The evolutionary theory is applied to explain a multitude of natural and social phenomena. In medicine, evolutionary biology and psychology enables us to take perspectives beyond the biomedical paradigm of disease. The evolutionary pathophysiology looks for the ultimate cause of disease rather than the proximate causes. The ultimate cause of disease lies in the evolved psychological mechanisms (EPMs). This recognition fundamentally alters the traditional view of pathogenesis that a disease is the result of alien pathogens invading our bodies. Especially in psychiatry, the insight that the pathologic and normal mind have a common basis and that discriminating between them solely by means of natural science is rather impossible, this makes us rethink the validity of current reductionistic approaches to psychiatric nosology. In this article (Part I), the authors introduce evolutionary biology and psychology. Detailed application of the evolutionary perspective to psychiatric disorders will be discussed in the continuing article (Part II).