A growing body of empirical evidence shows that experiences of nature provide people with psychological benefits (improved cognitive function and mental health). Although our knowledge of the proximate causes of such positive psychological responses of humans to nature has improved, their ultimate (evolutionary) drivers remain poorly understood. Here, we propose a new evolutionary psychological hypothesis – the greenery hypothesis – that integrates recent findings in evolutionary psychiatry with many experimental results that cannot be explained by several hypotheses/theories previously developed. Our proposed hypothesis states that humans have adapted to periodic severe environmental changes (severe drought and re-watering cycles) by establishing both negative and positive psychological responses to the absence or presence of greenery within the landscape as cues to optimize their own behavioral activity. We partially tested several predictions generated from the greenery hypothesis using a multi-country questionnaire survey of approximately 7,000 people, and obtained some supporting evidence. The greenery hypothesis has the potential to yield several insights into the fundamental understanding of the psychological responses of humans to the natural world and it could have important implications for a wide range of related fields, psychiatry, urban planning, biodiversity conservation, and animal welfare.