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.

中文翻译：

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灵长类动物毛发和汗腺特征独立进化的比较证据。

人类在许多方面与其他灵长类动物不同，但也许没有什么人类特征比我们裸露的皮肤更引人注目。长期以来，人们一直认为人类具有适应性，其独特的外观特征是毛囊和小汗腺图案的变化，导致毛发覆盖减少和汗腺密度增加。尽管这些特征非常显着并且具有潜在的进化重要性，但它们在灵长类谱系中如何进化尚不清楚。因此，我们收集并量化了三种灵长类动物（猕猴、黑猩猩和人类）皮肤五个区域的毛囊和小汗腺的密度。尽管人类的毛发覆盖度相对于我们的近亲来说大大减弱，但我们发现人类的毛发密度与黑猩猩相似，但明显低于猕猴。相比之下，人类的小汗腺密度平均比黑猩猩和猕猴高 10 倍，而黑猩猩和猕猴的密度惊人地相似。我们的研究结果表明，人类和猿类祖先的毛发密度减少后，人类的小汗腺密度增加，皮毛覆盖减少。这项工作回答了有关人类皮肤独特特征的长期存在的问题，并证实了一个模型，其中扩大的小汗腺密度的进化是人类谱系独有的。