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.

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 longstanding 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.

Background: Ambient particulate matter (PM) represents an environmental threat to which millions of people worldwide are exposed. Adverse effects of PM on human health have been reported, but its effect on skin aging has not been widely studied. Autophagy is a regulatory "self-eating" process and is one of the survival mechanisms for cells during extrinsic and intrinsic stress. Objectives: In this study, we aimed to examine the relationship between PM exposure, autophagy and skin aging. Methods: We compared the levels of autophagy in human skin fibroblasts before and after PM exposure. We also measured the changes in matrix metalloproteinase (MMP-1), procollagen I, interleukin (IL)-6, IL-8, and TGF-beta levels before and after PM exposure. Results: Transmission electron microscopy analyses of revealed the presence of PM within the cytoplasm and autophagosomes of the PM exposed fibroblasts. Western blot analysis showed a statistically significant increase in autophagic flux in fibroblasts after PM exposure. An increase in the protein expressions of MMP-1, IL-6, IL-8, and a decrease in procollagen-I and TGF-beta were also noted after PM exposure. Conclusion: Our findings suggest that the autophagy pathway is activated with PM exposure. Fibroblasts exposed to PM showed signs of aging which indicate that air pollution might influence skin aging.