FIGURE 3.11 The history of life, based on geological evidence, along with long-term oxygen, ice ages, and mass extinctions. Molecular data suggest that eukaryotic organisms (protozoans, algae, fungi, plants, and animals) share a common ancestor with Archaea.

also speed up the weathering that would have gone on in the absence of life. The ultimate control on the soil environment is probably climate; insufficient rainfall, for example, limits how fast both inorganic and organic chemistry can proceed. But on a global basis we now know that soil chemistry is powerful enough to affect climate by helping to regulate atmospheric carbon dioxide.

Similarly, we know that vascular plants have an enormous effect on Earth’s environment. Life on Earth originated nearly 4 billion years ago, but land plants are found in the geological record only during the past 400 million years or so (Figure 3.11). Several lines of geological evidence suggest that diversifying land vegetation changed the nature of continental weathering, erosion, and sedimentation, changing the physical stability of stream banks and even influencing the composition of Earth’s atmosphere (Berner and Kothavala, 2001). Roots break up rock and help transform it into soil. Deep roots also contribute carbon dioxide to soils, resulting in concentrations of soil CO2 that are 10 to 100 times higher than the modern atmosphere. The high CO2 concentrations in soil gas act to acidify soil water, which leads to increased rates of dissolution of minerals. Deeply rooted plants can also extract water from well below the surface and return it to the atmosphere via evaporation from leaves. This evapotranspiration has an important cooling effect on the land surface, as does the shade provided by the leaf canopy.

There is ample evidence that plants and animals also influence erosion rates, but there is still uncertainty about how important they are in the long-term evolution of continental surfaces and how their effects should be represented in new landscape evolution models (Dietrich and Perron, 2006). Erosion itself affects habitat conditions and can strongly influence biodiversity and ecosystem processes. Hence a central question is the extent to which life and landscape evolution are related. For example, do hillslope shapes and river forms reflect the presence of life, or would Earth’s land surface be more or less the same shape if



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