The recent theory that life and climate interact and that they have mutually altered each other over geologic history. The term was coined by the American biologists Paul R. Ehrlich and Peter H. Raven to describe the process whereby two or more species depend on the interactions between them. The coevolution of life and climate during the past 4,000,000,000 years of the Earth's history is an expression for the complex mixture of forces causing climatic change. The theory suggests that changes in social systems are best understood as mutual selection among components rather than a linear cause-and-effect sequence. For example, it has been argued that the origins of agriculture can best be understood by exploring the evolutionary forces affecting the development of domestication systems. Viewed this way, domestication is not seen as an evolutionary stage, but rather as a process and is the result of coevolutionary interactions between humans and plants.