Tracking prehistoric migrations
Prehistoric migrations, for many years out of favour among archaeologists, have recently come back into prominence. One major factor has been the very widespread acceptance of the ‘out-of-Africa’ explanation for the dispersal of our own species, Homo sapiens, as brilliantly documented by Cann, Stoneking and Wilson in 1987 on the basis of their study of mitochondrial DNA from a range of living human populations across the world. Another is the realisation that the term ‘migration’ can describe a range of demographic processes, some of which, like the demic diffusion proposed by Ammerman and Cavalli-Sforza in 1973, can be modelled quantitatively, sometimes with the aid of computer simulations.
Two new books, both taking the linguistic evidence as well as insights from DNA studies, have much to offer. Both seek to integrate inferences from molecular genetics with evidence from historical linguistics and prehistoric archaeology, although neither fully embraces the power of modern, computer-aided phylogeographic studies, such as that of Bouckaert and colleagues (2012). Each in its way is on the path to achieving a new synthesis between the three disciplines involved.
Peter Bellwood takes a worldwide view dealing first with hominin dispersals and then those of human hunter-gatherers. Readers of his The first farmers (2005) will not be surprised that he sees the farming revolution as a decisive factor in many areas, with dispersals of agriculturalists in the Holocene playing a major role in determining modern populations. There are many problems which remain to be resolved. For instance, Bellwood takes a migrationist view for the distribution of the Pama-Nyungan languages of Australia, seeing them as the result of a hypothetical language dispersal some 4000 years ago. Some linguists, such as Bob Dixon (1997), see the distribution of the Pama-Nyungan languages instead as the product of linguistic diversification, with local interaction and borrowing in the years since the initial colonisation of Australia. Bellwood’s account overall offers a remarkably coherent view of linguistic (and genetic) diversity worldwide, the first which has been offered which deals in some detail with the populations of all the continents of the world. One small but irritating feature of his otherwise ample bibliography, however, is his failure to name the full complement of the authors of multi-authored works, naming simply the first two, and relegating the others to an anonymous ‘et al.’. While that may be an understandable reaction to the long lists of authors who sometimes jointly contribute to papers on molecular genetics in Science or in Nature, it seems a needless economy of space in what is otherwise a very good bibliography.
A great merit of Bellwood’s survey is its global perspective. He deals carefully in each instance with the linguistic evidence as well as the archaeological position. His perspective is persuasive, and in its broad outlines is likely to be an enduring one. This is a significant contribution to our understanding of world archaeology.
Jean Manco in Ancestral journeys operates on a more restricted geographic scale, limiting herself to the continent of Europe. This does, however, allow her to deal in much greater detail with the genetic evidence. Indeed she offers no fewer than 20 distribution maps of specific mitochondrial DNA and Y-DNA haplogroups as they occur in living populations today, whose distributions she sometimes correlates with specific migratory events in the past. For instance, in fig. 75: “The distribution of Y-DNA R1bl-L21 suggests that it travelled down the Rhine and into the British Isles where it is now densest in the regions least affected by post-Roman arrivals” (p. 164). It may be that she sometimes is too ready to produce an over-literal interpretation of such frequency distributions, and so some of her conclusions are likely to be provisional. But these are hypotheses which are worth formulating. The smaller geographical range also permits her to go into much more detail over questions of time depth, which she addresses by including the evidence from ancient DNA (recovered from skeletal remains) which is now becoming available in Europe, especially in the cooler regions in the north. This not only confirms the significant genetic impact of the incoming populations from Anatolia and the Near East with the inception of farming (which had already been observed in the studies of DNA from living populations), but also the significant demographic changes which occurred at the time of the first impact of metal-use at the end of the Neolithic and the beginning of the Bronze Age.
In her review of the linguistic data she takes the once-conventional view of scholars in the tradition of Marija Gimbutas, such as Jim Mallory and David Anthony, in seeing the arrival of Proto-Indo-European speech in Europe as the result of migrations in the Late Neolithic or Early Bronze Age. Here she differs from Peter Bellwood, who follows the present reviewers in seeing Proto-Indo-European as a product of a farming/language dispersal from Anatolia. Her account does make a good case for fitting the Indo-European dispersal into the conventional, more restricted timescale. But she gets into difficulties with the Basques, where mitochondrial DNA haplogroup H has today a high frequency. This major type H is not found in ancient DNA from available northern European Mesolithic sites, and successful ancient DNA retrieval from Mediterranean regions has so far been limited. There is therefore still ample room for speculation over when and where the major European type H came from. Manco’s proposal that haplogroup H and therefore the Basques are a comparatively recent (i.e. Neolithic) arrival is a key to her prehistoric picture of Europe. We prefer the alternative assumption that the Basque population, like much of that of Western Europe, is a Palaeolithic relic, supplemented by some later genetic input from the incoming first farmers.
Time will tell. Our own view of the Indo-European problem has recently been set out in some detail (Forster & Renfrew 2014; Heggarty & Renfrew 2014). It differs radically from that of Gimbutas and Manco. That does not, however, detract from the utility of Manco’s book which sets out to reconcile the genetic evidence with that from other disciplines in a systematic way. Ancient DNA analyses are now becoming available at an increasing rate, and the picture may soon become clearer. Meanwhile this may be regarded as a pioneering attempt to grapple with some of these problems at a detailed level. We predict, however, that the conclusions of Bellwood, which are of a more general nature, are more likely to stand the test of time.