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Grand Prismatic Hot Spring, Yellowstone National Park, Wyoming. The striking array of colors reflects different communities of microorganisms that are very sensitive to water temperature. Water flowing away from the center of the spring (blue) cools down, causing a given community of microorganisms to be replaced by a different community that grows best at the new, lower temperature. The boardwalk visible in the lower part of the photo allows tourists to peer into its depths and provides a sense of scale.

GEOBIOLOGY: LIFE INTERACTS WITH EARTH

  • The Biosphere as a System 284
  • Microorganisms: Nature’s Tiny Chemists 288
  • Geobiological Events in Earth’s History 296
  • Evolutionary Radiations and Mass Extinctions 300
  • Astrobiology: The Search for Extraterrestrial Life 306

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GEOLOGY IS THE STUDY of the physical and chemical processes that control the Earth system, today and in the past. Biology is the study of life and living organisms, including their structure, function, origin, and evolution. As separate as geology and biology may seem, organisms and their physical environment interact in many ways. We have long recognized that biology and geology are intimately related, but until recently, we have not known exactly how. Fortunately, technological advances in both Earth and life sciences now allow us to ask and answer questions that were previously beyond our scope. Over the past decade, scientists working at the frontiers of both fields have begun to understand how several important geobiological processes work.

We know that organisms can change Earth. For example, Earth’s atmosphere is distinct from that of every other planet in having a significant concentration of oxygen—the result of the evolution of oxygen-producing microorganisms billions of years ago. Organisms also contribute to the weathering of rocks by releasing chemicals that help break down minerals; through this process, they obtain nutrients essential to their growth. Similarly, geologic processes can change life, as when an asteroid struck Earth 65 million years ago, causing a mass extinction that killed off the dinosaurs.

This chapter explores the links between organisms and Earth’s physical environment. It describes how the biosphere works as a system and what gives Earth its ability to support life. Next, it explores the remarkable roles microorganisms play in geologic processes, and it discusses some of the major geobiological events that have changed our planet. Finally, it considers the key ingredients for sustaining life and ponders the eternal question posed by astrobiologists: Is there life out there?

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