Comparative evidence for the independent evolution of hair and sweat gland traits in primates

Abstract

Humans differ in many respects from other primates, but perhaps no derived human feature is more striking than our naked skin. Long purported to be adaptive, humans’ unique external appearance is characterized by changes in both the patterning of hair follicles and eccrine sweat glands, producing decreased hair cover and increased sweat gland density. Despite the conspicuousness of these features and their potential evolutionary importance, there is a lack of clarity regarding how they evolved within the primate lineage. We thus collected and quantified the density of hair follicles and eccrine sweat glands from five regions of the skin in three species of primates: macaque, chimpanzee and human. Although human hair cover is greatly attenuated relative to that of our close relatives, we find that humans have a chimpanzee-like hair density that is significantly lower than that of macaques. In contrast, eccrine gland density is on average 10-fold higher in humans compared to chimpanzees and macaques, whose density is strikingly similar. Our findings suggest that a decrease in hair density in the ancestors of humans and apes was followed by an increase in eccrine gland density and a reduction in fur cover in humans. This work answers long-standing questions about the traits that make human skin unique and substantiates a model in which the evolution of expanded eccrine gland density was exclusive to the human lineage.

Ultra-endurance athletic performance suggests that energetics drive human morphological thermal adaptation

“…In contrast, humans do not have to couple breathing with running (Bramble and Carrier 1983) and an increased number and density of eccrine sweat glands facilitates rapid dissipation of heat by evaporation. Even in comparison with our closest relatives, the eccrine sweat density of humans is approximately 10-fold higher than in chimpanzees and macaques (Kamberov et al 2018). These differences, in tandem with reduced body hair promoting convective heat loss and an upright gait reducing water stress in hot environments, allow human endurance running in hot conditions (Lieberman and Bramble 2007;Wheeler 1992).…”

Water turnover among human populations: Effects of environment and lifestyle

“…In many primates and other mammals, water is also used for thermoregulation via sweating, which is one of the most effective mechanisms by which the body can cool itself (IOM, ). Humans have evolved a far greater capacity to sweat than other primates, with eccrine sweat gland densities that are 10 times higher than those of chimpanzees and macaques (Carrier, ; Kamberov et al, ; Lieberman, ). The remarkable human capacity to sweat is thought to reflect selection for thermoregulation in hot climates and when engaging in high levels of physical activity (Carrier, ; IOM, ; Kamberov et al, ; Lieberman, ; Popkin, D'Anci, & Rosenberg, ).…”

“…Humans have evolved a far greater capacity to sweat than other primates, with eccrine sweat gland densities that are 10 times higher than those of chimpanzees and macaques (Carrier, ; Kamberov et al, ; Lieberman, ). The remarkable human capacity to sweat is thought to reflect selection for thermoregulation in hot climates and when engaging in high levels of physical activity (Carrier, ; IOM, ; Kamberov et al, ; Lieberman, ; Popkin, D'Anci, & Rosenberg, ). When sweat is secreted onto the skin and is vaporized it transfers heat from the body to the environment through evaporative cooling (Jéquier & Constant, ; Lieberman, ; Sawka, ).…”

An Engrailed1 enhancer underlies human thermoregulatory evolution

“…Because fur reduces airflow over the skin (Al-Ramamneh et al, 2011), human furlessness is thought to be an adaptation that enhances sweat evaporation (Carrier, 1984;Lieberman, 2015;Montagna, 1963Montagna, , 1972. A second critical adaptation that also underlies humans' exceptional sweating capabilities is a high density of eccrine sweat glands in the skin (Folk and Semken, 1991;Kamberov et al, 2018;Montagna, 1963Montagna, , 1972. Eccrine glands secrete the water humans vaporize for evaporative cooling (Kuno, 1956) and their importance is underscored by the risk of hyperthermia in individuals with reduced numbers of these organs (Wright et al, 1993).…”

“…Eccrine glands secrete the water humans vaporize for evaporative cooling (Kuno, 1956) and their importance is underscored by the risk of hyperthermia in individuals with reduced numbers of these organs (Wright et al, 1993). Eccrine glands are the predominant appendages of human skin, with densities exceeding 200 glands/cm 2 in regions such as the face (Kamberov et al, 2018). In comparison, eccrine gland densities of macaques and chimpanzees are on average ten times lower than that of humans (Kamberov et al, 2018).…”