A straight line may be the shortest distance between two points, but it isn’t necessarily the fastest or easiest path to follow. That’s particularly true when terrain is not level, and now American and British researchers have developed a mathematical model showing that a zigzag course provides the most efficient way for humans to go up or down steep slopes. Trails evolve, among other reasons, because of physical differences in people and the differences in the biomechanics and energy cost of ascending and descending a slope.