The evolution of Gaia(s).

The Earth's biosphere, or 'Gaia', has increased its own persistence and flourishing through intense global cycling of essential elements, and net stabilization of atmospheric composition and climate. A key question is how has it acquired these remarkable properties? Did it involve some form of natural selection among parts of the biosphere? (And if so; which parts?) Or could these properties arise another way? A physical explanation is that in a complex system containing a source of variation and some 'memory' of past states, the most probable trajectory is towards a state of greater size, diversity and stability. Models show this state tends to be reached through a series of progressively more stable configurations, separated by system reorganizations, which become more difficult and thus less frequent over time. Such Gaia-level evolution can also be interpreted in terms of selection among ecosystems and nutrient cycles based on how well they persist and/or spread. Thus, what seem very different physical and biological paths to explaining Gaia are converging. The resulting theory makes testable predictions about how Earth's biosphere evolved that are consistent with available evidence. It also makes predictions of how biospheres in general evolve, which will become testable if/when exo-biospheres are detected.This article is part of the discussion meeting issue 'Chance and purpose in the evolution of biospheres'.