In the 1960s, ecological anthropology first appeared as a response to cultural ecology, a sub-field of anthropology led by Julian Steward. Steward focused on studying different modes of subsistence as methods of energy transfer and then analyzed how they determine other aspects of culture. Culture became the unit of analysis. The first ecological anthropologists explored the idea that humans as ecological populations should be the unit of analysis, and culture became the means by which that population alters and adapts to the environment. It was characterised by systems theory, functionalism and negative feedback analysis.