A chemical passport in bone
Saint-Jean-Pied-de-Port sits at the base of the Pyrenees, the traditional starting point of the Camino Francés, the most popular route of the Camino de Santiago. For centuries, this town has been a convergence point for pilgrims from across Europe. Archaeological science now provides a method to trace their origins, using chemical signatures locked within their bones. Isotopic analysis of human remains found in medieval cemeteries along the pilgrimage route, and at its destination in Santiago de Compostela, is a chemical passport.
The primary tool for this is strontium isotope analysis (⁸⁷Sr/⁸⁶Sr). Strontium is a naturally occurring element in bedrock, and its isotopic ratio varies depending on the age and type of the local geology. This specific geologic signature passes into the soil and water, up the food chain, and into the plants and animals that people consume. The strontium is then incorporated into human tooth enamel during childhood. Since enamel does not remodel after it forms, the ⁸⁷Sr/⁸⁶Sr ratio in a person's teeth provides a permanent record of the geological region where they grew up. Studies on remains from pilgrimage sites show a wide range of strontium values, indicating that individuals came from many different geological areas, including regions far beyond the Iberian Peninsula.
Reading a medieval diet
Isotopic evidence shows more detail about a pilgrim's life. Oxygen isotopes (δ¹⁸O), derived from drinking water, vary with climate, altitude, and distance from the coast. These values, also fixed in tooth enamel during childhood, can help differentiate between individuals from colder, northern climates and those from warmer, southern regions of Europe.
Stable isotope analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N) from bone collagen reveals long-term dietary patterns from the last decade or so of a person's life. These isotopes can distinguish between diets based on different types of plants (like wheat and barley versus millet or maize) and can show the proportion of marine versus terrestrial protein in a diet. Elevated δ¹⁵N values, for instance, show significant marine fish consumption. This dietary signature, when found in remains along the Camino, points to origins in coastal areas like Britain or Scandinavia. Combined, these multi-isotope studies confirm that the Camino de Santiago during the Middle Ages was a pan-European event, drawing people from distant homelands who all passed through gateways like Saint-Jean-Pied-de-Port.