Photodamage spares the distal digits owing to primate-like hand flexion

Sun-exposed areas of the body - including the face, neck, and extensor forearms - are chronically exposed to UV light and display signs of photoaging. The skin distal to the proximal interphalangeal (PIP) joint is, however, typically spared owing to the natural rest position of the hand. The inward curvature of the fingers, termed the finger flexion cascade, orients the skin distal to the PIP joint toward the ground as in knuckle-walking primates. The near constant protection of this area of skin from sun may reflect our primate ancestry, the daily activities of which included swinging through trees and knuckle-walking. Primates have elongated, inward curved fingers that were once advantageous for gripping branches during arboreal locomotion. Although natural selection has favored shorter, straighter fingers, the human hand continues to assume a natural shape of flexion at rest. The UV untouched, healthy skin at the ends of the fingers is a reminder of our primate heritage. This finding may provide a colorful, memorable means to reinforce to patients how photoaging is a manifestation of sun exposure rather than age.