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June 13, 2018

The days of the All-Fathers

Filed under: Father's Day,Genetics,History — Razib Khan @ 11:12 am
Citation: Zerjal et al.

“A man’s greatest joy is crushing his enemies.”

— Genghis Khan

There are many apocryphal quotes attributed to Genghis Khan. And there’s a reason for that — in a single generation he led an obscure group of Mongolian tribes to conquer most of the known world. His armies, and those of his descendants, ravaged lands as distant as Hungary, Iran and China. After the great wars, though, came great peace — the Pax Mongolica. But the scale of death and destruction were such that in the wake of the Mongol conquests great forests grew back from previously cultivated land, changing the very ecosystem of the planet.

It is no great surprise then that if there were ecological impacts of the Mongol conquest, there were also genetic ones. About ~10% percent of the men who live today within the former territories of the Mongol Empire at the death of Genghis Khan carry a particular Y chromosome lineage. About 15 years ago researchers tried to assess the relationship of these individuals on their Y chromosome, and were confronted by the reality that there wasn’t any neat relationship…the phylogeny was a “star.”

Citation: Zerjal et al.

What this means is that at some point in the past men who carried this Y chromosome underwent a very rapid expansion. So rapid that the genetic tree simply “explodes,” rather than accumulating mutations in a gradual manner which could outline different relationships between parental and offspring Y chromosomes. By looking at the pattern of diversity of the branches of the star lineage scientists concluded that this cluster must have expanded about ~1,000 year ago in the past.

Genghis Khan

What happened about ~1,000 years ago? It is notable that the lineage, the “star haplotype,” is most diverse and frequent in and around Mongolia. The conclusion was unmistakable: this Y-chromosome lineage comes down from the tribe of Genghis Khan, and its explosive growth occurred due to the explosive growth of the Mongol Empire.

Genes reflect history and social norms. The history of the Mongol expansion and the extermination of local elites across vast swathes of Inner Asia has left its legacy in the genomes of modern people, with the signature of explosive growth in the Genghis Khan Y haplotype, which stretches far and wide. The persistence and frequency of this lineage across nearly 1,000 years attests to the social prestige attached to be a direct male scion of Genghis Khan and his descendants.

The cultural importance of descent from Genghis Khan in Inner Asia can not be underestimated. Though he was a pagan through-and-through, among Muslim Turkic peoples descent from him became highly prestigious, and a mark that one was meant to rule.

Citation: Karmin et al.

In the case of the the Genghis Khan Y lineage there is a historical record that explains the cause of the genetic phenomenon. What about other Y-chromosomes?

It turns out that about 4,000 to 5,000 years ago a widespread bottleneck followed by an expansion occurred specifically on the Y chromosome for many lineages, not just one. This is particularly true of Eurasia.

For example, Y haplogroups R1a, R1b and I1 seem to have undergone expansion at this time after a population reduction. R1b is the most common haplogroup in Western Europe. R1a is the most common in Eastern Europe, Central Asia, and much of South Asia. I1 is the dominant haplogroup of Scandinavia. But 5,000 years ago ancient DNA tells us that R1a ad R1b were very rare where today they are common. I1 seems to be a relic of the Pleistocene hunter-gatherers of Europe, but it only began expanding at the same time as R1a and R1b.

Unfortunately 5,000 years ago most of the world was cloaked in prehistory.
Light war chariot

History provides few clues about why a few Y chromosomal lineages came to be so dominant. But we do know that this was around the time when pastoralism and horse-powered warfare, in the form of the light chariot, came into being. New research suggests that only theoretical models that rely on “inter-group competition” can explain the Y-chromosome pattern we see. That is, it can’t be polygyny, where a few men have many wives within the tribe. Rather, it has to be a tribe as conceived of as a patrilineal kinship unit. The victory of one tribe was total loss for the males in another tribe, and each tribe was represented by a particular Y-chromosomal lineage.

Which sounds awfully familiar to the descendants of Genghis Khan…

Interested in learning where your ancestors came from? Check out Regional Ancestry by Insitome to discover various regional migration stories and more!

The days of the All-Fathers was originally published in Insitome on Medium, where people are continuing the conversation by highlighting and responding to this story.

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