The great panmixia


Human inbreeding has decreased in time through the Holocene:

The history of human inbreeding is controversial. The development of sedentary agricultural societies may have had opposite influences on inbreeding levels. On the one hand, agriculture and food surplus may have diminished inbreeding by increasing population sizes and lowering endogamy, i.e. inbreeding due to population isolation. On the other hand, increased sedentism, as well as the advent of private property may have promoted inbreeding through the emergence of consanguineous marriage customs or via ethnic and caste endogamy. The net impact is unknown, and to date, no systematic study on the temporal frequency of inbreeding in human societies has been conducted. Here we present a new approach for reliable estimation of runs of homozygosity (ROH) in genomes with ≥3x mean coverage across >1 million SNPs, and apply this to 440 ancient Eurasian genomes from the last 15,000 years. We show that the frequency of inbreeding, as measured by ROH, has decreased over time. The strongest effect is associated with the Neolithic transition, but the trend has since continued, indicating a population size effect on inbreeding prevalence. We further show that most inbreeding in our historical sample can be attributed to endogamy, although singular cases of high consanguinity can also be found in the archaeogenomic record.

I think it is hard to think this is unrelated to decreased pairwise Fst between populations over the Holocene. Fst is a statistic that measures the proportion of genetic variation across two populations in relation to the total variance. In a Pleistocene world of small clans occupying a thinly populated landscape, one can envisage a scenario where gene flow is far more viscous than the more sedentary, but interconnected, world of agriculturalists.

Hunter-gatherers were probably not more xenophobic. Rather, increasing populations by an order of magnitude increases the number of potential geographically close mates a lot.

Another consequence of more gene flow and more partners is that inbreeding also declines, as people have more recent ancestors in their pedigree.

The main caveat I would put into this though is that this applies to dense Eurasian time transects. There is some reason to think that hominins on the northern Eurasian fringe were always on the knife’s edge of sustainability.

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4 thoughts on “The great panmixia

  1. “Hunter-gatherers were probably not more xenophobic.” – Sure looks as if they were, although I have no idea why.

  2. Well there was this incident which could have been xenophobic: https://www.nature.com/articles/s41598-020-58483-9

    Don’t know how common these were. There was the massacre of farmers in central Germany but that could have just been other farmers.

    However, all of these massacres being followed up with a male mediated HG input probably cancels out the xenophobia, since if they were truly xenophobic they wouldn’t have mixed with the farmer females. Maybe just ‘toxic masculinity’ by modern standards.

  3. Xenophobic is a bit of wooly term, hard to measure precisely. Intergroup warfare intensity can be somewhat more defined, so measureable, and there is some ethnographic description of that in small scale societies (Evolving Moloch blog, “War Before Civilization” etc). And some general theory predictions (tend to suggest that smaller scale societies engage more % individuals per intergroup conflict just bcos scaling rules make higher manpower investment less effective – both sides reach limits to scaling up “because the economy would collapse”, and do more damage than losing the conflict, as has so often been the reply to various recent muddleheaded “Let’s go all out!” schemes in the last year). Real world ethnographic lit useful for avoiding “out there” interpretations derived from picking out of archaeology (for example kooky “Old European matriarchy / gender egalitarian” ideas based on interpretation of arch already pretty dubious just from knowledge of overwhelming tendencies to gender roles in extant and historically documented small scale agricultural societies).

    Regardless, seems correct to note that elevated RoH in small scale HG most likely more due to population scaling and limited opportunity for outbreeding, than to different attitudes to contact or to warfare between groups.

    On paper, great to have another method also seeming to reconfirm the results in Ringbauer’s recent preprint (reducing RoH, consang generally rare in history among samples gathered to date).

  4. What we have to consider is the time frame. There were massive expansions which led to large scale panmixture and replacements between groups in hunter gatherer times too, they just happened, in most regions, and beyond the regional scale, just less frequently. So I think the grade of mixture between any two formerly separated groups would have looked very different already in Paleolithic times, depending on the exact time of the samples taken.
    The main thing which changed up until quite recently is indeed the magnitude and frequency of such events overall. Like the more developed humans became, the higher their population density, the more often happened conflicts and peaceful contacts alike. What happened in previous times lets say every 10.000 years, started to happen first every 5.000, then every 1.000 years and so on.
    What appears to be stable in nature, this is also what the naive ecologists don’t understand, is oftentimes just a snapshot which is too long for being recognised as such, since their time perspective is too limited for recognising the constant change.

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