ABSTRACT
Our Sun condensed from an accretion disk that itself resulted from gravitational collapse of interstellar matter derived from nearby supernova explosions (Bizzarro et al., 2007). Other components in the disk subsequently clumped together to form planetesimals of various sizes. And these in turn accreted ~4.6 × 109 years ago to form our Earth and the other three inner planets of our solar system, namely, Mercury, Venus, and Mars (Wood et al., 2006; Jacobsen et al., 2008; Wood and Halliday, 2010; Jackson and Jellinek, 2013). All four of these planets are rocky though the gas giants also probably have rocky interiors, as have some of their moons (Wilson and Militzer, 2012). Hydroxylated olivine particles and carbonaceous
chondrites were the major building blocks, bringing water as well as magnesium iron silicates to the planets and their moons very early in their formation (Alexander et al., 2012; Marty, 2012; Vattuone et al., 2013; Saraan et al., 2014). A collision of proto-Earth with Theia, a putative Marssized planet, produced a mostly molten Earth and Moon, accreted just beyond the Roche limit, perhaps around a distance of four Earth radii from our planet (Canup, 2012). There would have been many more days and months in the year in such an early conformation of the Earth-Moon system. Radiogenic, impact, and gravitational heating kept the Earth hot, resulting in the separation of silicates from iron, the latter differentiating and gravitating to the core, leaving a relatively oxidized
3.1 Beginning / 19 3.1.1 Origin of Life on Earth: Panspermia / 20 3.1.2Origin of Life on Earth: De Novo Appearance / 21 3.1.3Life from Abiotically Formed Organic Molecules in Aqueous Solution
(Organic Soup Theory) / 21 3.1.4“Pyrite-Pulled” Surface Metabolism Theory / 22 3.1.5Origin of Life at a Submarine Alkaline Hydrothermal Mound / 23 3.1.6What Happened to the RNA World? / 28
3.2Escape from the Mound / 29 3.2.1Early Evolution by Retrodiction / 29 3.2.2Emergence of the Methanogens / 29
3.3Evolution of Life through the Precambrian: Geological and Biochemical Benchmarks / 30 3.3.1First Stromatolites and Accompanying Microbes / 31 3.3.2Origin of Oxygenic Photosynthesis / 33
3.4Evidence / 36 3.5Unresolved Issues / 39 3.6Summary / 40 Acknowledgment / 41 References / 41
hot mantle in equilibrium with carbon dioxide (Wood et al., 2006). Mantle plumes dissipated this heat, culminating in volcanic activity that emitted vast quantities of carbon dioxide, water vapor, and lesser amounts of nitrogen, nitric oxide, sulfur dioxide, chlorine, and pyrophosphate (Yamagata et al., 1991; Zahnle et al., 2007; Sleep et al., 2014). The result was a hot volatisphere that soon differentiated to the primordial atmosphere and hydrosphere (Martin et al., 2007; Zahnle et al., 2007; Elkins-Tanton, 2008; Goldblatt et al., 2009; Trail et al, 2011; Sleep et al., 2014). Carbonated ocean crust foundered to depth, drawing down this greenhouse gas, though icy comets from beyond the frost line continued to contribute further minor contents of carbon dioxide and water to the atmosphere and ocean (Elkins-Tanton, 2008; Marty, 2012; Sleep et al., 2014).
