How inbred were our ancestors?

[Angela]

Or...

"https://www.biorxiv.org/content/10.1...912v2.full.pdf

"Human parental relatedness through time: Detecting runs of homozygosity in ancient DNA"

"Abstract

At present day, human parental relatedness varies substantially across the globe, but little is known about the past. Here we use ancient DNA to provide new insights, leveraging that parental relatedness leaves traces in the offspring’s genome in the form of runs of homozygosity. We present a method to identify such runs in low-coverage ancient DNA data using linkage information from a reference panel of modern haplotypes. As a result, the method facilitates analysis of a much larger fraction of the global ancient DNA record than previously possible. Simulation and experiments show that this new method has power to detect runs of homozygosity longer than 4 centimorgan for ancient individuals with at least 0.3× coverage. We used this new method to analyze sequence data from 1,785 humans from the last 45,000 years. Generally, we detect very low rates of first cousin or closer unions across most ancient populations. Moreover, our results evidence a substantial impact of the adoption of agricultural lifestyles: We find a marked decay in background parental relatedness, co-occurring with or shortly after the advent of sedentary agriculture. We observe this signal, likely linked to increasing local population sizes, across several geographic regions worldwide."

Basically, that boils down to the fact that hunter-gatherers practiced more first cousin marriage (and closer) than farmers, but from the paper itself it seems that much more recently there was an increase in inbreeding, and that it was more intense and continues to this day in certain parts of the world.

Very bad idea, imho.

Razib Khan opined:

"The major thing to note is that the estimates suggest cousin-marriage was far less common in the prehistoric and historic past than it is in some modern societies. In particular, it is far less common than it is today in the Islamic world and in India. "

​https://www.gnxp.com/WordPress/2020/...l-technologies

[bicicleur]

there is a study about the extinction of the last mammoths on a remote Island (I don't remember the name of the Isalnd) in the Arctic Sea north of eastern Siberia some 8 ka
they didn't go extinct because of predators nor because of lack of food
the died out because of inbreeding, as there were no other tribes of mammoths nearby

I think the same thing happened with HG who's founding fathers had found good hunting grounds in remote places
at first their tribes expanded, but they didn't find fit mating partners from other neighbouring tribes

The 50 cM threshold for sROHą20 can also
be surpassed in very small isolated populations, specifically, 34% of individuals
in populations of size 250 and 8% for size 500 (Fig. S14). Hereafter we refer to
this as the “long ROH” threshold, and individuals crossing it as having “long
ROH”.

Notably, 11 of the 54 individuals with long ROH are located on islands

there are many instances of HG communities where the young males got their wives from neighbouring tribes
we know this from isotope results
only tribes that used this practice could survive in the long run
that is how this practice became common

today there are communities where the males think they own the females in their own community
the females are not allowed to meet or date men outside the community
while the young men are proud to chase other women

maybe the megalithic communities did the same
and some communities in Rome
The only archaeological cluster (defined in annotations from the source dataset, modified for readability) with more than two
individuals is ‘Iron Age Republican Rome’, where 3 of 11 samples reported in
Antonio et al. (2019) fall above the long ROH threshold

[bicicleur]

Razib Khan

But as Matt notes, the results on the data are interesting too! The first author shared the r.o.h. table, so you can look at them yourself. The major thing to note is that the estimates suggest cousin-marriage was far less common in the prehistoric and historic past than it is in some modern societies. In particular, it is far less common than it is today in the Islamic world and in India. That being said, the r.o.h values do decrease with agriculture from hunter-gatherer periods, indicating that large farming societies were more exogamous…before a recent shift in some areas to endogamy. Wouldn’t this pose some issues in regards to the arguments The WEIRDest People in the World?

How to make sense of all this?

There are several dynamics going on. First, these results put paid to the notion promoted by some behavioral ecologists and anthropologists that most marriages in the past were cousin-marriages. This is just not true. Even hunter-gatherers tended to be exogamous. So was there a cultural change that led to the shift in r.o.h with agriculture? (and its increase with pastoralism in their dataset) No. I think one of the issues we have to remember are simple structural parameters that have nothing to do with ideology.

Hunter-gatherers were at a much lower density than farmers. Elevated inbreeding was almost certainly a function of this ecological circumstance, as gene flow across hunter-gatherer populations was less common simply due to the lack of regular interaction. In the data, the authors point out that a lot of their elevated r.o.h. samples are from islands. Islands don’t impose ideology, they impose limits to contact. The rise of pastoralism and mobility on the steppe in some ways recapitulated the lifestyle of hunter-gatherers (note that I do believe less habitual contact probably meant more xenophobia, so the structural parameter probably had an ideological consequence which amplified what we’re seeing here).

[Tamakore]

there is a study about the extinction of the last mammoths on a remote Island (I don't remember the name of the Isalnd) in the Arctic Sea north of eastern Siberia some 8 ka
they didn't go extinct because of predators nor because of lack of food
the died out because of inbreeding, as there were no other tribes of mammoths nearby
I think the same thing happened with HG who's founding fathers had found good hunting grounds in remote places
at first their tribes expanded, but they didn't find fit mating partners from other neighbouring tribes

I think it's going too far to say that the mammoths of Wrangel Island died out because of inbreeding. Even the authors of that paper only say that inbreeding may have contributed to their extinction.

If they were so debilitated by inbreeding, how come they survived for 6,000 years longer than any other population of mammoths? The probable answer is that the mammoths of continental Eurasia were hunted to extinction by humans, but the Wrangel Island mammoths were protected for a time by their isolation from humans.

The latest date for mammoths on Wrangel Island is 2000 BC. The earliest radiocarbon date for human occupation is 1700 BC. I think the closeness of those dates points to the real culprit. Humans probably hunted the Wrangel Island mammoths to extinction just as they did everywhere else, and they would have done so no matter how healthy or unhealthy the mammoths were.

Whenever humans arrived in a previously unoccupied land there was usually a wave of megafauna extinctions. It happened to the giant marsupials of Australia about 40,000 years ago. Humans arrived in my native New Zealand for the first time as recently as 750 years ago and within a hundred years all the large flightless bird species were extinct. However, some researchers seem determined to find other reasons for megafauna extinctions that were probably caused by human over-hunting.

I wonder whether there is any proven example of a species becoming extinct solely due to inbreeding?

[bicicleur]

I think it's going too far to say that the mammoths of Wrangel Island died out because of inbreeding. Even the authors of that paper only say that inbreeding may have contributed to their extinction.
If they were so debilitated by inbreeding, how come they survived for 6,000 years longer than any other population of mammoths? The probable answer is that the mammoths of continental Eurasia were hunted to extinction by humans, but the Wrangel Island mammoths were protected for a time by their isolation from humans.
The latest date for mammoths on Wrangel Island is 2000 BC. The earliest radiocarbon date for human occupation is 1700 BC. I think the closeness of those dates points to the real culprit. Humans probably hunted the Wrangel Island mammoths to extinction just as they did everywhere else, and they would have done so no matter how healthy or unhealthy the mammoths were.
Whenever humans arrived in a previously unoccupied land there was usually a wave of megafauna extinctions. It happened to the giant marsupials of Australia about 40,000 years ago. Humans arrived in my native New Zealand for the first time as recently as 750 years ago and within a hundred years all the large flightless bird species were extinct. However, some researchers seem determined to find other reasons for megafauna extinctions that were probably caused by human over-hunting.
I wonder whether there is any proven example of a species becoming extinct solely due to inbreeding?

Indeed the real cause of the mammoth extinction on Wrangel Island is not sure.
However it is clear that these mammoths did suffer from genetic deterioration due to long-time isolation.

I agree and I know of many instances worldwide of large megafauna - and other fauna - extinctions due to the arrival of the humans.
It is even so, e.g. on the Pampas in Argentina that the humans themselves went extinct after the extinction of the megafauna these same humans hunted.
I guess the same happened in Southern Australia, when humans arrived 46 ka and 43 ka the diprodoton went extinct.
But the best-known example is of course the people of the Clovis culture.
And it is not always megafauna that goes extinct because of arrival of humans, 125 ka it was the Giant Clam in the Red Sea : https://www.researchgate.net/publica...in_the_Red_Sea

I've also read a study involving DNA of mammoths.
The European mammoth went extinct in the paleolithic after the arrival of modern humans, but these mammoths were replaced by new arrivals of mammoths from Siberia and even Beringia.