For some scholars in the humanities and social sciences, old age is a period of abiding productivity. Upon reaching his retirement, Robert N. Bellah, the leading sociologist of religion of the last decades and one of the United States' most influential public intellectuals, trusted this would be the case, so he outlined the project of a global history of religion from its beginnings until the present. In 2011, Bellah published Religion in Human Evolution, which spans from the rituals of hunters and gatherers to the cults and myths of archaic states to the "axial age." This concept was advanced by the philosopher Karl Jaspers to capture the widely held assumption that the great religious and philosophical traditions that the world draws on to this day have their roots in innovations that took place around the middle of the first millennium BCE in the Jewish, Greek, Chinese and Indian civilizations.