Here’s a map which makes clear the spread of agriculture north and west from its point of origin around the Fertile Crescent. We’re talking about (at least) a ~3,000 year latency from the emergence of agriculture in the Near East until its penetration to the northern and western fringes of Europe. How did it spread? The general models are those of cultural diffusion, the migration of farmers along a wave of advance (basically a high fertility frontier expanding outward), or, a combination of both. Current estimates place the “Neolithic” contribution to European ancestry between 20% and 50%, leaning toward the lower estimate and dropping as one shifts north and west. Albert J. Ammerman and L.L. Cavalli-Sforza have long promoted a model of “demic diffusion” which posits that these expanding farming populations swept out of Anatolia because of population growth and brought the agricultural tradition to Europe via their physical migration. The finding that only 20% of Europeans were descended from Neolithic farmers was taken by some (e.g, Bryan Sykes) as a refutation of Ammerman and Cavalli-Sforza’s model, but the latter pointed that the original genetic signal in a wave of advance would of course be diluted over time. In other words, it was never one of total genetic replacement; rather, a synthesize between migrants and locals driven by the cultural innovation which the migrants possessed (agriculture). Ammerman’s recent paper, Tracing the Origin and Spread of Agriculture in Europe outlines these views rather well and elucidates the nature of the advance. But the dispute about how agriculture was spread is less important to me than the fact that it spread slowly over thousands of years. That means that north and west Europe experienced higher population densities considerably later, and gives a approximate time frame for when we might expect agriculture related adaptations to emerge….