« The reverential agnostic | Gene Expression Front Page | Teach both sides? Uh, but what exactly.... »
October 08, 2003

A tale of two scientists

Luca Cavalli-Sforza recently contributed a Demic Diffusion as The Basic Process of Human Expansions to the monograph Examining the Farming/Language Dispersal Hypothesis (links in this post). As some critics of this field have noted, it is politically charged, because it addresses and explores basic questions of the formation of peoples, their culture (language, artifacts, etc.) and biology (genes).

Over the past 100 years there has been a strong shift from one pole to another when it comes to the distillation of the question Cavalli-Sforza addresses in the paper above: do people spread languages by physically displacing others, or, do people acquire the language of others through exchange and interaction[1]. At the beginning of this century, language, nation and race were inextricably connected. So for instance in the context of the spread of the English language in Britain, it was assumed that hordes of Anglo-Saxons drove the native Celtic Britons to Wales and Cornwall. After World War II the pendulum swung back and cultural diffusionism was in the vogue, and as late as 2000 Tory Welsh historian Norman Davies could present an elite transmission model of the spread of Germanic speech amongst the Celtic British in his panoramic history of the British Isles as the null hypothesis. As noted on this blog, the question of whether the Anglo-Saxons came in full force, or whether a small elite conquered Britain and imposed its culture upon the natives can be illuminated greatly by genetic studies[2].

The man who truly brought modern genetics into the fields of archaeology, linguistics and history, has been Luca Cavalli-Sforza. For the more technically inclined reader I would suggest his History and Geography of Genes, while the more squeamish might like to pick up The Great Human Diasporas. Cavalli-Sforza is a heavy hitter, a google query will keep you busy for a long time.

In general, Cavalli-Sforza's work, reiterated in this paper, has emphasized that one must not neglect the movement and expansion of populations and focus just on cultural transmission, a professional bias of most archaeologists (who note the spread of culture in their excavations) and cultural anthropologists (who see culture as the dynamic element in the patchwork of human diversity). He notes that demic diffusion, the expansion of a population at higher relative rates than surrounding groups, can be observed today in Africa-as the Bantu expansion finally settles in to the last refuges of the pre-agricultural foraging peoples, the Pygmies & the Khoisan. In this case, Cavalli-Sforza notes that proponents of diffusionism have not taken up the "indigenist" model, because the process of demic diffusion is occurring in real time. In contrast, the indigenist model has been very strong in the context of European history & pre-history. I have read many scholarly works that dismiss the importance of volkswanderung in the history of the continent.

And so enters the concept of "demic diffusion." Cavalli-Sforza notes that he and his collaborator did not want to use the term "colonization," because of its contemporary connotations. Demic diffusion refers to the demographic expansion of population A into population B. Population A is usually characterized by the possession of an advantageous innovation that is difficult to transmit to population B, ergo, population A has a relatively higher rate of growth than B. In the context of this paper, it is especially used in reference to the spread of agriculture from the Middle East 10,000 years ago into Europe from the southeast. The basic thesis is that the ideas of agriculture did not spread by themselves, rather they were brought by agriculturalists. Colin Renfew, the archaeologist who has fleshed out many of the details of this spread looked at it originally from an anthropological perspective. Agriculture requires a suite of skills that is not easily transmitted or mimicked in a generation or two. In fact it took 4,000 years for agriculture to spread to northernmost Europe.

Cavalli-Sforza introduced the genetic methodology, showing that the gradients of certain genes (frequencies) could indicate the spread of peoples concomitantly. As noted above, and in further detail in the paper, there are several historical cases of this occurring. He notes though that the resultant admixture may be in various portions, and Cavalli-Sforza is not suggesting anything close to replacement, such as occurred in the New World in what became the United States.

He lingers upon this point to highlight a personal bone he has to pick with the geneticist Bryan Sykes, author of Seven Daughters of Eve, who spent a considerable part of his book rehasing what he perceived to be his victory over Cavalli-Sforza & co. Sykes showed that the mtDNA (maternal) lineages of Europeans are 80% Palaeolithic. The press simplified Sykes' theory into "Europeans descdended from Ice Age hunters." Cavalli-Sforza (and many others) caution that 1) there were multiple Palaeolithic lineages 2) assigning proportions is a dicey game. A later study that indicated 50% Middle Eastern farmer ancestry was greeted with the headlines "Europeans descended from Middle Easterners."

In any case, Cavalli-Sforza's beef seems to be two part. First, his demic diffusion thesis did not in the context of Europe specify a specific number, and Sykes' own work shows that the "Jasmine" lineage (the one that is Middle Eastern) is localized and concentrated in certain regions of Europe (southern littoral, southeast and some inland valleys). The 20% figure then can not be taken as the equal over all of Europe, but should be interpreted in the light of its localized concentrations, which are compatible with demic diffusion. Sykes seems to have wanted to pick a fight with the Great Man of Genetics for his own self-interest, at least that was the impression I got reading the book, because to my mind understated the localized concentrations of the Jasmine lineage to downplay the evidence for a genetic gradient in the eyes of the lay audience. Second, Sykes presented some juicey and highly flattering (for himself) anecdotes in his book. They went along the lines of "they admitted I was right after all!" Cavalli-Sforza asserts, with irritation quite obvious, that many of these personalized interactions are fictionalized vignettes, and that Sykes' work was in any case not in conflict with his original thesis in its broad outlines (researchers associated with Cavalli-Sforza found that Europeans were 20% Middle Eastern along the patrilineal line! Great back-up for Sykes' 20% number along the maternal line). I think Cavalli-Sforza is overstating his case, in his previous works I did get the impression that the "demic diffusion" into Europe was of greater importance than it seems to have been (assertion subject to revision!). Nevertheless, I also did get irritated by Sykes' shading of emphasis and rejiggering of definitions to make it seem like there was a big controversy when there wasn't-Cavalli-Sforza notes that he stands on more than genetic legs, rather, the pattern of archaeology & spread of agriculture seems far too congruent with the genetic markers to suggest anything but that peoples moved along with ideas. Overall, I suggest the paper if you have some background in this topic, otherwise, there's a lot of material germane to esoteric internal debates (it is a monograph!).

fn1. Those reasons could be "elite transmission" (English in Jamaica is an example) or cultural diffusion (the Dravidian speech of some Indian tribal peoples are examples).

fn2. In this case, the evidence seems mixed to me, though it is not helped by the fact that Celts of Britain and the German settlers were not genetically that different to begin with. The fact that there are still relatives of the Cheddar Man in England 9,000 years after his death in the local area indicates that some people do have rather deep lineages in England, Celt and Saxon notwithstanding.

Posted by razib at 01:15 AM

The 'Cheddar Man' story was misunderstood at the time. Because a man living near Cheddar was found to have the same Mito type as the Cheddar skeleton, people assumed that his ancestors had been living in the locality in unbroken succession for 1000s of years. But in reality he just had one of a number of paleolithic types that are found across Europe, and which happened to be the same as the bones in the cave.

Posted by: David B at October 8, 2003 03:04 AM

The 20% figure (both from mtDNA and Y chromosomes) is based on the assumption that the people who diffused were those with mtDNA haplogroup J (and subclades of T and U) and Y haplogroups J and E3b.

It is true that these are the lineages that probably entered Europe during the Neolithic. But that doesn't mean that ONLY these lineages came during the Neolithic. 20% J and E3b means 20% Neolithic contribution ONLY IF the first Neolithic migrants belonged entirely to haplogroups J and E3b, which is doubtful.

Demic diffusion could have been much more significant than the 20% figure indicates, since the people who diffused agriculture into Europe probably included other haplogroups than J and E3b even in Anatolia, and certainly they did so by the time the first farming communities were formed in the Balkans.

Posted by: Dienekes at October 8, 2003 03:54 AM

A small correction: Y haplogroup G may have also been involved in the Neolithic expansion. G has its highest frequency in the Caucasus.

Posted by: Dienekes at October 8, 2003 04:25 PM