Are we all at most 50th cousins?

[IceQube]

I've seen the figure 50 tossed around.

Is it true?

If not, then what is the actual number? An estimate would be fine.



(I wanted to share this odd factoid with my girlfriend but I decided that I better get it fact-checked first. I don't know, I just like making her feel awkward - "we're at most 50th cousins!! Incest!")

[Quartz]

Date of leaving Africa: 75K years. Average age per generation: 20 years. That's 3750 generations. 50 generations only takes you back to the middle ages. So the answer is no. And that's ignoring those who stayed in Africa. For the maximum, that is. The expected, mean, and mode figures are undoubtedly different.

[Jonathan Chance]

I think you'd need to find some means of dealing with consanguinuity to make that calculation accurate. You find that just a few people like me, with genetic stock pulling from:

Northern European
Middle Eastern
Sub-saharan Africa
Native American

And suddenly there's a LOT more ways to make it all come together with less than 50th-level cousins.

[President Johnny Gentle]

Originally Posted by Jonathan Chance

I think you'd need to find some means of dealing with consanguinuity to make that calculation accurate. You find that just a few people like me, with genetic stock pulling from:

Northern European
Middle Eastern
Sub-saharan Africa
Native American

And suddenly there's a LOT more ways to make it all come together with less than 50th-level cousins.

Exactly. Going back 50 generations would imply that we have about 1 quadrillion great*48 grandparents each. Current estimates say that there have been around 100 billion humans ever. The estimate I've seen for the world population in the year 1000 is 400 million.

The fact that the 1 quadrillion ancestors all have to be taken from this same group of 400 million (many of whom have no living descendants) makes the 50th cousin claim highly plausible.

[Quartz]

Originally Posted by Jonathan Chance

I think you'd need to find some means of dealing with consanguinuity to make that calculation accurate.

Doesn't matter. The OP is 'at most', so we have to look at the worst possible case.

[FuzzyOgre]

Originally Posted by IceQube

I"I don't know, I just like making her feel awkward - "we're at most 50th cousins!! Incest!")

"Guys? This is my girl friend Jenny. Actually, shes also my cousin. Yeah, thats incest. But we find it works for us. One time, Jenny said to me, 'I like being with you. It just feels right.' Right Honey? Haha, is nobody going to say anything? ..."

[al27052]

IIRC, the MRCA (most recent common ancestor) lived around 3000-5000 years ago, give or take. This is the person who was alive, most recently, that all humans are descended from. If a generation is 30 years, that's around 100-150 generations.

Here's the wiki on MRCA

http://en.wikipedia.org/wiki/Most_re...ommon_ancestor

[Derleth]

Originally Posted by FuzzyOgre

"Guys? This is my girl friend Jenny. Actually, shes also my cousin. Yeah, thats incest. But we find it works for us. One time, Jenny said to me, 'I like being with you. It just feels right.' Right Honey? Haha, is nobody going to say anything? ..."

In 1800, in a lot of (all of?) Europe and North America, at least, nobody would have looked at you sideways. The taboo about marrying your cousins (and I mean first cousins) is apparently more recent than a lot of people realize.

[Quartz]

Originally Posted by al27052

IIRC, the MRCA (most recent common ancestor) lived around 3000-5000 years ago, give or take. This is the person who was alive, most recently, that all humans are descended from. If a generation is 30 years, that's around 100-150 generations.

Here's the wiki on MRCA

http://en.wikipedia.org/wiki/Most_re...ommon_ancestor

And that goes to show that models must be taken with a huge pinch of salt until verified in the real world.

[Inner Stickler]

Cecil on the topic.

[Giles]

Originally Posted by al27052

IIRC, the MRCA (most recent common ancestor) lived around 3000-5000 years ago, give or take. This is the person who was alive, most recently, that all humans are descended from. If a generation is 30 years, that's around 100-150 generations.

200 years ago, the MRCA must have lived at least 8,000-10,000 years ago, since the Tasmanian Aboriginal people were genetically isolated from the rest of the world about 8,000 years ago. I suspect that in 200 years time, the MRCA will be no more than 2,000 years before the present, given the increasing genetic intermingling of previously separated populations.

[njtt]

Indeed, in many cultures, first cousin marriage is close to being mandatory in many cultures, and was common in 19th century Britain and N. America, and close to being the norm for the upper classes, probably even more than for the rural poor (probably much the same applies to most other European cultures too).

I believe all cultures where the matter has been studied regard parent-child sex to be incest, and icky, and nearly all agree about sibling sex. Incest taboos at this level are probably biologically innate in humans. (That is not to say that it does not occasionally happen in certain families - obviously it sometimes does - but it is never approved by the wider community.) However, incest taboos that go much beyond this, like the recent USAian stigmatization of cousin marriage and sex, are cultural oddities (and to extend the taboo beyond first cousins seems to me to be based more on a failure to understand the concept of cousin than anything else). The genetic risks of cousins breeding (unlike sibling and parent-child breeding) are fairly negligible.

I have heard that the medieval Catholic Church officially disapproved of cousin marriages up to the level of (IIRC) 17th cousins, but this rule was probably honored more in the breach than the observance.

[Derleth]

Originally Posted by njtt

I believe all cultures where the matter has been studied regard parent-child sex to be incest, and icky, and nearly all agree about sibling sex. Incest taboos at this level are probably biologically innate in humans.

The Westermarck Effect is usually cited here, as is its slightly lesser-known inverse, genetic sexual attraction. In brief, the Westermarck Effect prevents people who were raised together, regardless of genetic similarity, from becoming attracted to each other, whereas genetic sexual attraction is the effect whereby two closely-related people who were not raised together have a higher chance of being attracted to each other assuming their genders and sexualities are compatible.

However, incest taboos that go much beyond this, like the recent USAian stigmatization of cousin marriage and sex, are cultural oddities (and to extend the taboo beyond first cousins seems to me to be based more on a failure to understand the concept of cousin than anything else). The genetic risks of cousins breeding (unlike sibling and parent-child breeding) are fairly negligible.

Indeed, and this deserves to be emphasized: Inbreeding doesn't magically cause genetic diseases to appear. It just makes what's there already more likely to manifest, with the likelihood being determined by precisely how close the parents are.

I have heard that the medieval Catholic Church officially disapproved of cousin marriages up to the level of (IIRC) 17th cousins, but this rule was probably honored more in the breach than the observance.

The medieval Catholic Church defined beavers to be fish, which, in this thread, sounds awesomely sexual but in reality was just about satisfying the urges of a bunch of cloistered monks.

[Shakester]

Originally Posted by Giles

200 years ago, the MRCA must have lived at least 8,000-10,000 years ago, since the Tasmanian Aboriginal people were genetically isolated from the rest of the world about 8,000 years ago. I suspect that in 200 years time, the MRCA will be no more than 2,000 years before the present, given the increasing genetic intermingling of previously separated populations.

There are living full-blood Australian Aborigines, and while they were never completely isolated from the rest of the world in the way that the Tasmanian sub-group were, they were mostly isolated from everyone except for near neighbours from their arrival in Australia at a very minimum of 40 000 years ago, and most likely 50 to 60 000 years ago.

So there's no way they have have a recent common ancestor with modern European/African/Asian populations. Anyone claiming they do is tripping. There is no way on Earth that they can possibly share an ancestor with modern Europeans any more recently than way over 40 000 years back.

[AK84]

I remember reading a study which attempted to locate the MRCA of several married couples. In several cases the MRCA was only a century hence and in one within the living memory of elderly family members. None of the couples were aware of any relationship before the study.
Another took random people from the disparate parts of the world and in many cases MRCA was reletvely close.

[njtt]

Originally Posted by Derleth

The Westermarck Effect is usually cited here, as is its slightly lesser-known inverse, genetic sexual attraction. In brief, the Westermarck Effect prevents people who were raised together, regardless of genetic similarity, from becoming attracted to each other, whereas genetic sexual attraction is the effect whereby two closely-related people who were not raised together have a higher chance of being attracted to each other assuming their genders and sexualities are compatible.

The Westermarck Effect is rather different from the incest taboo, though. The former is about people raised together (regardless of kinship) not wanting to have sex with each other. The latter is about third parties, or a society as a whole, disapproving when closely related people (regardless of whether they were raised together) do have sex.

The fact that it turned out that unrelated, opposite-sex kids raised together in kibbutzim turned out not to have much sexual interest in one another was probably quite a disappointment to the original, idealistic kibbutz leaders.

[foolsguinea]

Because people often marry within their ethnic group, even to the point of preferring to marry first and second cousins in some tribal cultures, the MRCA model cited (being invented by relatively exogamous Yanks, I suspect) is useless.

The actual MRCA for all Homo sapiens is way, way back. For some isolated groups it could be thousands or myriads of years ago. Even excluding various isolated aboriginal groups and the whole Amerindian set, you have several genetically distinct groups in sub-Saharan Africa that may not have a particularly recent common ancestor with outsiders.

But for two people from the same ethnicity, like, say, two WASPs, yeah, more recently than 50 generations is reasonable.

[Blake]

Originally Posted by Shakester

There are living full-blood Australian Aborigines,

Do yo have a reference for this claim? Because unless things have changed, no genealogist or geneticist makes such a claim.

"Mixed race" Aborigines made up over 50% of the continental population over 150 years ago. That fact alone makes it incredibly unlikely that anyone with 0% outside admixture could exist. It would have required deliberate efforts to avoid interbreeding, combined with an ability to trace genealogy to prove ancestry. Because the population was so small, without such efforts a single person with any outside ancestry sneaking into the gene pool more than 3 generation ago would pretty much guarantee outside ancestry within the past 200 years.

...and while they were never completely isolated from the rest of the world in the way that the Tasmanian sub-group were, they were mostly isolated from everyone except for near neighbours from their arrival in Australia at a very minimum of 40 000 years ago, and most likely 50 to 60 000 years ago.

So there's no way they have have a recent common ancestor with modern European/African/Asian populations. Anyone claiming they do is tripping. There is no way on Earth that they can possibly share an ancestor with modern Europeans any more recently than way over 40 000 years back.

Not only is there a way, it is almost certain that they share a common ancestor much more recently. Lets assume for a second that you can verify your claim of people with no recent outside ancestry. I'm not sure you understand how genetic dispersal works. Everyone on the planet was mostly isolated from everyone except for near neighbours from 40, 000 years ago until the past 50 years or so.

But that doesn't prevent genetic dispersal. If I marry a person from the village 50 miles to the west, then my children will share a common ancestor within 2 or 3 generations with everyone from that village. And if my child marries someone from the village 50 miles to the east, then there children will almost certainly share an ancestor within 2 or 3 generations with everyone within a hundred mile radius. And that is just two generations. Every two subsequent generation that radius will increase by 50 miles, meaning that within 400generations /8000 years someone in Ireland will be the ancestor of every single person in Europe and Asia.

Australians were no different. In fact the long trade routes and regular super-clan meetings where spouses were exchanged almost certainly lead to a relatively faster dispersal than Europe over the past 10, 000 years or so. While most farmers in Eurasia were living and dying within a few miles of where they were born. almost all Aborigines were meeting people from hundred of miles away many times during their lives. And at those meetings a few marriages almost always occurred. So every Australian would almost certainly have shared a common ancestor with every person living within 2, 000 kilometres within the past 4 generations, and with every other person on the continent within half a dozen generations.

Australia is about 4, 000 kilometres to a side. So if even one person each generation married someone who lived even 500 km away, and everyone else married someone from an area within 50 km, then every person on the continent would share a blood relative within 8 generations/160 years. Aboriginal populations were small, so common ancestry might lag that by 6 generations at most. So at the outside, all Aborigines "updated" their common ancestor every 300 years.

Once you combine that with the fact that Australians were in regular contact with Asia for at least 5, 000 years, and you will see why Australians can't be a significant outlier. As far as we can tell, 100% of Aborigines in The Top End have Indo-Malyan ancestry within the past 1,000 hundred years or so. But let's assume that even one of the IndoMalayans who brought dingoes to Australia, or one the continuous contacts since then, produced a surviving descendant in Australia. Just one single descendant any time in the past 5, 000 years of provable contact and trade, prior to 1712.

Given an ancestry mingling time of 300 years, a single descendant any time from the arrival of the dingo traders to the arrival of the Dutch would pretty much guarantee that Aborigines share the exact same common ancestry ass the rest of the Old World.

So if we accept that there is a recent common ancestor of IndoMalayans and the rest of the planet, then that must also be the ancestor of Aborigines. The only way that could fail to be true is if every IndoMalayn who interbred with an Australian came from a now extinct bloodline, which seems ridiculous.

Far from being no way on Earth that Aborigines could share a recent common ancestor with the rest of the Old World, it would take an extraordinary series of events for them not to share a common ancestor within the past few thousand years.

[Blake]

Originally Posted by foolsguinea

Because people often marry within their ethnic group, even to the point of preferring to marry first and second cousins in some tribal cultures, the MRCA model cited (being invented by relatively exogamous Yanks, I suspect) is useless.

It isn't, because you seem to be assuming that you need 50% exogamous marriages to produce a common ancestor. You don't. Even in populations of millions, just a single exogamous marriage each generation will pretty much guarantee a common ancestor with the rest of humanity within the past few thousand years. See President Johnny Gentle's post about pedigree collapse for why this is the case.

Also note that while many groups will prefer that women marry within group few, if any, care if men sire children with outsiders, And common ancestry works just as well through the male line as the female. Just a single man each generation fathering a child with an outsider will lead to recent common ancestry.

The actual degree of exogamy isn't really important. In fact within small populations, when endogamy is the rule but exogamy occurs regularly, that will actually speed the pedigree collapse because once a line in in the pool, it can only be excised by physical extinction, it can never "marry out".

As a result, exogamous Yanks where a million people annually engage in exogamous marriages aren't going to be much more nodal than a group where just one individual every twenty years reproduces exogamously.

For some isolated groups it could be thousands or myriads of years ago.

Could yo name one of these groups, with some evidence for this claim?

Even excluding various isolated aboriginal groups and the whole Amerindian set

It's a virtual certainty that 100% of Amerindians have both European and West African Ancestry within the past 400 years.

...you have several genetically distinct groups in sub-Saharan Africa that may not have a particularly recent common ancestor with outsiders.

Do you have a name in mind? Because groups like the San, Hadza and Mbuti are the most genetically distinct groups on the continent, but they all engage in regular intermarriage with their nieghbours and always have done.

I can't really think of any groups in Africa that are even approximately isolated.

[Shakester]

Originally Posted by Blake

Do yo have a reference for this claim? Because unless things have changed, no genealogist or geneticist makes such a claim.

I've met Aborigines who have told me that they are full-blooded, and I see no reason to doubt them. All I've seen from you is a propensity to argue on the internet, so I'm just going to say this: I read what you wrote, I disagree, and I'm leaving it there.

[Colophon]

Originally Posted by Shakester

I've met Aborigines who have told me that they are full-blooded, and I see no reason to doubt them. All I've seen from you is a propensity to argue on the internet, so I'm just going to say this: I read what you wrote, I disagree, and I'm leaving it there.

But how would they possibly know if their great-great-great-great-grandmother was one-sixteenth Malayan?

[Blake]

Originally Posted by Shakester

I've met Aborigines who have told me that they are full-blooded, and I see no reason to doubt them. All I've seen from you is a propensity to argue on the internet, so I'm just going to say this: I read what you wrote, I disagree, and I'm leaving it there.

Well, if you have a second hand anecdote from a party with a personal stake, that's completely different. Clearly there is no need for any other evidence. When science and logic contradict baseless anecdote, clearly we should believe the anecdote

Originally Posted by Colophon

But how would they possibly know if their great-great-great-great-grandmother was one-sixteenth Malayan?

Never mind that. Literacy rates among "full blooded" aborigines just 60 years ago were less than 10%, and most births were undocumented because Aborigines weren't included in any census.

So it would be extraordinary if anyone could even positively identify 8 Aboriginal Great Grandparents, let alone be sure of their ancestry beyond that point.

We aren't talking Great-great-great-great-grandparents here. I am highly skeptical that anyone even knows the name of more than half their Aboriginal great grandparents, and that's assuming a 20 year generation span.

[Ludovic]

Originally Posted by njtt

The Westermarck Effect is rather different from the incest taboo, though. The former is about people raised together (regardless of kinship) not wanting to have sex with each other. The latter is about third parties, or a society as a whole, disapproving when closely related people (regardless of whether they were raised together) do have sex.

I don't think it's rather different. It seems clear to me that the incest taboo most likely arose from a moralization of the Westermarck Effect (i.e. "I would never want to mate with my sister, that's gross! No one else should, either!")

[Blake]

Originally Posted by Ludovic

"I would never want to mate with my sister, that's gross! No one else should, either!")

But dude, your sister's hot.

[mr. jp]

Originally Posted by Shakester

I've met Aborigines who have told me that they are full-blooded, and I see no reason to doubt them. All I've seen from you is a propensity to argue on the internet, so I'm just going to say this: I read what you wrote, I disagree, and I'm leaving it there.

I'm wondering what you are even doing on the General Questions section of the Straight Dope Message Boards. Arguing on the internet is what we do here, believing anecdotes over evidence is what we don't do.

[al27052]

No one has mentioned the Sentinelese.

Also, let's remember that, even if the MRCA lived at the time of Christ, there are plenty of people that have absolutely none of his genes, because there've been so many generation between then and now.

[Nava]

Originally Posted by njtt

However, incest taboos that go much beyond this, like the recent USAian stigmatization of cousin marriage and sex, are cultural oddities (and to extend the taboo beyond first cousins seems to me to be based more on a failure to understand the concept of cousin than anything else).

Not necessarily: the half of my family which doesn't consider first-cousins taboo is from a place where there is a lot of genetic variation. The half which considers second-cousins taboo is from a place with a lot less genetic variation: avoiding "mixing blood that's too close" requires going further away when you know that climbing any branch of the family tree will lead to a common pair of ancestors eventually (just as a completely random piece of data, a study of the family trees of my 40-students 11th-grade class, where each tree was available going up to 1700, found common ancestors for any of us with two to six other students; we were in the area the no-cousins side of my family hails from).

[Hari Seldon]

Even MRCA can be misleading since a group of people could all be 50th cousins at most, while not having a common ancestor much earlier than that. Of course, any two would have to have a common ancestor within the past 50 generations, but that is not the same as the whole population having a common ancestor within 50 generations.

Perhaps an example would help. Imagine just three people, Alice, Bob, and Charles. Alice's parents are Don and Edith. Bob's parents are Edith's brother Frank and Gloria. Charles's parents are Gloria's brother Hank and Don's sister Irene. Assuming no other relationship, Alice and Bob, Bob and Charles, and Charles and Alice are all first cousin's but they have no ancestor closer than three generations ago.

[jbaker]

I tend to be sceptical of arguments for a relatively recent MRCA. These arguments disregard the existence of "fat tails," statistical outliers that are more extreme than analyses would suggest - the same reason why once-a-millennium events happen in the financial markets every few years. They also disregard the existence of radically isolated populations, such as the Sentinelese and some of the peoples of New Guinea, and the likelihood that at least some Native Americans have no European or African ancestry (e.g., in some of the uncontacted or minimally contacted tribes of South America). Consider also that there are still purebred Hawaiians, and while the pre-James Cook Hawaiians were not radically isolated, they were still pretty isolated.

As far as the OP's question is concerned, however: I doubt if either he or his girlfriend are from radically isolated populations. They're sure to be much closer than 50th cousins. The chances that they are closer than 10th cousins are very high.

[Kimstu]

Originally Posted by Derleth

The medieval Catholic Church defined beavers to be fish, which, in this thread, sounds awesomely sexual but in reality was just about satisfying the urges of a bunch of cloistered monks.

So what? How on earth is that a GQ response to the previous poster's tentative assertion?

In any case, the actual answer seems to be that medieval canon law starting in the 9th century prohibited marriages within the 7th degree of kinship, not 17th.

[John Mace]

Originally Posted by Shakester

There are living full-blood Australian Aborigines, and while they were never completely isolated from the rest of the world in the way that the Tasmanian sub-group were, they were mostly isolated from everyone except for near neighbours from their arrival in Australia at a very minimum of 40 000 years ago, and most likely 50 to 60 000 years ago.

So there's no way they have have a recent common ancestor with modern European/African/Asian populations. Anyone claiming they do is tripping. There is no way on Earth that they can possibly share an ancestor with modern Europeans any more recently than way over 40 000 years back.

First of all, Australia was not genetically isolated for that long. We know there was immigration from parts of Asia 4-5K years ago, at least. But further, once Europeans arrived, it would be extremely difficult to prevent gene flow to even the most remote tribes given 300 years of interbreeding.

I've met Aborigines who have told me that they are full-blooded, and I see no reason to doubt them.

I would have no reason to believe they had any way of knowing that.

[ClintO]

So jesus and I are cousins?

[Shagnasty]

Originally Posted by ClintO

So jesus and I are cousins?

On which side? His mother's or his father's?

[Blake]

Originally Posted by jbaker

I tend to be sceptical of arguments for a relatively recent MRCA. These arguments disregard the existence of "fat tails," statistical outliers that are more extreme than analyses would suggest - the same reason why once-a-millennium events happen in the financial markets every few years.

The problem with that argument is that it makes the alternative even more likely. If "fat tails" make a recent ancestor unlikely, it makes the presence of individuals with no common ancestors even more likely, simply because there are 6 billion ways to find a common ancestor, and only one way to avoid doing so.

They also disregard the existence of radically isolated populations, such as the Sentinelese and some of the peoples of New Guinea..

No people of New Guinea are isolated, radically or otherwise. Some of the agriculturalists were isolated from other agriculturalists, but they all maintained contact and interbred with adjacent HGs.

The Sentinelese have also never been truly isolated. They are just too close geographically to other people. We know that Sentinelese words occur in other Andamanese langauges and we know from British records in the 19th century that there were Sentinelese actually living amongst the other Andamanese peoples. So long as one such event occurred every couple of generations, the Sentinelese are going to have the same ancestor as everyone else.

Given the tiny population of the Sentinels, if even one outsider found their way into the bloodline any time prior to 500 years ago, then the entire group will be descended from them. This is further exacerbated by the effect of disease and the evolutionary advantage of outside genetics providing immunity.

.. and the likelihood that at least some Native Americans have no European or African ancestry (e.g., in some of the uncontacted or minimally contacted tribes of South America).

That would be astonishingly unlikely. The uncontacted tribes of South America are all village farmers and all are in contact with their neighbours, or were until 50 years ago. More importantly, they all seem to be descendants of the great agricultural kingdoms that were decimated from European disease. They aren't people who have always lived like that, they are people whose ancestors just 500 years ago lived in cities under hereditary kings. There seems to be no way that they could be isolated.

Consider also that there are still purebred Hawaiians..

Do you have a reference for this claim? It would be interesting to see how anyone would establish such a fact, given that the Hawaiians had no written language. If an Hawaiian had a Tahitian ancestor born in 1066, how would they ever know? No English commoner could prove that they had no single French ancestor from 1066, and that is a literate culture with some of the world's best written records. I can't see how an Hawaiian could ever be certain that they had no Tahitian ancestry from the same date.

...and while the pre-James Cook Hawaiians were not radically isolated, they were still pretty isolated.

And that is the reason why Hawaiians can't be described as being in any way isolated. We know they weren't radically isolated, to the extent that Hawaiian is mutually intelligible with other Polynesian languages. From both linguistic and archaeological evidence we can be sure that Hawaiians were in constant cohabitation, not just occasional contact, with the outside world until just 800 years ago. It seems likely that the last common ancestor of Hawaiians and other Polynesians is less than 1, 500 years back. That would makes the Hawaiians less isolated than Europeans.

[Chronos]

Quoth Kimstu:

In any case, the actual answer seems to be that medieval canon law starting in the 9th century prohibited marriages within the 7th degree of kinship, not 17th.

And "7th degree of kinship" doesn't mean 7th cousin, either. Kin in the 7th degree would be, for example, third cousins once removed (or second cousins 3 times removed, etc., though that makes marriages rather unlike just by virtue of age).

[Chronos]

Correcting myself: 2nd cousins once removed. Which, yeah, is a bit further than most modern cultures take it, but really not all that far.

[the_diego]

Hmmm... the magic-50 I remember was used for a different statistic: "There are only 50 'real' people on earth." By that they meant there are only 50 distinct facial features in adult humans identified. This cuts across race, mind. It was an expansion of the separated-at-birth concept. Yep, read it from time when I was young.

[Chessic Sense]

All this talk of MRCA is unnecessary. That unknown person is the mother or father of all humanity, but that's not what the OP is asking about. The OP wants to know what generation, i.e. what set of ancestors, would be enough to find some path from each living person to every other living person.

The difference is that a single ancestor needn't be present in every "relation chain" from living person to living person, like the MRCA demands. If we mapped out every lineage going back only, say, 4 generations and had enough people to connect everyone in at least one manner, then we'd say everyone is 2nd cousins even if the MRCA isn't in that generation.

An example would be two couples who have kids and then switch partners to have more kids. Every kid would share at least one parent with every other kid, but no common ancestor would exist within this set. And since humanity's female MRCA never knew the male MRCA, you can't disqualify this set of kids just because they're not full siblings.

[Chronos]

And since humanity's female MRCA never knew the male MRCA

Nitpick: You mean the female-line MRCA and male-line. Everyone's father's father's father's ... father was Y-Adam, and so he was most certainly a common ancestor of all humanity, but his mother was also, therefore, a common ancestor of all humanity. And the mixed-line MRCA was much more recent than either Y-Adam or Mitochondrial Eve.

[Blake]

Originally Posted by Chessic Sense

All this talk of MRCA is unnecessary. That unknown person is the mother or father of all humanity, but that's not what the OP is asking about. The OP wants to know what generation, i.e. what set of ancestors, would be enough to find some path from each living person to every other living person.

The difference is that a single ancestor needn't be present in every "relation chain" from living person to living person, like the MRCA demands. If we mapped out every lineage going back only, say, 4 generations and had enough people to connect everyone in at least one manner, then we'd say everyone is 2nd cousins even if the MRCA isn't in that generation.

An example would be two couples who have kids and then switch partners to have more kids. Every kid would share at least one parent with every other kid, but no common ancestor would exist within this set. And since humanity's female MRCA never knew the male MRCA, you can't disqualify this set of kids just because they're not full siblings.

I can't make sense of this.

To start with, I can't see any possible way that every kid would share at least one parent with every other kid. It seems mathematically impossible since there are a minimum of 4 children, but a maximum of 2 permutations of parents

John and Jane have a kid: Bob
Kath and Ken have a kid: Carol

Kath and John then have a kid: Ted
Jane and Ken then have a kid: Alice

Ted doesn't share any common parentage at all with with Alice. Ted's parents are Kath and John. Alice's parents are Jane and Ken. Ted and Alice are genetically utterly unrelated.

In order to find a path from each living person to every other living person within this group, you will need to go back and find an ancestor that is shared by both Ted and Alice. If John's Grandfather and Ken's Great Grandfather are the same person, Engelbert, then you will have a path linking all these people. But that would make that Engelbert the MRCA .

If we mapped out every lineage going back 4 generations, the only way we I can see that we could have enough people to connect everyone in at least one manner is if the MRCA occurred within four generations.

I'm guessing I'm missing something here.

Perhaps you are working form a position that "step sibling" or adopted sibling or other relationships that have no objective or genetic basis are sufficient to establish a relation chain. But I don't think that's what the OP wants. Since he is talking about incest, relationships that only exist culturally, with no genetic relationship at all, hardly seem to fit.

[njtt]

Originally Posted by Ludovic

I don't think it's rather different. It seems clear to me that the incest taboo most likely arose from a moralization of the Westermarck Effect (i.e. "I would never want to mate with my sister, that's gross! No one else should, either!")

It is possible that the incest taboo arose this way, I suppose, and certainly it seems likely that it is intertwined with the Westermark effect in some way, but they are still not the same thing. They sometimes apply to different people: e.g., the Westermark effect will not apply to siblings raised apart, but the incest taboo still does; the Westermark effect, but not the incest taboo, applies to unrelated kibbutzim raised together.

Also, my impression is that the Westermark effect does not make the relevant relations seem disgusting so much as just uninteresting, whereas violations of incest taboo typically do evoke reactions of disgust (from third parties - presumably at least one of a pair violating the incest taboo will actually be enjoying it).

[Derleth]

Originally Posted by njtt

it seems likely that it is intertwined with the Westermark effect in some way, but they are still not the same thing.

Who claimed they were?

[foolsguinea]

Originally Posted by mr. jp

I'm wondering what you are even doing on the General Questions section of the Straight Dope Message Boards. Arguing on the internet is what we do here, believing anecdotes over evidence is what we don't do.

I haven't seen any evidence, just unsupported assertions, bad math, and fuzzy, "of course we're all related," thinking. MRCA for an entire continent in four generations? That's ridiculous on its face!

Is there any proof that the exogamists are more fertile than average? If not, perhaps the one scion who married out this century, of a given endogamous tribe, left no fertile grandchildren. Oops.

Shall we assume that no tribe practices forced endogamy? Ooh, let's! We can go on to assume that no one eats pork or wears eye makeup, while we're at it!

Some claims are too ridiculous to be entertained.

[foolsguinea]

Originally Posted by Blake

The Sentinelese have also never been truly isolated. They are just too close geographically to other people. We know that Sentinelese words occur in other Andamanese langauges and we know from British records in the 19th century that there were Sentinelese actually living amongst the other Andamanese peoples. So long as one such event occurred every couple of generations, the Sentinelese are going to have the same ancestor as everyone else.

But it didn't. You have no evidence that it did. You're just asserting that it must have.

It seems likely that the last common ancestor of Hawaiians and other Polynesians is less than 1, 500 years back. That would makes the Hawaiians less isolated than Europeans.

Hawai'ians less isolated from [other] Polynesians than Europeans from what? Not other Europeans.

That said, yes, it's fair to say that the OP is at most a 10th cousin (if I were betting in a pool, let's say 8th or less) of his girlfriend, unless their heritages are from other sides of the planet or something.

[Aspidistra]

Originally Posted by Blake

I can't make sense of this.

To start with, I can't see any possible way that every kid would share at least one parent with every other kid. It seems mathematically impossible since there are a minimum of 4 children, but a maximum of 2 permutations of parents

John and Jane have a kid: Bob
Kath and Ken have a kid: Carol

Kath and John then have a kid: Ted
Jane and Ken then have a kid: Alice

Ted doesn't share any common parentage at all with with Alice. Ted's parents are Kath and John. Alice's parents are Jane and Ken. Ted and Alice are genetically utterly unrelated.

In order to find a path from each living person to every other living person within this group, you will need to go back and find an ancestor that is shared by both Ted and Alice. If John's Grandfather and Ken's Great Grandfather are the same person, Engelbert, then you will have a path linking all these people. But that would make that Engelbert the MRCA .

If we mapped out every lineage going back 4 generations, the only way we I can see that we could have enough people to connect everyone in at least one manner is if the MRCA occurred within four generations.

I'm guessing I'm missing something here.

Perhaps you are working form a position that "step sibling" or adopted sibling or other relationships that have no objective or genetic basis are sufficient to establish a relation chain. But I don't think that's what the OP wants. Since he is talking about incest, relationships that only exist culturally, with no genetic relationship at all, hardly seem to fit.

His point is valid, even though the example given may not work exactly. The simplest example of it that I can devise involves 15 people:

Generation 1:
Alan and Barb have two kids - Geoff and Heather
Carol and Doug have two kids - Ian and Jane
Emma and Fred have two kids - Karen and Luke

Generation 2:
Heather and Luke have a kid - Mark
Geoff and Jane have a kid - Nick
Ian and Karen have a kid - Olivia

Now Generations 1 and 2 die. Everyone left (Mark, Nick and Olivia) is a first cousin to any other person, even though the MRCA is more than 2 generations back - in fact, an arbitrary number of generations greater than two back, since we know nothing about the genetic separation between the people in gen1.

Obviously the modelling of this over the entire population of the world is incredibly complex, but I would pick 30 or less as the most likely number that constitutes a factual answer to the OP's question. 30 generations back gives you a number of ancestors (1 billion) which is roughly double the world population at the time (about half a billion or less in the 15th century) - I would call that more than enough to ensure that at least one of those ancestors was shared between every two people on earth, even if it's not the same ancestor for every pair.

[Hari Seldon]

As I explained in my previous post, MRCA is irrelevant. FWIW, the MRCA appears to go back about 250,000 years, maybe 10,000 generations.

Incidentally, I was wrong in my previous post (#28) in stating that Alice, Bob, and Charles, although first cousins to each other, shared no ancestor later than the third generation preceding. Actually, their MRCA could be indefinitely far back in the scenario I envisaged.

Incidentally, the question of whether second cousins thrice removed could be of marriageable ages is not clear. I was thinking about my mother's first cousin cohort that ranged over 44 years, from her older brother born in 1904 to her first cousin born in 1948, the same year as my sister. Now if you go to second cousins, the age distribution could get very large and, anyway, a 60 or 70 year old man can still father a child.

[John Mace]

Originally Posted by Hari Seldon

As I explained in my previous post, MRCA is irrelevant. FWIW, the MRCA appears to go back about 250,000 years, maybe 10,000 generations.

No, it doesn't. Where are you getting that number from?

[spikebrennan]

[quote=Derleth;15521647
The medieval Catholic Church defined beavers to be fish, which, in this thread, sounds awesomely sexual but in reality was just about satisfying the urges of a bunch of cloistered monks.[/QUOTE]

This is true only if by "medieval" you mean the 17th century, and by "cloistered monks" you mean Quebecois.

[septimus]

Originally Posted by Blake

I can't make sense of this.

Several examples have been posted. Here's one from Real Life!

My present neighbors include two brothers from another province who came here and married unrelated women. One of the wives has a brother married to the other wife's sister. All three of these couples had children. (In fact, I've been to weddings for four of the children from these marriages.) All of these children are either siblings or first cousins with each other, yet share no known common ancestor. (Those connections don't involve us, but my wife has relatives who are cousins both on her mother's side and on her father's side. And, no, none of this involve inbreeding -- in all this genealogy I can't think of a single instance of known relatives marrying.)

To answer OP's question (but with the numbers 12th, 20th, 30th, just wild, probably conservative, estimates):
* I guess that a typical American is 12th cousins (or closer) with most other typical Americans.
* That same American is 20th cousins with 99% of living humans, and 30th cousins with 99.99%.
Getting that 99.99% up to 100% may be fraught with uncertainty, due to outliers like the Sentinelese.

[TriPolar]

Originally Posted by Quartz

Doesn't matter. The OP is 'at most', so we have to look at the worst possible case.

It's not very interesting or enlightening to dismiss this concept based on two people on earth who may 51st cousins instead of 50th. The measure of this is the closeness of relation between the vast majority of people on earth. Is it around 50 or not? If there are some outliers that would be interesting in of itself, but even getting to the point of saying it's more likely than not that you are no more than a 50th cousin of everyone on earth is something of note.

I'm also getting the impression reading this that a small isolated group doesn't change things much. Members of that group will all be closely related, and any one of those members who are related to someone outside of the group will bring the whole group in close relationship with the rest of the world.