Life as a Planetary Response

Plate tectonic theory revealed how the dynamics of Earth's interior are manifest at the surface, and sanctions mechanistic hypotheses for how the surface affects the deep interior. Molecular phylogenies show us the fundamental biochemical relatedness of all life, and that Earth's earliest life did not rely on sunlight, which means it was not confined to the surface. The discovery of exoplanets helps us realize that Earth may be one of innumerable examples of a populated planet, especially as we relax the notion of life bound to planetary surfaces. These revolutions frame biogeoscience research as substantively planetary. So, what does life do? Why would a planet support life? Setting aside phototrophy, which is highly productive on Earth but dependent on the Sun, lithotrophic life depends on energy sources supplied by the planet on which it resides. Those energy sources emerge as the approach to thermodynamic equilibrium is stalled by sluggish kinetics, which is inevitable on planets undergoing cooling to conditions where liquid water or ice form at the surface. Life emerges in response to the cooling trajectories of planets as a means to release trapped chemical energy that will remain confined without catalysis. As a complex adaptive system, with enormous metabolic flexibility derived from small compositional shifts in biomolecules, life is poised to cover every catalytic option that geologic processes present. At the same time, the biochemistry we have is one the Earth allows, and geobiochemical outcomes reflect the intermingling trajectories set by bulk composition, heat generation, and the dependence of relative rates of organic and inorganic reactions. Options for metabolic strategies expand in response to technological advances that provide novel energy sources and evolutionary stresses. An emerging geobiochemical focus for the next century may be how global microbial evolution is actively redirected by human activities and political decisions.