surface fundamentally from a highly reducing environment to an oxidized environment. The evolution of photosynthetic organisms beginning approximately 3.5 billion years ago established present-day oxygen concentrations and radically changed physical, geological, chemical, and biological processes on our planet (see Figure 3.1). Increased oxygen affected geologic processes by causing the oxidation of minerals as well as biology by creating conditions favorable for oxygen-breathing organisms. Life even affects the weather and helps to regulate the temperature of Earth. For example, some algae or phytoplankton in the ocean emit dimethyl sulphide that is capable of nucleating raindrops and causing rain. As the sun shines, more phytoplankton grow. These in turn nucleate clouds, which in turn control the temperature (http://www.oceansonline.com/gaiaho.htm).

Biological processes affecting Earth work on very small scales and in short time frames. It is the sheer magnitude of the amount of biomass and the cumulative effects of these processes over long time frames that shape Earth. It is estimated that the approximately 350 to 550 × 1015 grams

FIGURE 3.1 The biogeological time line. Approximate timing of major events (billion years from present day) in the history of life on Earth. SOURCE: Image courtesy of Dr. Bharat Patel, http://trishul.sci.gu.edu.au/courses/ss13bmm/introduction_MAM.pdf.



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