# Hey, are you my long lost tenth cousin?March 16, 2008 6:19 PM   Subscribe

What's the chance that we're related?

I had read somewhere (I forget where or the details) that researchers had determined through mathematical and Human reproductive analysis that all living Humans are likely to have had a shared ancestor that lived around 2,000 years ago. Don't quote me on the exact numbers.

I'm curious, how many years would I have to go back in order to predict that a random stranger and I have a 25% chance of a common ancestor?

OK, that last sentence was worded kind of ambiguously and confusingly); let's see if I can state it more mathematically. For any two randomly selected Humans, there is a 25% chance that they have a common ancestor within the last X years. What is X?
posted by brandnew to Science & Nature (16 answers total) 4 users marked this as a favorite

Actually, mathematical models show that the shared ancestor could be as recent as 1200 AD.

References

Some simulations run by Mark Liberman of Language Log
posted by vacapinta at 6:25 PM on March 16, 2008

According to this site, it's a royal Egyptian from 1600 BC. However, that's assuming there's no Aboriginals in Australia left without dna input from Caucasian ancestors, seeing as they (the aboriginals) came across 40,000 years ago.
posted by b33j at 6:28 PM on March 16, 2008

And here's a story about research on this topic done by someone at Yale.
posted by b33j at 6:30 PM on March 16, 2008

And Obama and Cheney are eighth cousins.
posted by fshgrl at 7:32 PM on March 16, 2008

The date must be some kind of floating mean that gets more and more recent as more and more peoples become not-isolated.

And yet must go longer and longer into the past as you get a larger and larger sample, because you are more likely to find some island where nobody has left for 40,000 years. Bam, that ancestor just got way more ancient.

This is an interesting statistics type of question, because it's an exception to the usual "sample 1000 people and extrapolate" kind of statistics, since every person has the chance to be a block buster, and that . Kinda like trying to take a survey to find a lottery winner or predict an election.
posted by gjc at 7:58 PM on March 16, 2008

According to this site, it's a royal Egyptian from 1600 BC.

That site is 100% crazy. Some other conclusions: "Quite likely everyone in the West descends from the Prophet Muhammad, c. 600 AD."
posted by damn dirty ape at 9:34 PM on March 16, 2008

That site is 100% crazy.

Its using crazy examples, yes, but the math is sound.
posted by vacapinta at 9:41 PM on March 16, 2008

"Humans are likely to have had a shared ancestor that lived around 2,000 years ago."

Er, not 2,000 years ago. More like 80,000.

However, within an inter-breeding population (e.g, all Japanese), after n generations you reach a point where all persons of generation m are likely to have as ancestors all persons of generation n (n < m) who are still ancestors. At that point, the proportion of n who are still ancestors is likely to be about 80%. What do I mean by still ancestors? That is, they have descendants in generation m.

This is how it works. Someone in generation m who had no children is nobody's ancestor. Someone in generation n who had children (generation n + 1) who had no children was an ancestor to generation n + 1, but is not an ancestor to anyone in generation m. And so forth. Once we reach generation m, about 20% of the people in generation n's lines have died out. That leaves 80%, and enough generations of mixing that anyone in n who is an ancestor to anyone in m has a high likelihood of being an ancestor to everyone in m.

How many generations that takes depends on how thorough the mixing is (e.g., how much each sub-population is exogamous), the size of the population, and the penetration of the population in after generation n of "foreign" genes. On average for European populations that are not highly endogamous (Jews, Roma, etc.), 1000 years is about right.

So if you're of exclusively European stock, anyone living in Europe in 1000 AD who has descendants living today is likely your ancestor.
posted by orthogonality at 10:05 PM on March 16, 2008

Er, not 2,000 years ago. More like 80,000.

No, 2,000. With only minor mixing assumptions. Thats the core of Chang's work. Read the paper. He's a statistician.

So if you're of exclusively European stock, anyone living in Europe in 1000 AD who has descendants living today is likely your ancestor.

Thats just it. You dont have to be exclusively of European stock. All you need is one European ancestor. And you inherit his/her entire geneaology. Thats pretty easy to see, I think.
posted by vacapinta at 10:35 PM on March 16, 2008

It's been too long for me to even attempt the math, but I do recall that the "Mitochondrial Eve" was 200,000 estimated to live years ago (the wikipedia link says 140,000).
posted by kisch mokusch at 4:20 AM on March 17, 2008

ugh, place the 200,000 after years!
posted by kisch mokusch at 4:21 AM on March 17, 2008

A factoid of related interest: the eruption of a supervolcano in Indonesia about 71,500 years ago created a volcanic winter during which the human race may have dwindled to as few as one to ten thousand breeding pairs. A breeding bottleneck like that makes for a lot of cousins.
posted by Zed_Lopez at 10:07 AM on March 17, 2008

kisch mokusch: if you'd actually looked at that wikipedia article, it says (the bold is mine):

Mitochondrial Eve is the most recent common ancestor (MRCA) of all humans via the mitochondrial DNA pathway, not the unqualified MRCA of all humanity. All living humans can trace their ancestry back to the MRCA via at least one of their parents, but Mitochondrial Eve is defined via the maternal line. Therefore, she necessarily lived much longer ago than the MRCA of all humanity.

This question is about the unqualified MRCA.

Regarding the original question. Chang provides the math to figure this out but I dont have the time today to get the OP his answer. I'll play with this later though...
posted by vacapinta at 11:02 AM on March 17, 2008

Oh, I looked at it. Personally, I would've bolded that she necessarily lived much longer ago than the MRCA of all humanity.
Which indicates an upper limit to the question. The MRCA lived less than 140,000 years ago.

I didn't look at your link very closely, but I did notice: Apparently [Nei and Roychoudhury, 1982] and [Goldstein et al, 1995] use DNA to estimate ages for the MRCA of 116,000 and 156,000 years ago. One wonders if they are aware that DNA cannot be used to estimate the MRCA.

So you're left with a probability argument to determine the MRCA, which I haven't read (sorry). But the solid evidence puts it at less than 140,000 years. It's not a perfect answer, and but a little context helps, I think.
posted by kisch mokusch at 1:22 PM on March 17, 2008

Well, I read Chang's Nature paper, but I think you might have misread a few things.

His simple model does give the MRCA at AD 1400, however, as the authors write:
"The dynamics of human subpopulations are much more complex than those in the simple graph model discussed above."

Their more complicated model (which is pretty impressive, I will admit) has the MRCA date at 1400 BC:

With 5% of individuals migrating out of their home town, 0.05% migrating out of their home country, and 95% of port users born in the country from which the port emanates, the simulations produce a mean MRCA date of 1,415 bc and a mean IA date of 5,353 bc. Interestingly, the MRCAs are nearly always found in eastern Asia. This is due to the proximity of this region to both Eurasia and either the remote Pacific islands or the Americas, allowing the MRCA's descendants to reach a few major world regions in a relatively short time.

They go on to say that a more realistic number is AD 55:

Arguably, this simulation is far too conservative, especially given its prediction that, even in densely populated Eurasia, only 55.3 people will leave each country per generation in ad 1500. If the migration rate among towns is increased to 20%, the local port users are reduced to 80%, and the migration rates between countries and continents are scaled up by factors of 5 and 10, respectively, the mean MRCA date is as recent as ad 55 and the mean IA date is 2,158 bc. The predictions of the simple ten-node graph model sketched earlier fall somewhere between these dates and those of the more conservative computational model.

Comments in the paper regarding isolated groups:

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.

No large group is known to have maintained complete reproductive isolation for extended periods. The populations on either side of the Bering Strait appear to have exchanged mates throughout the period documented in the archaeological record14. Religious isolates such as the Samaritans occasionally have absorbed migrants from outside the group15. Even populations on isolated Pacific islands have experienced occasional infusions of newcomers16.

Note: IA here is an abbreviation for Identical ancestors: "Thus, among all individuals living at least Un generations ago, each present-day human has exactly the same set of ancestors. We refer to this point in time as the identical ancestors (IA) point."
posted by kisch mokusch at 4:39 PM on March 17, 2008

I didn't misread it. I agree with what you just said, which is that we can do much better than 140,000 years with a few simple assumptions.

I also meant to include this link to a Metafilter post I made on the subject.
posted by vacapinta at 4:57 PM on March 17, 2008

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