An “in-fill” framework for the expansion of peoples in Europe: beakers, beakers everywhere!

In the 1970s A. J. Ammerman and L. L. Cavalli-Sforza argued for the validity of a model of Neolithic expansion of farmers into Europe predicated on a “demic diffusion” dynamic. This is in contrast to the idea that farming spread through the diffusion of ideas, not people. The formal theory is inspired by the Fisher wave model, but empirically just imagine two populations with very different carrying capacities due to their mode of production, farmers, and hunter-gatherers. In a Malthusian framework, the farmer carrying capacity in a given area of land might be ~10× greater than that of hunter-gatherers. Starting at the same initial population, the farmers will simply breed the hunter-gatherers out of existence.

As the farmers reaching their local carrying capacity, migration outward will occur in a continuous and diffusive process. For all practical purposes, the farmers will perceive the landscape occupied by hunter-gatherers as “empty.” This is due to the fact that hunter-gatherers often engage in extensive, not intensive, exploitation of resources. In contrast, even slash and burn agriculturalists leave a much bigger ecological footprint. They swarm over the land.

The beauty of the demic diffusion process is that that it’s analytically elegant and tractable. Families or villages engaged in primary production to “fill up” a landscape through simple cultural practices which manifest on the individual scale that allow for aggregate endogenous growth. And this model underlies much of the work by Peter Bellwood in First Farmers and Colin Renfrew’s theories about the spread of Indo-European langauges. You can call it the Walder Frey theory of history.

I didn’t really think deeply about this theory because I didn’t have much empirical knowledge until I read Lawrence Keeley’s War Before Civilization. In this book, Keeley observes that the archaeological record suggests that there was violent conflict between the first farmers and hunter-gatherers in northwestern Europe, near the North Sea. He reports that there seems to have been a broad front of conflict, presumably a prehistoric “no man’s land.” Not only that, but Keeley claims that the spread of agriculture stopped for a period. The barrier between hunter-gatherer occupation and farmer territory was not permeable. Not diffusion.

As a stylized fact, the demic diffusion framework treats all farmers as interchangeable and all hunter-gatherers as interchangeable. On the face of it, we know that this is wrong. But the assumption is that to a first approximation this axiom will allow us to capture the main features of the dynamics in question. This may be a false assumption. The fact is we know that some hunting and gathering populations can engage in intensive resource extraction and remain sedentary.

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The sons of Japeth divide the world between them


Most “old hands” in the discipline of historical population genetics remember when grand narratives were constructed out of Y chromosomal haplogroup distributions. One of the most distinctive ones is that of haplogroup R1b, which exhibits very high frequencies in the west of Europe, as high as more than 80% among the Basques. Because the Basques are the only non-Indo-European population which exists today in Western Europe, it was presumed that they are more ancient than other groups. And, their high frequency of R1b (along with other peculiarities such as a high frequency of Rh-), was taken to indicate that they reflected the genetics of Europe’s aboriginal hunter-gatherers when farming arrived.

This turned out to be wrong in a lot of details. Genetically the Basques are quite like the European farmers from Anatolia who replaced the original hunter-gatherers. Less so than the Sardinians, as they have more hunter-gatherer ancestry. But instead of being the language of European hunter-gatherers, it seems plausible that the Basque language descends from that of the Cardial culture.

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Cave bears live on in brown bears, and were vegetarians (what!?!?!)


Unless you’ve been asleep you are aware of another megafauna ancient DNA discover, Partial genomic survival of cave bears in living brown bears:

Although many large mammal species went extinct at the end of the Pleistocene epoch, their DNA may persist due to past episodes of interspecies admixture. However, direct empirical evidence of the persistence of ancient alleles remains scarce. Here, we present multifold coverage genomic data from four Late Pleistocene cave bears (Ursus spelaeus complex) and show that cave bears hybridized with brown bears (Ursus arctos) during the Pleistocene. We develop an approach to assess both the directionality and relative timing of gene flow. We find that segments of cave bear DNA still persist in the genomes of living brown bears, with cave bears contributing 0.9 to 2.4% of the genomes of all brown bears investigated. Our results show that even though extinction is typically considered as absolute, following admixture, fragments of the gene pool of extinct species can survive for tens of thousands of years in the genomes of extant recipient species.

A sad thing about this publication is that brought to my attention that these ancient cave bears were mostly herbivores. It makes me view The Clan of the Cave Bear differently!

I assume most readers of this weblog are not surprised. We know that various extant and extinct members of the elephant lineage have mixed. By a strange coincidence (or perhaps not?) the fraction of cave bear DNA in modern brown bears seems very similar to the fractions of Neanderthal DNA we seen in modern lineages. The authors infer that the gene flow may also have been bidirectional, so various bear lineages had multiple and complex interactions over hundreds of thousands of years. Something notable is that the divergence between cave and brown bears is considerably deeper than that between Neanderthals and modern humans. If the latter can be dated to around 750,000 years ago, with large intervals on either side, the bears apparently separated into separate species 1.2 to 1.4 million years ago.

The greater time depth is of interest to the authors. From The New York Times:

“We did not expect to find this at all because they’re really quite diverse in terms of their evolution,” Dr. Barlow said.

The team was also able to determine that the genes flowed both ways between species, with the cave bears also carrying some brown bear DNA. The most recent transfer of genes came from the cave bear to the brown, the study found.

Brown bears are more closely related to polar bears than they were to cave bears from whom they diverged more than a million years ago, he said. Cave bears were largely herbivores, while brown bears are meat-eaters and about 20 percent smaller than cave bears, with more delicate bones. A brown bear would probably have looked “wimpy” next to a cave bear, he said.

The expectation here is conditional on the idea that bears which occupy different ecological niches probably won’t hybridize even if they overlap in range.

All that being said, when Greg Cochran started talking about archaic admixture into modern lineages in 2005 I read up on the mammalian hybridization literature and came to the conclusion that a priori there was no reason why Neanderthals and modern (African) humans couldn’t have produced fertile offspring. Big mammals tend to occupy a lot of territory, and different big mammal lineages overlap. It seems rather common for gene flow to occur between them. There is evidence of jackal and coyote introgression into Eurasian wolves, for example.

So I  guess I’m not that surprised. And David Quammen’s new book, The Tangled Tree, presents a rather non-revolutionary message from where I stand. Though perhaps it hasn’t gotten out to the “public.” The complexity and multi-textured reality of the “species problem” is pretty clear to any biologist who work’s on population-level data.

A celebration of Cheddar…Man


It’s been a lot of cheddar the past few weeks. Or should I say Cheddar Man, the 9,150 year old Mesolithic subfossil from the area of Cheddar Gorge in England. This individual is important because it’s the oldest remain of such high quality found in Great Britain. And, in the late 1990s, as reported in Bryan Sykes’ Seven Daughters of Eve and elsewhere, the Cheddar Man subfossil was genotyped for mtDNA, the maternal lineage. There were, and are, lots of controversies about the validity of that result due to contamination being common in those early years of ancient genetics.

But today we have Cheddar Man’s whole genome. The preprint is finally out, and I’m digesting. Additionally, there has been a Channel 4 documentary, and a few weeks of media hype all around the implications of Cheddar Man.

This is an exciting time for genetics, history, and heritage. Since Britain is a major center of interest for these topics, it’s not surprising that Cheddar-mania has taken off. To mark this occasion DNA Geeks commissioned a design of Cheddar Man using Prince as a model. That might seem strange, but it probably is appropriate given Cheddar Man’s other-worldly and ambiguous appearance. You can get t-shirts and framed prints.

I’ll probably be posting about the Cheddar Man preprint, which really transcends Cheddar himself, tonight or tomorrow.

Understanding prehistory through genetic inference and ancient DNA

Before David Reich’s book, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past, I highly recommend a new preprint from Pontus Skoglund and Iain Mathieson*, Ancient genomics: a new view into human prehistory and evolution.

It’s basically at the sweet spot for a lot of readers: doesn’t overemphasize methods or archaeological minutiae that’s hard to follow. That being said I do think you would benefit if you read two things which would complement in those directions, First Farmers: The Origins of Agricultural Societies, and Ancient Admixture in Human History.

* I have to say, I consider Iain a friend, but am I the only one a bit perplexed by how a British person can have such a difficult to spell version of his name? I always have to look it up!