Michael: There’s a lot of diversity in human hair shape and texture – straight strands, gentle waves, tight ringlets, and thick afros. And it’s all thanks to a combination of chemistry, physics, and genetics. There are two main components to hair: fibers, which are the strands you see, and follicles, which lie beneath the skin.

Hair fibers are mostly made of a kind of keratin: long, thin proteins formed from two helical strands that coil around each other and are held together by different bonds. There are weak, temporary bonds like van der Waals interactions and hydrogen bonds, and stronger ones called disulfide bridges. But with hair shape, the root of the matter [wink] is in the hair follicles, the little bulb-shaped organs in your skin that produce the hair fibers.

Straight-hair follicles tunnel directly down into the skin, and the fibers grow out with a circular, symmetrical cross-section.

But curly-hair follicles are more diagonal, with a hook or “kink” at the skin’s surface. This makes oval hair fibers with keratin distributed unevenly. The edges with more keratin fibers become the inside of the curl, because there are more bonds between fibers, so it bunches up a bit. Repeat this over and over, and you get waves or ringlets.

Now, there’s a strong genetic component to hair curliness. People tend to inherit it from their parents. It seems like our human ancestors in Africa probably had curly hair, and the straight hair seen in European and East Asian people evolved independently, though we’re not really sure why. Straight hair in Europeans comes from a variant of a gene called TCHH. It encodes for a protein called trichohyalin that strengthens part of the hair follicle. On the other hand, people from East Asia with straight hair tend to have a specific form of EDAR, a cell signalling receptor gene. But if you don’t want to be locked into straight or curly hair – you can change your ‘do with heat. That’s how hair curlers and straighteners work.

The intense heat breaks weak hydrogen bonds in your hair, so you can shape the fibers, and let the bonds reform. They’ll stay that way, at least for a while... Unless you do something like step in the rain! Water molecules can disrupt hydrogen bonds too, letting your hair fibers go back to their natural shape. Or if you had your hair up in a ponytail or under a hat, it’ll stay all kinked!

For a more permanent fix, some people turn to perms for long-term curls, or permanent relaxation for straight strands. These treatments use chemicals to disrupt the much-stronger disulfide bridges. And once these bonds reform, hair can be locked into place for weeks or months.

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