Gaia Theory
Gaia theory, or geophysiology, is arguably one of the most powerful scientific theories since Darwin's theory of natural selection, and promises to be one of the most important ideas of the 21st century. It is the scientific study of Earth's planetary-scale metabolism, a self-regulating physiological system conferring homeostasis (a system-level property that regulates its internal environment to maintain stable, steady-state conditions). Specifically, the system automatically regulates the temperature and chemical composition of Earth's atmosphere and oceans. Gaia theory has evolved during the last 4 decades, and is taught in Britain and parts of Europe as a "mainstream" science.
Gaia is the most complex system that humans have ever attempted to study and understand with science. Similar to your own metabolism, physiology and homeostasis - but vastly larger - it is a hugely complex system composed of strongly linked (integrated) living and non-living systems, all subject to the principles of systems sciences. Therefore, understanding Gaia requires understanding the basic principles of systems sciences and biology, which are our other two foci.