In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets—Biosignatures—Habitability markers—Photosynthesis—Planetary surfaces—Atmospheres—Spectroscopy—Cryptic biospheres—False positives. Astrobiology 18, 663–708.

Coexisting methane and carbon dioxide in atmospheres of habitable planets represent a disequilibrium biosignature.

The emergence and expansion of complex eukaryotic life on Earth is linked at a basic level to the secular evolution of surface oxygen levels. However, the role that planetary redox evolution has played in controlling the timing of metazoan (animal) emergence and diversification, if any, has been intensely debated. Discussion has gravitated toward threshold levels of environmental free oxygen (O 2 ) necessary for early evolving animals to survive under controlled conditions. However, defining such thresholds in practice is not straightforward, and environmental O 2 levels can potentially constrain animal life in ways distinct from threshold O 2 tolerance. Herein, we quantitatively explore one aspect of the evolutionary coupling between animal life and Earth's oxygen cycle-the influence of spatial and temporal variability in surface ocean O 2 levels on the ecology of early metazoan organisms. Through the application of a series of quantitative biogeochemical models, we find that large spatiotemporal variations in surface ocean O 2 levels and pervasive benthic anoxia are expected in a world with much lower atmospheric pO 2 than at present, resulting in severe ecological constraints and a challenging evolutionary landscape for early metazoan life. We argue that these effects, when considered in the light of synergistic interactions with other environmental parameters and variable O 2 demand throughout an organism's life history, would have resulted in long-term evolutionary and ecological inhibition of animal life on Earth for much of Middle Proterozoic time (∼1.8-0.8 billion years ago).oxygen | animals | evolution | Proterozoic A long-standing and pervasive view is that there have been intimate mechanistic links between the evolution of complex life on Earth-in other words, the emergence and ecological expansion of eukaryotic cells and their aggregation into multicellular organisms-and the secular evolution of ocean−atmosphere oxygen levels (1). Molecular oxygen (O 2 ) is by far the most energetic of the abundant terminal oxidants used in biological metabolism (e.g., ref.2). When this energetic capacity is harnessed by mitochondria in eukaryotic cells, the energy flux supported by a given genome size increases by a factor of ∼8,000 (3), potentially paving the way for increased complexity at the cellular level (but see ref. 4). Oxygen is also a crucial component of enzymatic pathways leading to the synthesis of regulatory membrane lipids (5) and structural proteins (6) in eukaryotic organisms and provides a powerful shield against solar UV radiation at Earth's surface through stratospheric ozone production. Furthermore, O 2 is the only respiratory electron acceptor that can meet the metabolic demands required for attaining the large sizes and active lifestyles characteristic of metazoan life (e.g., ref. 7). There is thus ample support for the view that Earth's oxygen cycle provided a crucial evolutionary and ecological constraint on the road to increased biotic complexity, both at the cellular level and on the road ...