Chapter 6 195 ago when humans started to evolve into different races, tightly coiled hair was prevalent in the hotter, equatorial climate. The theory further suggests that the more tightly curled hair provided more efficient cooling by providing a layer of trapped air between the scalp and the environment, thereby facilitating more efficient sweat evaporation than that of straight hair. Genomic research over the previous decade resulted in the discovery of single nucleotide polymorphisms in the EDAR and FGFR2 hair formation genes, which are related to thickness of Asian hair.8,9 A similar finding related to straight European hair identifies the TCHH gene, which is expressed in the inner root sheath and shows a high frequency in Northern Europeans.10,11 To date, no such work related to curly hair types has been reported. Biology While the debate over the evolutionary selection of curly hair continues, science has begun to unravel its biological differentiation. Early on, the relationship of follicular shape and geometry of the emerging hair fiber was suggested.12 It is now thought that the configuration of hair is based on the geometry of the follicle from which it grows and the reported asymmetry of various cell differentiation programs within it.13,14 Curved follicles continue to produce curly hair irrespective of their in vitro dermal environment, thus demonstrating the control of the bulb over the process. Thibaut et al. indicated that there is an asymmetry of cell differentiation programs in the proliferative compartment of the bulb and both the inner and outer root sheaths (IRS and ORS) irrespective of the ethnic origin of the curly hair (Figure 1). Fluorescence microscopy of antibody stained sections indicated that differentiation started early on the concave side based on Ki-67 and expression of K14 and transglutamase-1 in the IRS and ORS. However, other theories for the biological control of the growth of curly hair exist. Swift suggests that current theories cannot explain both curl and ellipticity, and thus their strong correlation as demonstrated later in this chapter. He proposes that one consider an asymmetric pressure exerted by