Steward’s concept of cultural evolution rested on two key concepts: “First, it postulates that genuine parallels of form and function develop in historically independent sequences or cultural traditions. Second, it explains these parallels by the independent operation of identical causality in each case” (Steward 1973b:14). Thus, understanding cultural evolution involved discovering “parallels and similarities which recur cross-culturally” and proposing “lawlike” statements about the causes of such parallels .
Steward’s approach was to find and explain similarities between societies without assuming that all societies passed through identical stages of development. Multilinear evolution, he wrote, “deals only with those limited parallels of form, function, and sequence which have empirical validity. What is lost in universality will be gained in concreteness and specificity” (1973b:19). For example, Steward compared the prehistoric patterns of developments in five independent centers of ancient civilization—Mesopotamia, Egypt, China, Mesoamerica, and the Andes. These centers shared “parallels of form, function, and sequence” based on having developed in arid and semiarid environments in which the economic basis was irrigation and floodwater agriculture. Agriculture created food surpluses that allowed for non-subsistence activities and population growth. When population growth reached the limits of agricultural productivity, competition over natural resources intensified, warfare ensued, and political leadership shifted from temple priest to warrior king. As some communities prospered and others suffered, empires were forged that instituted strong political controls over vast regions.
Steward traced the evolutionary similarities in the five ancient civilizations. Although the chronology of events differed, Steward argued that there were striking parallels in the pattern of cultural evolution, not because there were universal stages of cultural development or due to the diffusion of civilization between regions, but because these five cultural traditions emerged in similar arid and semiarid environments where agriculture had been able to flourish. This development of agrarian civilizations in arid and semiarid environments was one “line” of the multilinear evolution that Steward proposed. Thus, Steward’s multilinear evolution intentionally avoided sweeping statements about culture in general, applying more limited models to specific sets of cultures.