Broadly speaking, natural systems thinking is defined as a way of thinking that endeavors to conceptualize the functioning of living organisms as dependent on predictable forces at work within and around them. Systems concepts help to bring the function of those variables and life forces into better view. Psychiatrist Murray Bowen over the course of several years and a major research project at the National Institute of Mental Health (NIMH) developed a theory of the family as a system. He considered his theory a natural systems theory, "... designed to fit precisely with the principles of evolution and the human as an evolutionary being" The human family system, a network of relationships, linking each family member to every other, responds dynamically to its environment and the conditions to which all members must adapt. Each family member's behavior influences that of every other to some degree. Although ideas of a general system theory and cybernetics were developing at the same time, Bowen reported that he knew nothing about those ideas at the time he developed his thinking. He believed that his systems orientation derived from his study of systems in nature and not from the "systems thinking" of the period. An emerging systems paradigm in biology and evolutionary thinking focuses on collective behavior and appears consistent in principle with Bowen's thinking about the family. The collective behavior of the family unit cannot be understood by looking at the characteristics of the individuals who comprise it. The human family presents a highly integrated, interactive system of adaptation. Its roots extend along the path of hominid evolution and share common elements with other evolved collectivities. The complex development of the human brain appears to have co-evolved with the interactional processes of the family. The Bowen theory provides the potential for an integrative theory of human behavior reaching beyond the focus on the physiology and psychology of the individual to the operation and influence of the family system. Such an integrative theory can offer broader explanatory and investigative pathways for understanding physical, emotional, and social problems as they emerge in human activity.