Social theory developed in the 19th century, which had fundamental influence on sociological and anthropological thinking up until the First World War (see structural functionalism). Evolutionism postulates that societies develop from simpler to more complex organizational forms, a simple formulation, which hardly anyone would disagree with, even today. In the 19th century, however, one often also imagined that development proceeded by necessity toward morally "superior" and more "civilized" conditions (a view that was widely abandoned after the First World War). In more modern variants, evolutionism is often tied to theories of modernization and scale, ecological anthropology, and research on development and underdevelopment. Lévi-Strauss has shown that movements from "primitive" to "modern" thought not only imples increasing complexity, but a change in the type of complexity (see bricoleur).
The impact of 19th century evolutionism on modern anthropology lies, first and foremost, in the grand comparative projects it attempted, which have inspired a general ideal of comparative studies in anthropology. Secondly, the ideal of cross-cultural comparison implied a postulate of the racial homogeneity of all humankind - i.e. to be comparable, all people, regardless of color, culture or cuisine, were basically of the same kind, a position that was championed by all the major 19th century evolutionists. This postulate, of "the psychic unity of mankind", has been fundamental to later anthropology, which has (like the Victorian evolutionists) consistently resisted all attempts to reduce culture to biology.
In the post-Second World War years, the American anthropologist Julian Steward developed a neo-evolutionist theory, which influenced his many students, and was an inspiration for American ecological anthropology (cultural ecology), subtantivism in economic anthropology, and peasant studies throughout the the next decades. Steward built on his teacher Robert H. Lowie's theory of multilinear evolution (which posited that cultures could evolve along several parallel, though different, lines), and developed a true materialist evolutionary theory, in which a society's evolutionary stage could be measured quantitatively, as the amount of energy, per capita, it could extract from the environment. Later, Steward's student, Morton Fried, would distinguish between pristine (self-originating) evolution and secondary evolution (imposed from without), and Steward and Fried's student, Marshall Sahlins, would distinguish general and specific evolution; i.e. a universal, long-term trend towards greater and greater energy production, and a local trend, towards more and more efficient utilization of a given resource base (this is often referred to as adaptation).