hardly ever travelled, and usually did not marry people who grew up more than a few kilometres away. So the average marrying couple was probably much closer related than that for most of history. Even today plenty of people marry someone from their own region and cultural group. I suspect having a MCRA 300-400 years ago is very common.

Meanwhile, biologically, marrying siblings is problematic, but even full cousins isn't that bad, as long as you don't make a habit of it for multiple generations in a row. And anybody separated by more than a few generations is not problem at all.
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Re: Most recent common ancestor between strangers?

Postby ahammel » Wed Mar 21, 2012 4:30 am UTC

I imagine 17th century is pretty close to the average for two strangers whose immediate ancestors lived in similar parts of the world. Don't worry about it.
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Re: Most recent common ancestor between strangers?

Postby Xanthir » Wed Mar 21, 2012 10:50 pm UTC

goodscoop wrote:Hi all,
I found this blog about MRCA and I had to jump in. I am dating someone and we can trace our MRCA to 1655/1660. Would you have any concerns marrying this person given this information? It sounds like I would be marrying the average stranger from a few of the posts on here. I'd love to hear your thoughts or potentially an idea on where I might research this further.
Any help is appreciated and thanks.


If your MRCA is more than a hundred years or so back (which corresponds to being third or fourth cousins), you're completely fine. As Diadem says, you can get closer than that as long as you don't do it for too many generations in a row (though some areas have laws against marrying within a certain ancestral distance).
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Re: Most recent common ancestor between strangers?

Postby SunAvatar » Thu Mar 22, 2012 4:53 am UTC

Karantalsis wrote:There is a mountain of evidence for this. The most obvious being that every living thing has the same genetic code. The same amino acids are used.

I remember reading somewhere that this is only almost true: a few species of microörganism have been discovered that have variant genetic codes. These variations are pretty minor, so it it still sounds more likely than not that all cells have a single common ancestor. I find it surprising that any variation in a genetic code already in use could work out well enough to be selected for, but Wikipedia says "slight variations on the standard code had been predicted earlier," so I'll trust the biologists on this.
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Re: Most recent common ancestor between strangers?

Postby ahammel » Sat Mar 24, 2012 1:26 am UTC

SunAvatar wrote:
Karantalsis wrote:There is a mountain of evidence for this. The most obvious being that every living thing has the same genetic code. The same amino acids are used.

I remember reading somewhere that this is only almost true: a few species of microörganism have been discovered that have variant genetic codes. These variations are pretty minor, so it it still sounds more likely than not that all cells have a single common ancestor. I find it surprising that any variation in a genetic code already in use could work out well enough to be selected for, but Wikipedia says "slight variations on the standard code had been predicted earlier," so I'll trust the biologists on this.

Human mitochondria use a slightly different genetic code than human nucleii. They originated as endosymbionts.

On the "differing genetic codes were predicted earlier" bit, be aware that the wikipedia citation for that points to Francis Crick and Leslie Orgel's infamous "Directed Panspermia" paper. From the abstract:
Crick and Orgel wrote:It now seems unlikely that extraterrestrial living organisms could have reached the earth either as spores driven by the radiation pressure from another star or as living organisms imbedded in a meteorite. As an alternative to these nineteenth-century [?] mechanisms, we have considered Directed Panspermia, the theory that organisms were deliberately transmitted to the earth by intelligent beings on another planet.

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