“Historically, evolution has been depicted as linear and branching, like a tree, or cross-linked, like a web. Although those metaphors certainly capture many evolutionary processes, others are much more sinuous—even circular. Again and again, life and environment alter each other through feedback loops. Through their behaviors and byproducts, living creatures make lasting changes to their surroundings that partly determine the fate of their descendants and of other species. Microbes can seed clouds. Forests on one continent can make it rain on another. Breath can sway a planet.”