A veritable confusion: use and abuse of isotope analysis in archaeology

Bell Beaker pottery spread across western and central Europe beginning around 2750 BCE before disappearing between 2200–1800 BCE. The forces propelling its expansion are a matter of long-standing debate, with support for both cultural diffusion and migration. We present new genome-wide data from 400 Neolithic, Copper Age and Bronze Age Europeans, including 226 Beaker-associated individuals. We detected limited genetic affinity between Iberian and central European Beaker-associated individuals, and thus exclude migration as a significant mechanism of spread between these two regions. However, migration played a key role in the further dissemination of the Beaker Complex, a phenomenon we document most clearly in Britain, where the spread of the Beaker Complex introduced high levels of Steppe-related ancestry and was associated with a replacement of ~90% of Britain’s gene pool within a few hundred years, continuing the east-to-west expansion that had brought Steppe-related ancestry into central and northern Europe 400 years earlier.

Strontium isotopes are a powerful tool which provide information about provenance directly from the tissues of humans rather than the grave context and burial goods. Geographical variation in strontium isotopes is primarily controlled by the underlying geology but there are many other factors that need to be considered before migratory individuals can be identified. Consequently, despite many studies which have shown that the method works well, it is clear that much remains to be clarified and it will not work for every question or in every place. It rests on the assumption that people were sourcing their food locally and that there is a measurable strontium isotope difference between the place the person migrated from and the place they migrated to. As migrants may deliberately seek out familiar soil types and terrains in their new homeland, some questions surrounding major migration events may prove intractable for this technique. Other factors that can create heterogeneity or homogeneity leading to false positives or false negatives, such as human choices or coastal subsistence, are explored and the metabolism of strontium into human tooth enamel is discussed. Several models of land-use choices by humans are presented to highlight the subtleties inherent in the isotope data and these are used to interpret archaeological human isotope ratios from three studies.