Isotope analysis
A recent development in forensic science is the isotopic analysis of hair strands. Hair has a recognisable growth rate of 9-11mm per month or 15 cm per year. Human hair growth is primarily a function of diet, especially drinking water intake. The stable isotopic ratios of drinking water are a function of location, and the geology that the water percolates through. Sr, Sr and oxygen isotope variations are different all over the world. These differences in isotopic ratio are then biologically 'set' in our hair as it grows and it has therefore become possible to identify recent geographic histories by the analysis of hair strands. For example, it could be possible to identify whether a terrorist suspect had recently been to a particular location from hair analysis. This hair analysis is a non-invasive method which is becoming very popular in cases that DNA or other traditional means are bringing no answers.
Long hair
Humans, horses, orangutans and lions are among the few species that may grow their head hair or manes very long. Humans are believed to have lost their fur 2.5–3 million years ago as hominids when transitioning from a forest habitat to the open savanna, as an effect of natural selection, since this development made it possible to run fast and hunt animals close to the equator without getting overheated. An exception was however head hair, which was kept to provide thermal insulation of the scalp from the sun, to protect against ultra-violet radiation exposure, and also to provide cooling (when sweat evaporates from soaked hair). The ability to grow straight hair has been observed among Homo sapiens sub-groups in less sunny regions further away from the equator. Relative to kinked Afro-textured hair, straight hair allows more UV light to pass to the scalp (which is essential for the production of vitamin D, that is important for bone development).