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D. J. Des Marais (Exobiology Branch, Ames Research Center)
To understand our own origins and to search for habitable environments and biospheres elsewhere, we must discriminate between attributes that are universal among all biospheres versus those that represent principally the outcomes of long-term survival specifically on Earth. Certain key agents that drive long-term environmental change (e.g., stellar evolution, impacts, geothermal heat flow, tectonics, etc.) can help us to discern ancient climates and to compare their evolution among populations of Earth-like planets. The early, tectonically "hyperactive" Earth provided abundant chemical energy for life through oxidation-reduction reactions. Most examples of present-day biochemical machinery that harvest chemical energy from the environment are more pervasive and ancient among our microbial ancestors than are the present-day examples of photosynthesis. The geologic rock record indicates that, as early as 3.5 billion years ago (3.5 Ga), microbial biofilms were widespread within the coastal environments of microcontinents and tectonically unstable volcanic islands. Non oxygen-producing within the coastal environments of microcontinents and tectonically unstable volcanic islands. Non oxygen-producing (non-oxygenic) photosynthesis preceded oxygenic photosynthesis, but all types of photosynthesis contributed substantially to the long-term increase in global primary biological productivity. Evidence of photosynthesis is tentative by 3.5 Ga and compelling by 2.7 Ga. Evidence of oxygenic photosynthesis is strong by 2.7 Ga and compelling by 2.3 Ga. These successive innovations transformed life from localized entities that survived principally by catalyzing chemical equilibration to a globally dominant agent that created and sustained pervasive chemical disequililbria. Major biogeochemical perturbations ca. 2.3 to 2.1 Ga, 1.3 Ga and 0.8 to 0.6 Ga contributed to the irreversible oxidation of the global environment and perhaps also triggered evolutionary innovations that became the foundation of our modern biosphere. Understanding the nature and timing of the ascent of life on Earth is crucial for discerning our own beginnings. This understanding also empowers our search for the origins, evolution and distribution of life elsewhere in our solar system and beyond. This work was supported by the NASA Astrobiology Institute. beyond. This work was supported by the NASA Astrobiology Institute.
The author(s) of this abstract have provided an email address for comments about the abstract: David.J.DesMarais@nasa.gov
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.