Major depressive disorder is not a single, uniform condition. Different causes of depression produce distinct symptom patterns, creating discrete subtypes. The human capacity for mood variation evolved because it once offered adaptive benefits, but rapid cultural change has created a mismatch between ancestral and modern environments, making some traits that used to be beneficial maladaptive in contemporary environments. An evolutionary framework suggests that systemic inflammation, chronic stress, and gut dysbiosis can intensify symptoms and prolong adaptive mood states into maladaptive depression episodes.This book demonstrates that lifestyle interventions can be effective in both preventing and treating depression. After critically evaluating current treatments, Markus J. Rantala and Severi Luoto argue for individualized treatment approaches. They propose a new hypothesis for depression founded in evolutionary science. Written for researchers, clinicians, and informed readers, the book challenges how depression is currently diagnosed and treated.