What are the main factors that contribute to hair being naturally curly or wavy?

Human hair comes with all sorts of colors, textures and shapes. Notably, African hair is more coiled and dry; Asian hair is straighter and thicker; and Caucasian hair is somewhere in between with around 45% having straight hair, 40% having wavy hair, and 15% having curly hair. These variations are determined by many genes and genetic polymorphisms in combination with environmental factors (and not in the least by hair stylists!). At this point there is much more that we don’t know than what we do know.

Here are some interesting research results that shed some light in this area:

The gene for curly hair in Caucasians

It has been long established that curly hair is a dominant trait in Caucasians and straight hair is recessive. This means two things: 1) if a person carries one allele for curly hair and another for straight hair, this person will have curly hair; 2) curly hair is a simple trait and is most likely determined by one single gene. However, a single gene has yet to be found to be solely responsible for the curly (or straight) hair trait in Caucasians.

A recent genome wide association scan has found a SNP (single nucleotide polymorphism) called rs11803731 in the TCHH gene which accounts for about 6% of hair curliness. The TCHH gene encodes a protein called trichohyalin, which is known to be expressed at high levels in hair follicles and has been shown to be involved in the cross-linking of the keratin filaments found in hair. The ancestral allele of this SNP (the A-allele) is present in the worldwide population. Sometime during human history, a mutation lead to the emergence of the T-allele (called the derived allele in Fig. 1A). The T-allele causes an amino acid to change from leucine to methionine at position 790 of the TCHH gene. The function of this change is not clear. Nevertheless, Caucasians carrying the T-allele are more likely to have straight hair (about 70%) than those without the T-allele (about 50%) [1]. From this we can infer that curly hair is the ancestral trait while straight hair evolved much later.

The genes for thick straight hair in Asians

Most people of East Asian descent have thick, straight hair. This corresponds with a SNP (rs3827760) in the EDAR gene which is involved in hair follicle development. The ancestral allele of this SNP is the A-allele. The G-allele is the newly derived allele that leads to the thick, straight hair. In certain parts of Asia, almost all people have the G-allele (see Fig.1B). People with the GG genotype at this SNP have thicker hair compared to those with the AA genotype due to the modification of a single amino acid in the protein. Those with the AG genotype have hair slightly thinner than those with GG, but still thick when compared to Europeans and Africans (likely AA) [2, 3].

Figure 1. The allele distribution of SNP rs11803731 of the TCHH gene (A) and SNP rs3827760 of the EDAR gene (B)

Another gene that contributes to the thick-hair trait in the Asian population is FGFR2. The T-allele of the SNP rs4752566 in the FGFR2 gene increases the expression level of FGFR and leads to an increased hair follicle growth and thicker hair [4].

Does the thickness of the hair make it difficult to curl? Or do genes make the hair thick AND straight? We will have to wait for more research results to answer these questions.

Does straight hair provide any selection advantage?

Straight hair seems to be a new trait in human evolution. It is associated with the newly emerged alleles in both the TCHH gene and EDAR gene. The distributions of these straight hair related alleles (see figure) support the hypothesis that our human ancestors had curly hair; the straight hair found in East Asians and Caucasians likely developed independently.

So what is so good about straight hair? It must provide some biological advantage for hair health. It has been observed that oily hair is typically straight and that individuals with both curly and oily hair are extremely rare. One theory is that the oils secreted into the hair shaft by the sebaceous glands can travel down the shaft of straight hair more easily.

Studies indicate that the G-allele of the SNP rs3827760 in the EDAR gene responsible for the East Asian hair type arose during the past 65,000 years, when early humans were migrating ‘Out of Africa’ into Europe, then Asia. During the Ice Age, thicker hair might have been advantageous to protect against the cold. The EDAR gene is also involved in skin gland function. A study found that increased EDAR activity can cause enlarged skin glands (sebaceous glands found in abundance on face and scalp) and enlarged eyelid glands (meibomian gland) among other effects. These enhanced gland functions, may result in increased skin lubrication/protection and decreased evaporation which might have been positively selected during the cold, dry Ice Age [5].

If you're a curly-haired individual or just an envious straight-haired person, you may wonder what exactly causes some strands to swirl whilst others simply stay smooth. It turns out there are two theories, according to a 2018 study published in the Journal of Experimental Biology.

The first hypothesis states that curls can be explained by a greater number of hair cells on the convex side of the hair follicle — that is, the outside edge of the curl — and fewer on the concave side — the inside edge. That relatively smaller number of cells on the inside creates a shorter edge, which pulls the follicle inwards, creating the curl. 

The other theory suggests that differences between the cell lengths on the convex and concave sides of the strand of hair could explain curls. Similar to the first theory, the relative size difference between elongated cells on the outside and compact cells on the inside creates the curl. 

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Curly hair is thought to be better at keeping mammals warmer than just straight hair alone. In fact, straight hairs interwoven with curls creates the last line of defense against heat loss.

"The typical mammalian coat structure is that of a forest with shrubs," said study lead researcher Duane Harland, a senior scientist at AgResearch, one of New Zealand’s largest corporate government research institutes. Straight hairs stick up to “create a space near the skin" and "the finer curly hairs fill the space and trap air," Harland told Live Science.

Harland's study, however, only addressed curls in Merino sheep. It's hard to know for sure if insulation is what also drove some humans to develop curly locks. "The simple answer is that nobody knows specifically about human hair,” Harland said. "Our social aspect and ability to develop technology that replaces functions originally covered by biology, such as hats, make it hard to pin down." 

That said, we can still learn a lot about the origins and biology of our own follicular features from our more furry friends, because if you go back far enough, our hair evolved from the same genetic origins. "Mammalian hair is ancient,” Harland said. It probably developed before the dinosaurs, Harland noted. (An analysis of 29 fossil skulls of archaic mammal relatives, the therapsids, suggests that these mammalian predecessors had fur, according to a 2016 study in the journal Scientific Reports (opens in new tab).)

To test the two theories, Harland and his colleagues used sophisticated microscopy techniques to zoom in on sheep wool fibers and measure the differences between the number and size of the cells on the inside of the curve and those on the outside. Almost immediately, the team found evidence dispelling the idea that the number of cells creates the curl.

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"We found evidence contradicting the theory that curvature results from there being more cells on the side of the fiber closest to the outside," he said. That’s because, in all cases, he found the cells on the outside of the curve were longer, "which supports the theory that curvature is underpinned by differences in cell type length," Harland said.

Alas, it's not that simple. The story, as is so often the case with science, doesn't end here. "We have not got completely to the bottom of curly hair," Harland said. His study looked only at individual cross sections of sheep hair fibers under a microscope. That snapshot of hair could result from curvature, but it could also have some twisting forces, confusing the results, he said. 

Even if those cross sections were good representations of hair strands as a whole, it doesn't mean the other theory — the one suggesting that cell numbers are the cause — is wrong. It could be that different hair on different animals is curly for different reasons. 

"It would be pure hubris to declare that the situation is universal," Harland noted.

"So, there is scope for further discovery," he said. "It'd be great if other scientists out there were busy replicating and building on our study. Perhaps, they will find we got something wrong, hopefully only small things if anything, but that's science."

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