Ancient hair samples act as biological time capsules that reveal the diets and health of people from thousands of years ago.

Hair is an exceptional forensic tool because it grows at a predictable rate of approximately 1 centimeter per month. This growth pattern allows bioarchaeologists to map out a person's life history month by month by sectioning a single strand. Because keratin is highly resistant to decay, it can survive in arid or frozen environments for thousands of years while locking in isotopes and trace elements.Stable isotope analysis is the primary method used to decode these samples. For example, nitrogen isotopes can distinguish between a diet heavy in meat versus plants, while carbon isotopes reveal the types of grains consumed, such as maize or wheat. A famous study of the 5,300-year-old 'Ötzi the Iceman' used hair analysis to show he had high levels of copper and arsenic, suggesting he was involved in copper smelting.Beyond diet, hair reveals environmental hazards and social status. Research on 2,000-year-old Peruvian mummies by the University of Western Ontario found varying levels of cortisol, a stress hormone, preserved in hair shafts. This provided direct evidence of the physiological stress experienced by individuals before their death. Modern mass spectrometry can now detect even minute traces of pharmaceuticals or heavy metals like mercury and lead in these ancient samples.