A chemical autobiography
In the arid foothills of the Hissar Mountains, a burial mound known as a kurgan holds a silent biography. This story is not written in text, but in the atomic composition of a skeleton from the Iron Age. Scientists can read this story using isotope analysis, a technique that turns bone and tooth enamel into a geological record. The main element is strontium (Sr), a chemical element that the human body absorbs from food and water. The ratio of two strontium isotopes, strontium-87 (⁸⁷Sr) and strontium-86 (⁸⁶Sr), varies depending on the age and type of underlying bedrock in a geographic area. These unique geologic signatures are passed up the food chain and become locked into a person's skeleton.
The analysis hinges on a simple difference between teeth and bones. Tooth enamel forms during childhood and its isotopic composition does not change for the rest of a person's life. It is a permanent record of where an individual grew up. Bone, in contrast, is a dynamic tissue that remodels itself over a period of roughly seven to ten years. Its strontium ratio reflects the geology of the place an individual lived in the years leading up to their death. A mismatch between the ⁸⁷Sr/⁸⁶Sr ratio in tooth enamel and bone from the same skeleton is definitive proof of migration.