Microbial Evolution and the Rise of Oxygen: The Roles of Contingency and Context in Shaping the Biosphere through Time

Abstract

We are shaped by our environment, but we then shape it in turn. This interplay between life and the Earth, and how these interactions have shaped both parties through time, is the heart of the discipline of geobiology. My research is fundamentally motivated by a desire to understand how life and the Earth have changed together through time to reach the state that they’re at today, and to understand from this history how the coevolution of planet and life may be different on other worlds. The focus of my work has been on how the structure and productivity of the biosphere across time and space has been shaped by the metabolic opportunities provided by the environment—as a result of both biotic and abiotic factors—and the metabolic pathways that are available to life, as a result of evolutionary contingency in the evolution of pathways and their inheritance and horizontal transfer. The biosphere on Earth today is incredibly productive due to the coupled dominant metabolisms of oxygenic photosynthesis and aerobic respiration, yet these can’t always be assumed to have been present—considering life more broadly, for instance in the context of the early Earth and other planets, we have to grapple with how evolutionary contingency and planetary environments interact to constrain the metabolic opportunities and rates of productivity available to the biosphere. In this dissertation, I broadly consider how the size and structure of Earth’s biosphere has changed through time as surface environments evolve and metabolic innovations accumulate. These investigations make use of information gleaned from the rock record of the early Earth, as well as the biological record of the history of life as preserved in the genomes, biochemistry, and ecology of extant organisms. These coupled records provide opportunities for constraining estimates of the opportunities for life throughout Earth history and elsewhere in the universe.