Epidermal Keratinocyte Sensing and Processing of Environmental Information Together with the Brain’s Simulation and Prediction Abilities Helped to Enable <i>Homo sapiens’</i> Evolutionary Success

Abstract

Among terrestrial mammals, Homo sapiens has evolved a very specific anatomical feature—very little body hair—thus, the skin surface is exposed directly to the environment. We and others have demonstrated that skin epithelial cells, called keratinocytes, express not only functional sensory systems for a variety of environmental responses, but also a series of neurotransmitter receptors that play key roles in information processing in the brain. Furthermore, the brain cortex is particularly large in Homo sapiens , which has a higher ratio of brain to whole-body weight than any other mammalian species. Here we propose that the evolutionary success and global spread of Homo sapiens are due at least in part to the existence and interaction of these two systems; i.e. the epidermis and brain cortex. First, we discuss the role of the epidermis as a sophisticated organ with multiple sensory inputs and information-processing capabilities, and then we consider the putative requirement for a large brain to carry out simulations and predictions based on input from multiple epidermal systems. We also present some other examples where a functionally sophisticated epidermis is associated with a large brain size. Finally, we discuss possible reasons why Homo sapiens has emerged as the sole surviving human subspecies.