Hair carries its history

The hair shaft carries a unique historical record of everything that has happened to it during its long growth. As a hair grows, the year-on-year accumulation of the effects of drying, dyeing, coloring and so forth are

recorded in its structure, and the record can be 'read 'under the microscope.
    One way of looking at how long this record stays on the hair is to measure the length of the hair (assuming that it grows by a centimetre a month) and marking past world events along its length.