This review summarizes and assesses a series of papers presented at the Geological Society of America Annual Meeting in 2011 on the changing composition of Earth's early atmosphere. This is a developing field, with differing views, due largely to the facts of an incomplete rock record and negligible preservation of the gaseous components of the atmosphere. Nevertheless, there are constraints, available through geological proxies in the form of chemical sedimentary rocks that reflect the composition of the hydrosphere, and because the two are directly linked, the atmosphere. A review of the geological constraints on atmospheric conditions is presented for early Earth, from its formation at 4.56 Ga up to 1.8 Ga, followed by a developing model that links changing atmosphere/hydrosphere conditions and biosphere evolution to changes in planetary tectonics, including the evolving supercontinent cycle.

Discoveries in geobiology have dramatically shaped our understanding of the nature, distribution, and evolutionary potential of terrestrial life, paving the way for new exploration strategies to search for life elsewhere in the Solar System. Genomic studies, applied over a broad range of geological environments, have revealed that the vast proportion of species on Earth are microbial. Studies of the fossil record indicate that this has been the case for >75% of our planet's history. Microbial life has been shown to occupy a stunning array of environmental extremes, seemingly only limited by the distribution of liquid water and its chemical activity, nutrient availability, suitable energy sources, radiation, etc. Advances in geomicrobiology have revealed important contributions of microbial processes to many global biogeochemical cycles, and in the evolution of Earth's atmospheric and surface composition. The discovery of a subsurface biosphere, fueled by inorganic chemical energy and able to tolerate extremes in temperature and salinity, has been especially important in opening up new horizons for the astrobiological exploration of Mars, as well as icy satellites of the outer Solar System. Although the environment of life's origin remains uncertain, molecular studies suggest that the last common ancestor of life probably lived in hydrothermal environments where it utilized simple compounds of carbon, hydrogen, and sulfur as sources of chemical energy. This general view is consistent with what we know about late Hadean to early Archean environments on the Earth, as well as model-based interpretations of late, giant impacts that could have exterminated early mesophilic (and possibly photosynthetic) surface life forms, leaving behind only deep subsurface chemotrophic thermophilic microbial communities to re-populate the biosphere. These and related discoveries have contributed extensively to the view that life could be much more broadly distributed, within the Solar System and beyond, than once thought. We now believe it possible that life may have become established in surface environments on Mars during the first half billion years of the planet's history, when liquid water was widespread there. Furthermore, a subsurface hydrosphere on Mars (suggested by both models and geomorphic evidence) may have provided a continuously habitable zone for life over most of Martian history and could still support an active, deep biosphere on Mars today. Exploration of the outer Solar System supports the presence of saline brines (perhaps oceans) beneath the icy crusts of Europa, Callisto, and possibly Ganymede, along with plausible energy sources for life based on chemical disequilibria between oxidized and reduced compounds. It also appears that interior zones of liquid water may also exist on Enceladus, a moon of Saturn, while hydrocarbon oceans of liquid methane discovered on Titan may provide alternative solvents for novel life forms completely unlike anything found on Earth. Ongoing efforts to systematically explore potentially habitable environments elsewhere in our Solar System have helped catalyze the development of astrobiology, an emerging interdisciplinary science that seeks to understand the origin, evolution, distribution, and future of life in the cosmos. Geobiology, which studies interactions of biological and physical-chemical systems and how they have evolved over the history of Earth, is a central focus of astrobiology, providing fertile ground for the growth of conceptual models and new technological tools needed to implement the search for extraterrestrial life elsewhere in the Solar System.