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Origin and Evolution of Earth: Research Questions for a Changing Planet
conditions favorable to life. We are far from understanding how much of evolution is purely biological and how much has been forced by Earth processes; nor do we know exactly how much of Earth’s environment is determined by the presence of life. And yet these questions have suddenly become more urgent as we find ourselves in an era when—presumably for the first time—Earth’s surface environment can be manipulated by a single dominant life form, Homo sapiens, that is capable of making choices about the effects of its actions.
How Does Life Affect Geological Processes?
Life affects Earth’s planetary processes in several ways. At the microscopic scale, life is an invisible but powerful chemical force. Organisms can catalyze reactions that would not happen in their absence, and they can accelerate or slow other reactions. The chemical reactions they enhance have a specific character; in general they extract energy from Earth and from sunlight to fuel life processes. These reactions, compounded over immense stretches of time by a large biomass, can generate changes of global consequence. An example of this global influence is the processing of carbon and oxygen. Weathering reactions on land, combined with organic precipitation of carbonate shells in the oceans, remove carbon from the atmosphere and convert it to carbonate minerals on the seafloor (Question 7). Photosynthesis also extracts carbon from the atmosphere, converting carbon dioxide into oxygen plus organic material. Some of this organic carbon is stored in soils, ocean sediments, and the living biomass of the continents and oceans, while the oxygen is delivered to the atmosphere. Larger animals and plants also have physical effects on Earth, such as promoting soil formation and moderating erosion.
Beyond these generalities, we understand little about the details of biologically mediated chemical processes in the environment, especially those of the distant past. Like many fields of science, however, this one is being revolutionized by powerful new analytical tools and computational techniques. For example, new ultrahigh-resolution microscopes can now be used to observe microorganisms in the environment and in laboratory experiments (Figure 3.10). Synchrotron X-ray techniques can be used to study the chemical processes of these microorganisms. Innovative isotopic techniques are being used to help understand the complicated chemical processing that organisms can achieve. DNA sequencing methods have brought a new dimension to studies of microbiological processes. In the past it was difficult to identify the organisms in natural samples because many could not be cultured. Today, organisms do not need to be cultured; their identity can be determined directly from their DNA. Computational chemistry (see Question 6) also shines a strong new light on natural biochemical processes, bringing the possibility of calculating from quantum mechanical theory how atoms and molecules will behave in the microenvironments surrounding tiny organisms.
Soils represent a particularly clear example of how multiple fields, including inorganic chemistry, physics, and hydrology, can wrest new insights from geobiological processes. Inorganic weathering of minerals and organic carbon in the soil environment releases nutrients and carbon. The rate of release and the types of nutrients define the environment in which life can exist and control the range and abundance of life forms that can survive. In addition, the roots of land plants, as well as bacteria, fungi, and animals such as earthworms, can accelerate the weathering of mineral and organic matter in soils. Such biological catalysis of weathering processes can enhance the suitability of soil for life and
FIGURE 3.10 High-resolution images of (A) a cell (outer cell wall indicated by white arrows) and associated mineralized filaments (white) and nonmineralized fibrals (gray) and (B) FeOOH-mineralized filaments filtered from water. SOURCE: Chan et al. (2004). Reprinted with permission from AAAS.