Major Evolutionary Transitions as Phase Transitions in the History of Life

Evolutionary biologists have identified a series of major evolutionary transitions (METs) between the origin of life and the spread of human language, including the origin of eukaryotes with endosymbiotic organelles, multicellularity with cellular differentiation and social societies. In the canonical view, each of these transitions is associated with repackaging of information so that individuals at one level become components of a larger entity. Considerable research has addressed how the evolution of these new ‘evolutionary individuals’ has changed the locus of selection; less well appreciated is that they change the nature of the variation presented to selection as well. This and similar packaging associated with the evolution of metazoan gene regulatory networks demonstrates that there is substantial, temporally asymmetric changes to the nature of evolutionary variation over time. Moreover, any of these METs represent another sort of phase transition, associated with biologically-mediated changes in the environment. Indeed the success of the METs may have been largely mediated by these ecosystem engineering aspects.  The advent of oxygenic photosynthesis before 2.4 billion years ago led to the ‘Great Oxygenation Event’, changing the redox state of the atmosphere, and arguably facilitating the subsequent rise of eukaryotes about 2.0-1.8 Ga. Increased oxygen levels increased global ‘carrying capacity’, freeing microbes from a ‘Malthusian trap’ imposed by the redox state of the Earth.  Similarly, a second expansion of oxygen levels about 600 million years ago, during the early diversification of animals, may have been mediated by organisms as well (although the evidence is less overwhelming). Thus at least some of the METs are associated both with asymmetric changes in the nature of evolutionary variation, and phase changes in the environment, which greatly expand the Earth’s potential biomass, or carrying capacity.