What I'm thankful to know about genetics and history in 2020

A generation after History and Geography of Human Genes

Over the summer of 1998, I read the unabridged version of The History and Geography of Human Genes by L. L. Cavalli-Sforza. It is an incredible book. A life-changing book. But there is so much we know now that we couldn’t know then, and a lot to learn from the great researchers active since Cavalli-Sforza wrote his magnum opus. I’d like to stop to reflect briefly today and run through an overview of just some of the knowledge that has accrued in little more than two decades of discoveries.

The two great transformations since then have been:

1. Genomics (whole genomes and computing power to analyze the data)

2. Ancient DNA (filling in the human past with data rather than inference)

Below is a list of things we’ve learned (not exhaustive)…

• The major human population clusters across the world are relatively recent products of massive mixings between very different populations. How recent? Mostly over the last 4,000 to 10,000 years. In 4,000 BC there was no one with the genetic profile of modern Northern or Southern Europeans and no one with the genetic profile of modern South Asians. The genetic profile of West Asians and East Asians did not exist. The islands of Japan were inhabited by relatives of the Ainu. Much of Africa was occupied by hunter-gatherers that resembled the San Bushmen. This all outlined in David Reich’s book:

  

• Modern human populations likely look very different from people who lived during the Ice Age, or even during the Neolithic. Europeans, for example, seem to have been under continuous selection for lighter skin, hair, and eyes, for the last 5,000 years. Ice Age Europeans were very genetically different, and much darker-skinned than modern Europeans (though quite possibly blue-eyed!). Many farming populations have lost the robustness that characterized their ancestors. Selection favored those with fragile bones!

• The “Ice Age European” was actually a set of very distinct populations. Some of these groups cluster tightly with archaeological constructs, such as “Magdalenian,” while other constructs, such as “Gravettian” seem very diverse and heterogeneous. Archaeology has a lot of insight, but genetics is helping fill in many details.

• The Bell Beaker people of Bronze Age Western Europe seems to have originated in Southwest Europe but spread to Central Europe among people who were genetically and culturally very different. It is this second group of people who brought the culture to Ireland and Great Britain. This illustrates the sometimes surprising connections (or lack thereof) between genetics and culture.

• Caste and jati are genetically real constructs and date to more than 1,000 years ago. Genetically, a Tamil Brahmin is more like a Brahmin from North India than they are their neighbors in Tamil Nadu.

• The arrival of German barbarians into the post-Roman Empire was due to a real migration of a distinct people, not simply an ad hoc culture. Peter Heather was right!

• The Paleo-Siberian ancestors of Native Americans separated from the ancestors of modern Siberians more than 15,000 years ago. Clovis First is definitely wrong.

• Modern indigenous Australians descend from the first settlers who arrived ~50,000 years ago. They have not been subject to as much population turnover as people in Eurasia, the New World, or Africa.

• There was a massive migration out of the Eurasian steppe 4,000 to 5,000 years ago. There was also migration back into the Eurasian steppe from Eastern Europe at the tail end of this period.

• The Huns were genetically distinct from local Europeans. They were probably East Asian in origin.

• The arrival of farming in Europe was mostly through the migration of people from the Near East, who slowly and fitfully mixed with local hunter-gatherers.

• Some groups in the Amazon seem to have a distant connection with people in Southeast Asia, something not seen in any other New World populations.

• Overall, the mass migration of the Roman period didn’t have a long-term impact on Western Europe because of low fertility in cities, where the migrants were concentrated.

• Uralic people, such as the Finns, seem to exist due to a migration out of the east.

• The population bottleneck that impacted all non-Africans 50 to 100 thousand years ago did not impact ancient Africans (though there was a gentler bottleneck before 100 thousand years ago). Modern Africans descend from many local populations, along with some groups who migrated back from Eurasia. Hunter-gatherers, such as the Mbuti Pygmy and San Bushmen, have more of the “deep” diverged ancestry, while the farmer and pastoral groups have more “mixed” recently differentiated ancestry related to Eurasians.

• Speaking of San Bushmen, they seem to be subject to the smallest bottleneck effect of all modern human populations.

There’s a lot more we’ll learn. The next decade will be interesting. But it’s been a privilege to be an eye-witness to so much science that has answered so many open questions to various degrees of certainty.