Inbreeding Declined In West And Central Eurasia During The Holocene Era

A new paper published in the journal Cell shows that inbreeding has declined over the last 12,000 years, as demonstrated with 411 ancient DNA samples from West Eurasia (defined as Europe) and Central Eurasia (defined as Southwest Asia, the Caucasus and Central Asia).

Extreme consanguineous matings did occur among agriculturalists but were rare, while inbreeding was more common in hunter-gatherers and declined more gradually as farming societies grow more complex. Modern Europe, which has far more people, has even less inbreeding.

The regressions and images above go from the present on left, to the past, on the right. F(ROH) (for runs of homozygosity) is a measure of inbreeding with a higher number indicating more inbreeding in the lineage of the individual whose DNA is examined.

Essentially, the larger the size of the communities involved, as improving food production technology makes possible, the less inbreeding is found.

The detailed numerical data is summarized in the chart below. The key data points (with median F(ROH)) are:

* Hunter-gathers (N=40) 0.0633

* Simple agriculturalists (N=102) 0.0286

* Early complex agriculture (N=230) 0.0250

* Advanced complex agriculture (N=160) 0.0160

* Modern Central Eurasia (N=309) 0.0156

* Modern Europe (N=139) 0.0039

Central Eurasia is less inbred than Europe among hunter-gathers, simple agriculturalists, and advanced complex agricultural societies, but is modestly more inbred in the era of early complex agriculture, although the statistical significance of the differences in these areas is modest.

In the modern era, Central Eurasia with an inbreeding coefficient of 0.0156 (which is more than in Central Eurasia during the advanced complex agriculture phase) is much more inbred than modern Europe which has an inbreeding coefficient of 0.0039. The combined modern inbreeding coefficient is 0.0066.

Prevalence Of Moderate Inbreeding With Some Cousin Marriage

100% of hunter-gatherers, 95% of farmers prior to the advanced complex agriculture phase, 77% of farmers in an era of advanced complex agriculture, and 14% of modern Europeans are as inbred as the median modern Central Eurasian (0.0117). This is what one would expect to arise from endogamy within a community with some cousin marriage (including remote cousins) that is not predominant.

Prevalence Of Cousin Marriage

93% of hunter-gatherers, 5-15% of ancient farmers, and 2% of modern Europeans are as inbred as 23% of modern Central Eurasian are (0.0391), a level typical for first cousin marriages. According to the paper:

Samples from modern groups like the Balochi, the Bedouin, or the Sindhi from Pakistan have the highest proportions of individuals with FROH > 0.0391 (50%, 41.3%, and 33.3% respectively).

Prevalence Of Highly Inbred Mating

10% of hunter-gathers, 1.9% of simple agriculturalists and modern Central Eurasians, 0.4% of farmers in early complex agriculture, and 0% of farmers in advanced complex agriculture and modern Europeans has an inbreeding coefficient of 0.0932. 

This is a level of inbreeding in excess of otherwise unrelated double cousin marriages, and similar to that of marriages between an uncle or aunt and that person's niece or nephew, between half-siblings, between a grandparent and a grandchild, or offspring of closer matings. 

Even remote cousin over many generations in a small community, however, can elevate the inbreeding coefficient of mere cousin marriages or of a double cousin marriage, above this threshold. The genetic evidence indicates that this sort of genetic drift was involved in the early hunter-gatherer communities, while cousin and/or closer marriage was a major factor in other genomes studied.

The high inbreeding coefficients in modern "Central Asians" are largely a product of Islamic law's acceptance of cousin marriage and of traditional cultures in some (but not all) Islamic societies, that favor high rates of cousin marriage of 10% to more than 50%, as shown on the map below. 

Cousin marriage is uncommon, however, despite being at least nominally legal under Islamic law, in countries that are predominantly Muslim where Muslims predominantly follow the school of Sunni Islamic jurisprudence that is predominant in Southeast Asia (i.e. the Shafi'i school of Sunni Islamic law).

Some concluding comments of the paper bear repeating in their entirety so as not to lose the nuance of these important caveats to its findings:

Three points further deserve mention regarding mating patterns in human societies. 

One is the seeming contrast between the high levels of drift-driven autozygosity (panmictic inbreeding) we report for ancient hunter-gatherer societies and ethnographic studies showing low levels of inbreeding among modern-day hunter-gatherers. For instance, a comparison of inbreeding patterns in a worldwide sample of contemporary hunter-gatherers with Amazonian horticulturalists reported lower inbreeding in hunter-gatherer groups. Hill and colleagues also report low levels of relatedness within modern-day hunter-gatherer bands. However, the mentioned ethnographic findings rely on genealogies and report the prevalence of inbreeding by consanguinity, not inbreeding by drift. In fact, we also find consanguinity to be rare among early Holocene Eurasian hunter-gatherers relative to agriculturalists, consistent with widespread exogamy in modern-day hunter-gatherers. This raises the possibility that reciprocal exogamy and consanguinity avoidance traditions may have been predominant among human foragers since prehistory (but possibly not in archaic hominins).

Second, our results lend support, albeit with limited data, to the hypothesis that extreme consanguinity may have become more common with farming. This result parallels higher within-group marriages among modern-day horticulturalists than foragers. It is also consistent with singular reports on ancient agriculturalist genomes, such as evidence for consanguinity identified in an early Neolithic farmer from Iran, a first-degree incest case from Neolithic Ireland, as well as a recent report on close-kin unions in the central Andes after 1000 CE. 

In our analysis, among the seven individuals with the highest level of inbreeding (with FROH > 0.125), all four hunter-gatherers appear autozygous by drift, while all three agriculturalists appear autozygous by consanguinity. This appears unlikely to happen by chance (Fisher’s exact test, two-sided p = 0.029). These results are consistent with the view that consanguineous traditions could have thrived in class-based agricultural societies with private property more readily than in more egalitarian hunter-gatherer groups.

Finally, we report higher consanguinity in Central versus West Eurasia in contemporary societies, in parallel with earlier work. This is consistent with widespread first- or second-cousin marriage practices in agricultural societies in Middle Eastern and North African countries and in South Asia, including Muslim and Jewish groups, as documented by ethnographic or genomic studies. We note that cousin marriages were also common among royal dynasties and upper classes of Europe until the 20th century, and many prominent European scientists of that period are known to have married their first cousins, including Charles Darwin and Albert Einstein. These traditions are thought to have arisen through various social factors, including the inheritance of property in class societies.

Interestingly, we do not observe the relatively high rates of consanguineous marriage observed in modern-day Central Eurasia in any of the past societies we studied, in Antiquity or earlier. We naturally prefer to remain cautious, especially given the limited sample size of our advanced complex agriculturalist samples from West and Central Eurasia (n = 9 and n = 30, respectively). Nevertheless, it appears possible that present-day cultural patterns may have emerged relatively late in time.

Mole Rat Mind Control

From PNAS via The Atlantic “Naked-Mole-Rat Queens Control Their Subjects by Having Them Eat Poop":

And according to a new study from Japan, naked-mole-rat queens use their hormone-rich poop to govern their subordinates. When the subordinates eat the hormone, it turns them into attentive caretakers of the queen’s own pups. It’s mind control, via poop. 

Naked mole rats had interested Kazutaka Mogi, a biologist at Azabu University, because of their unusual social structure. Like ants and bees, but unlike almost all other mammals, naked mole rats live in large colonies where the queen is the only female that reproduces. Her subordinates take care of the pups, and they never make sex hormones of their own or become sexually mature. Mogi and his team had investigated parenting in mice, and they knew that hormones play a key role in triggering parental behaviors in mammals. If the bodies of the subordinate naked mole rats aren’t making any hormones, how do they become such attentive caretakers—to pups that aren’t even their own? 

… 

The team collected fecal pellets from pregnant queens and gave them to a handful of subordinate females, which soon became much more responsive to the cries of pups. Then they repeated the experiment to make sure the hormones were really the key component of the poop. This time, they took fecal pellets from nonpregnant queens and added estradiol—a type of estrogen—to only half of the pellets. Only the naked mole rats that ate the estradiol-supplement poop became more responsive to pup cries. 

Mogi was excited. He had never seen hormones work like this before. Hormones are powerful mediators of behavior, but their effects are normally limited to the body of the animal making them. Here the queen seems to be making hormones to alter the bodies of totally separate animals. Insect colonies have sometimes been called superorganisms for the way thousands of individuals behave as one unit; in this case, hormones seem to be acting on naked-mole-rat colonies as a single superorganism.

This is pretty amazing stuff that I would not have expected in mammals.

Bad Sportsmanship At Science (the Magazine).

Sabine Hossenfelder's new book, "Lost in Math" (although I like the German title, "The Ugly Universe" better), will arrive on my porch tomorrow afternoon. The review of that book at the magazine "Science" is unsporting, in bad taste, and does not adhere to the standards of civility we ought to expect in reputable professional science:

Science magazine has a review. For some reason they seem to have decided it was a good idea to have the book reviewed by a postdoc doing exactly the sort of work the book is most critical of. The review starts off by quoting nasty anonymous criticism of Hossenfelder from someone the reviewer knows on Facebook. Ugh.

Via Not Even Wrong (Bee's makes a pointed rebuttal to it here).

Bee also took the high road in writing this book with a human subjects committee "best practices" worthy approach to her interviews. The same post has a delightful anecdote recounting the arrival of the finished product at her door:

The cover looks much better in print than it does in the digital version because it has some glossy and some matte parts and, well, at least two seven-year-old girls agree that it’s a pretty book and also mommy’s name is on the cover and a mommy photo in the back, and that’s about as far as their interest went.

I'll reserve a substantive review until I've read the book itself.

Babies As Footballs

Sometimes evolutionary hypotheses are a bit too powerful.

You Get More From Your Parents Than Parenting

Lots of people are raised for substantial parts of their lives by people whom they are not genetically related to, and a new study of Korean adoptees from the 1950s and 1960s has revealed once again that a rather surprisingly large share of the way that kids turn out is congenital rather than a product of the way that they were raised.

Having a college educated mother increases an adoptee’s probability of graduating from college by 7 percentage points, but raises a biological child’s probability of graduating from college by 26 percentage points. In contrast, transmission of drinking and smoking behavior from parents to children is as strong for adoptees as for non-adoptees. For height, obesity, and income, transmission coefficients are significantly higher for non-adoptees than for adoptees. In this sample, sibling gender composition does not appear to affect adoptee outcomes nor does the mix of adoptee siblings versus biological siblings.

Factors like religious affiliation and linguistic accent are also very strongly cultural. Nobody is born Christian or Buddhist, or speaking Spanish or English. Genetics may influence you attitude towards religion, or your likelihood of having a learning disability related to reading, but it won't determine the content of your religious beliefs or the language that you will learn.

But, parental income at the time of adoption has virtually no effect on adopted child income. Parental education does have a modest effect on adopted child educational level (parents who went to graduate school have adopted kids with half a year more of education than those who merely graduated from high school), but the effect is really strong only for parents who don't have at least a high school diploma (those kids get dragged down). In contrast, parental income and education have a rather pronounced effect on the the incomes and educational levels of biological children suggesting that these outcomes are mediated by factors like hereditary IQ and personality effects.

This doesn't necessarily mean that your parents alone are your destiny. Quite extensive studies have shown that there is a fairly wide range of stable differences in all sorts of traits between even full blooded siblings on factors from IQ to personality and that those differences impact life outcomes. Almost no intelligence or personality or mental health traits are 100% hereditary or even much more than 85% hereditary, and most hover at the 50% level or less.

There is also a fair amount of evidence to show that really bad parenting and early childhood deprivations or physical traumas can screw anyone up no matter how much potential that person may have had at birth. Genuine poverty, malnutrition, lead poisoning, prolonged isolation, and abuse are all bad for kids no matter how promising they are (although there are even genes that govern how well kids can cope with some of these adversities, a trait sometimes described as resilience).

But, given the intense amount of effort the parents put into deciding precisely how to raise our children, parenting choices make less of a difference than one would intuitively expect. Princes raised by peasants really do generally grow up to be princes, royal bastard are going to tend to outperform their ordinary siblings, and kids raised in circumstances far more favorable than those they were born in because they are adopted generally underachieve relative to the biological children of their adoptive parents and have less in common with their siblings than you would expect from a shared upbringing. It may have a ring to the monarchist propaganda that Disney likes to produce, but there is truth to it. The myth of Achilles, who was a prince raised by farmer parents as their own who rises to prominence, is closer to real life than the Prince and the Pauper.

Indeed, there are at least one or two studies out there that show that decent parenting from just about anyone is fine. A good step-parent or adoptive parent or extended family member in lieu of a parent can raise you well enough to reach essentially the same potential you could have if raised by a biological parent - or better if a biological parent's exceptionally bad parenting was factor that could have dragged you down.

It is hardly surprising that assortive mating should strength social class divides. If people who have what society rewards have kids with other people who have the same traits, and people who lack that have kids with other people who lack that, we expect that there kids are more likely to reproduce their status than in a society where couples form at random or are deliberately unequal in traits that society rewards, which will tend to produce more variability between siblings and more averaging out.

More paradoxically, each generation that society is meritocratic should strength class divideds in future generations. Some people from modest beginnings will always be more talented than their parents, and some people with talented parents will always have less of what made their talents successful. Regression towards the mean is to be expected, and the less talent matters the less talent will be associated with social class and the more vulnerable the social class system will be to being upset by talented people born in a lower socio-economic class usurping less talented people born to privilege.

By all accounts, American marriages are more assortive than they used to be, and we have had roughly two full successive generations of a relatively meritocratic society. The longer this continues, the more stark social class divisions can be expected to become, with really no obvious limits. A few centuries of carefully preserved meritocracy and assortive mating should paradoxically greatly reduce social class mobility and make the gradations within social classes more fine, to the extent that social class divisions are a product of individual personal traits rather than the nature of the slots the arise from our current technology and economic system. A hunter-gather society, for example, can support only so much social class stratification.