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9.11 Coevolution of Life and Earth G 9.11 Coevolution of Life and Earth G. J. Retallack, University of Oregon, Eugene, OR, USA ª 2007 Elsevier B.V. All rights reserved. 9.11.1 Introduction 295 9.11.2 Barren Worlds as Null Hypotheses 298 9.11.2.1 Moon 298 9.11.2.2 Venus 298 9.11.2.3 Mars 298 9.11.2.4 Asteroids 299 9.11.3 Biosignatures of Earth 299 9.11.3.1 Inflammable Atmosphere 299 9.11.3.2 Permanent Liquid Water 300 9.11.3.3 Carbon-Based Life 301 9.11.3.4 Spheroidal Weathering 303 9.11.3.5 Granitic Rocks 304 9.11.3.6 Convex Slope 304 9.11.3.7 Calcic Horizon 305 9.11.3.8 Argillic Horizon 305 9.11.3.9 Mollic Epipedon 306 9.11.3.10 Civilization 306 9.11.4 Origin of Life on Earth 307 9.11.4.1 Experimental Studies 308 9.11.4.2 Geological Records 308 9.11.4.3 Soil 309 9.11.4.4 Sea 309 9.11.4.5 Deep-Sea Vent 309 9.11.4.6 Other Worlds 309 9.11.5 Coevolutionary Histories 310 9.11.5.1 Life and Air 310 9.11.5.2 Life and Water 312 9.11.5.3 Life and Soil 314 9.11.5.4 Life and Rocks 315 9.11.6 Conclusions 317 References 317 When I hear the sigh and rustle of my young wood- very significant differences between our planet and lands, planted with my own hands, then I know that I other planetary bodies in our solar system. Such pro- have some slight share in controlling the climate – cesses as the growth of trees are commonly regarded Anton Chekhov (1899) as mundane, in the sense of commonplace or trivial, but they can be mundane also in the more important 9.11.1 Introduction sense of global, as apparently understood by Chekhov (1899). The media which elevate biological processes Earth is one of the oldest words in our language (from to global scope are air, water, and soil. Air and water Old English ‘eorde’) meaning both our planet (custo- are important to the coevolution of life and earth marily capitalized in this sense) as well as the soil (Nisbet and Sleep, 2001), but we will take the path beneath our feet. Here I explore both ends of its less traveled here, and present a perspective on coe- meaning by emphasizing how small-scale biological volution of life and earth from my research experience and soil-forming processes have played a role in the with ancient and modern soils. 295 296 Coevolution of Life and Earth Soil is Anglo-French word, introduced from the alterations can be anticipated. A broad definition of ancient Roman ‘solium’ (seat) or ‘solum’ (ground). soil obviates the need for such terms as regolith or Soil is so central to human existence that it is as the distinction between physicochemical and biologi- difficult to define as ‘home’ and ‘love’. To some soil cal weathering (Taylor and Eggleton, 2001). More scientists, soil is material altered by the action of organ- important to our present purpose, there is no longer isms, especially plant roots. To engineers, soil is an assumption that life created soil. The questions now material moved by mechanized graders without become when and how did life evolve in soil? Did soil recourse to blasting. To farmers, soil is the surficial nurture the origin of life, or did life evolve in the ocean organic layer that nurtures crops and feeds livestock or volcanic hot springs, then later adapt to soil? These (Hole, 1981). We prefer a wider definition of soil as questions remain open (Nisbet and Sleep, 2001). materials on the surface of a planetary body altered in When soil, air, and water became alive, unlike the place by physical, biological, or chemical processes. apparently sterile but discernably altered soils of the This definition has also been adopted by NASA Moon, Venus, and Mars (Figure 1), it must have (National Aeronautic and Space Administration of been an important time in the history of our Earth. the USA) press releases on Lunar and Martian soils, Soils gave life access to fundamental nutrients such as for the compelling reason that they wished to be phosphorus at their source in the weathering of rocks. understood by the general public. In any case, the Soils allowed life to colonize a significant fraction of surface of the Moon and Mars is now altered locally Earth’s surface, thus enhancing their volumetric by robotic landers and footprints, and our future effect on surficial fluids. Life gave soils increased No atmosphere Mesosiderite Moon Possible evolutionary pathways Carbonaceous chondrites Thin atmosp here Mars Venus Thick atmosphere Earth Silicate sand High temperature Silicate crystals Metallic iron Clay glaze Impact melt Rock fragments Sulfate Oxidation rind spheres Basaltic rocks Chondrules Agglutinates Root traces Figure 1 A schematic comparison of soil formation on Earth, Moon, Mars, Venus, and some hypothetical meteorite parent bodies. From Retallack GJ (2001a) Soils of the Past, 2nd edn., 600pp. Oxford: Blackwell. Coevolution of Life and Earth 297 depth and stability to enhance rates of physicochem- Horses evolved cursorial limb structure for predator ical weathering. Life also gave soils a variety of escape over open county and high-crowned teeth to biosignature horizons and deep weathering products withstand abrasion from leaves studded with opal (Retallack, 2001a). phytoliths (MacFadden, 1992). Grasses and grazers The evolution of life and soil can be viewed as a coevolved to produce ecosystems, such as grasslands, coevolutionary process, like the coevolution of and soils, such as Mollisols, that became biotic grasses and grazers, as first proposed by Kowalevsky planetary forcings of the atmosphere and hydro- (1873). Coevolution is the coordinated evolution of sphere (Retallack, 2001b). Because coevolution is phylogenetically unrelated organisms (in this case, directed more toward other organisms than to plants and animals), which coevolved to enhance physicochemical environments, Earth’s environment their mutual interdependence. Grasses evolved sub- coevolved with life far from primordial conditions terranean rhizomes, basal tillering of adventitious exemplified by other planetary bodies (Figure 1). roots, intercalary meristems, telescoped internodes, The coevolution of life and soil is evident from and opal phytoliths to withstand grazing (Figure 2). functional intricacies of modern ecosystems, but the Low albedo Coevolutionary consequences High albedo High transpiration Low transpiration Low C storage High C storage Dry soil Hyracotherium grangeri Miohippus obliquidens Moist soil Early Eocene (50 Ma) Early Oligocene (31 Ma) Willwood Formation Brule Formation Powell, Wyoming Badlands National Parahippus agatensis Pliohippus nobilis Park, South Dakota Early Miocene (19 Ma) Late Miocene (7 Ma) 5 m Anderson Ranch Formation 5 m Ash Hollow Formation ystem Agate, Nebraska Ellis, Kansas Reconstructed ecos 50 cm Paleosols 50 cm Planar Calcareous Organic Brown color Root traces Mollic epipedon Bedding Nodules debris = = = = High-crowned Low-crowned, Horse Horse molars, 3-toes, small molars, metacarpals Very high-crowned molars, Coadaptations 4-toes on front feet, Low-crowned, longer metacarpals small medium-size molars, 1-toe, metacarpals as long 3-toes on front feet, as femur Sheathing leaves, metacarpals long intercalary nodal Abundant abrasive meristem opal phytoliths on leaves Telescoped terminal Rhizome with meristem ensheathed fine adventious roots Grass Grass by leaves Coadaptations Figure 2 An example of coevolution, grasses and grazers, exemplified by paleosols and fossil horses from North America: grass evolutionary stages are less well constrained by the fossil record. Data from Retallack GJ (2001b) Cenozoic expansion of grasslands and global cooling. Journal of Geology 109: 407–426. 298 Coevolution of Life and Earth details of when and how this happened are shrouded grains (Figure 1). Well developed lunar soils in the mists of time. It is still uncertain, indeed largely 5–10 cm thick take some 200 million years to form, hypothetical, when and how life first came to soils. judging from rate of micrometeoroid bombardment There were changes in atmospheric oxidation and (McKay and Basu, 1983) and the succession of paleo- water salinity on the early Earth, but it remains sols in the radiometrically dated Apollo 15 core uncertain which of these are due to soil colonization, (Heiken et al.,1976). The most profound soil-forming as opposed to oceanographic, volcanic, or tectonic event in the history of the Moon was the footprints, changes. Fortunately, there is a remarkable record tracks, and excavations of Apollo astronauts. of paleosols in very ancient rocks on Earth land (Rye and Holland, 1998) and on different planetary 9.11.2.2 Venus bodies (Retallack, 2001a), which furnish evidence on past conditions of climate and life. This supplements Venus is slightly smaller (0.8) than the mass of Earth, the geological record of life and environments in but its thick (93 bar) atmosphere is a toxic mix of marine, volcanic, and metamorphic rocks (Schopf, greenhouse gases (97% CO2) creating surface tem- 1983; Schopf and Klein, 1991). Paleosol development peratures of about 470C. Water is not stable in the and geochemistry also provide relevant tests for com- soil and water vapor is rare (0.1–0.4%) in the atmo- peting hypotheses of the development of continents sphere. Soil formation on the basaltic parent material and plate tectonics. may be a rapid process akin to pottery glazing (Figure 1), in which salts are melted to obscure the hard edges of rocks and sediments (Barsukov et al., 9.11.2 Barren Worlds as Null 1982). Hypotheses 9.11.2.3 Mars Uniqueness of our planet is best considered by com- parison with other planetary bodies, which turn out to Venus and our Moon stretch concepts of soil forma- have a variety of common features, such as basalt and tion almost to breaking point, but Mars has more CO2-rich atmosphere.
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