To our microbiome, the human body must seem like an entire planet, full of different ecosystems. This is especially true for those that live on our skin. At the microscopic scale, the hairy, moist surface of your armpits is as different from the smooth, dry skin of your forearms as a rainforest is to a desert.

In a thorough survey of our skin microbiome, Elizabeth Grice identified species from at least 205 different genera. Your forearm has the richest community with an average of 44 species, while your nostril, ears and inguinal crease (between leg and groin) are the most stable habitats. Grice also found at bacteria from a specific body part have more in common than those from a specific person. Your butt microbes have more in common with mine than they do with your elbow microbes.