Are we all at most 50th cousins?

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!”)

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.

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.

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

“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? …”

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

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.

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

Cecil on the topic.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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.

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.