Hair refers to naturally growing, thread-like projections made from bundles of keratin – a protein also found in teeth and nails (which is why hair removal products can damage all three). Although organisms like insects and spiders sport similar filamentous structures, hair is unique to mammals.
Hair is produced by follicles, narrow cavities found in the skin. Mammals express hair on all non-glabrous skin – the glabrous skin on lips, eyelids, palms, soles, back of the ears, and parts of the genitals lack follicles. Each follicle’s sebaceous glands produce oily sebum that lubricates and waterproofs the skin and hair. A hair matrix at the follicle's base generates hair through rapid cell division; cancer treatments such as chemotherapy specifically target rapidly dividing cells, causing hair loss side effects.
By 22 weeks post-conception, a human fetus has a lifetime supply of hair follicles – though follicle number is fixed, the type and amount of hair produced is not. Fetuses have a thin coat of downy lanugo hair, usually shed around 35 weeks of gestation. Fine, short, lightly colored vellus hair replaces it – except on the scalp, eyebrows, and eyelashes where darker, thick terminal hair grows. At puberty, a surge of androgens (“male” hormones expressed by all humans) causes additional terminal hair – or androgenic hair – to replace vellus on the arms, legs, armpits, and pubic area, and the chest, stomach, and face in men.
Hair texture (curliness versus straightness) results from the shape of the hair follicle. Keratins’ molecular sulfur-sulfur interactions also play a role: disulfide bonds cause the hair to bind to itself, causing tighter curls. A perm hairstyle utilizes chemicals to break disulfide bonds (relaxing curly hair) and reform them (curling straight hair).
The pigment melanin gives hair its color. Humans evolved for sun-rich regions have dark, melanin-rich hair, which shields the head from solar radiation. Additionally, afro-hair’s kinky, coiled texture allows heat to escape the body because it doesn’t cling to the scalp when wet with sweat. Conversely, the finer hair of those suited to colder climes express less melanin, and lighter – or more transparent – hair that lets sunlight through to promote natural vitamin D production.
A handful of folks have pheomelanin-rich, red hair. Redheads historically hail from the northern and western fringes of Europe, though the trait is found among Ashkenazi Jews, the Moroccan Berbers, and some of Persian descent. Popular myth credits redheadedness to interbreeding with Neanderthals; although these hominids had the trait, its genetic causes are different in humans.
In some cultures, redheads suffer discrimination. Accusations of “gingerism” make UK headlines; the South Park joke, “Gingers have no souls,” spawned Internetmemes; and popular media alternately portrays redheads as bullies or victims. Although gingerism is taken lightly by some, the roots of anti-redheadedness run deep: medieval Europeans believed red hair meant moral degeneration, or the sign of a witch, werewolf, or vampire. This bizarre prejudice seems evident of an unflattering human condition: a little bit of pigment can mean a whole lot of hate.