How human lives are ruled by symbolism. Metaphors, similes and parables hold enormous power over us. Throughout history, we have lived for them, died for them and even now we dedicate our lives to them. Moreover, the early use of symbols enabled us to forge bonds and societal rules as our species grew increasingly complex and competitive. Robert Sapolsky, a professor of biology and neurology at Stanford University, examines how human brains evolved and explains how — despite our use of metaphors and other symbols setting us apart in the animal kingdom — we may actually be dealing with a double-edged sword.