Academic journal article
By Gkiasta, Marina; Russell, Thembi; Shennan, Stephen; Steele, James
Antiquity , Vol. 77, No. 295
There is a long tradition of using radiocarbon dates to map the spread of farming and the arrival of Neolithic cultures across Europe. Clark (1965) was the first to do this, plotting only the earliest settlements in each territory; he noted "the need, in view of the element of uncertainty inherent in individual determinations, to dispose of samples numerous enough to yield convincing patterns" (1965:66). He was able to discern a pattern of spread into Europe along the Danube from an origin in the South East, with a long delay before farming reached the North European Plain, south Scandinavia, and other places in western Europe from the Alps to northern Britain and Ireland (ibid.:67).
Ammerman and Cavalli-Sforza (1971) used regression methods to describe the average rate at which farming spread. They also used the correlation coefficient (r) to assess the extent to which regional rates of spread differed from that overall average. They reported an average diffusion rate from an assumed origin in Jericho of about 1 km/year, and found a high value for r (0.89) in their sample of 53 Neolithic sites--suggesting that this rate was quite representative of the process generally. However, they also noted evidence for regional variation in rates (from 0.7 km/year in the Balkans, to 5.6 km/year for the Bandkeramik culture). Subsequently, they used spatial interpolation methods to generate isochron maps that plotted the mean rates of spread of farming (and of the disappearance of hunting and gathering) in two dimensions (Ammerman & Cavalli-Sforza 1984). They conjectured that the pattern observed may have been produced, not by cultural diffusion (the adoption of cultural traits), but by a gradual process of spatial population expansion and replacement. They found support for this `demic diffusion' model in a synthetic gene map, generated from the SE-NW gradient in the first Principal Component of variation in allele frequencies of modern Europeans. In their monograph, they reported that such a cline (trend) in gene frequencies was expected where farming had spread by demic diffusion. The steepness of the cline was modelled as a function of the rate of reproductive mixing with hunter-gatherers; when this rate was very low, the cline would be relatively flat, such that gene pools near the origin of the diffusion would contain about 90% of initial farmers' genes, and gene pools at the periphery would contain 75% of them.
Subsequent work has transformed this simple picture. It has been shown mathematically that identical travelling waves for the spread of farming can be generated by demic expansion, demic diffusion, or by trait adoption-diffusion (Aoki, Shida & Shigesada 1996). Archaeologists have pointed to the very different rates of the spread of farming in different regions of Europe, and have challenged the use of synthetic gene maps to validate the demic diffusion model (since such maps contain no information about the chronology of dispersals). The methodology of generating synthetic gene maps has also been challenged, since it can potentially produce clines even in spatially random data (Sokal, Oden & Thomson 1999). Furthermore, it is now recognized that genuine clines in gene frequencies can be produced by population replacement with successive founder effects (cf. Barbujani et al. 1995), or by demic diffusion with acculturation (cf. Rendine et al. 1996), or by gradients in duration of natural selection when the selection pressures are initiated by adoption of a new economic strategy, rather than by population replacement (Fix 1996, 1997).
Some recent genetic studies have found distinctive European mtDNA matrilineages that have an apparently Upper Palaeolithic or Mesolithic common ancestor; it has been estimated by Richards et al. (1996, 1998, 2000) that female immigrant farmers contributed only about 20% of the modern European mitochondrial gene pool. A similar conclusion has been reached with respect to male genetic contributions as measured from Y-chromosome markers (Semino et al. …