A number of theories have been put forward to explain why some people have curly hair and others straight. For many years, it was believed that this was determined by the shape of the hair shafts: those with a round cross-section were straight, those with an oval shape were wavy, and those with a flat profile were curly. Another theory was that in curly hair, one side grows more quickly than the other, causing it to curve. Scientists currently think that the type is largely determined by the shape of the hair follicle, which seems to be controlled by a gene.
The Structure and Chemistry of Hair
It is known that curly hair differs from straight in a number of ways aside from its shape. It is extremely rare to find someone with hair that is both curly and oily. Biologically, this makes sense, because the oils secreted by the sebaceous glands at the follicle can more easily travel down a straight shaft. Anyone with this kind of hair knows that it can be dry, hard to manage, and often frizzy. The good news is that split ends are less common.
Another difference, at the chemical level, is in the structure of the proteins of which hair is composed. It consists mainly of a protein called keratin — the same material that makes up the skin and nails — and molecules of this compound bundle together to form fibers. The protein molecule contains sulfur atoms that can sometimes bond with one another, forming what is called a disulfide bond. When these form between atoms that are quite far apart, it causes curving. The more disulfide bonds there are, the curlier the hair will be.
The Influence of Follicle Shape
Hairs grow out of tiny, tube-like cavities in the skin known as follicles. People with curly hair appear to have a hooked, rather than a straight, follicle shape. It is thought that this shape forces the parts of the keratin molecules containing sulfur atoms closer together, making it more likely that they will form disulfide bonds.
The Origins of Differing Types
Naturally curly hair is determined genetically. Some genes are said to be dominant over others; this means that, when an individual inherits two different genes for the same trait, one is more likely to express itself rather than the other. For example, the gene for long eyelashes is dominant over that for short eyelashes, so if someone inherits one of each from her parents, she will have long eyelashes. The non-dominant gene is said to be recessive, and will only be seen if it is inherited from both parents. The gene for curly hair is said to have incomplete dominance over that for straight, so an individual inheriting one straight and one curly gene may have intermediate, wavy hair.
It is not clear why these different types evolved. It has been theorized that, as early humans spread out of Africa into cooler, cloudier regions, straight hair evolved to give greater protection from cold, at the expense of increased exposure to ultraviolet light. Some experimenters have claimed that curly hair gives better protection against ultraviolet, while straight is better at trapping air close to the scalp, providing an insulating layer for a part of the body that would otherwise be very prone to heat loss. There does not seem to be any consensus about this, however.
Altering Hair Type
Not all curly hair is naturally so. Many people choose to change their type by applying various treatments. One method is to wind the hair around rollers while it is wet. This causes hydrogen bonds to form between keratin molecules, giving a curved shape dependent of the size of the rollers. Hydrogen bonds, however, are much weaker than the permanent disulfide bonds that cause natural curls, and the effect is lost when the hair gets wet again.
Chemical methods can be used to create disulfide bonds in straight hair, giving a more lasting effect, but new growth will be straight, so, again, the effect is eventually lost. Scientists are looking into ways of changing the follicle shape to achieve a permanent effect. This involves studying the way the curly hair gene works in detail, which is still an area of active research.
Care
The biggest challenges most people with this type of hair face are dryness and unmanageability. Since many use chemical and thermal products, the dryness tends to become worse. Curly hair requires different care from straight in order for it to maintain its natural health. It does not require washing every day and will benefit from an occasional break from chemicals and heat. Each time it is washed, it should be deep conditioned, and if dry, styling products containing alcohol should be avoided. Proper attention to hair care will improve its overall health and manageability.