Chapter 5

Life on Earth & its limits


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Synopsis

Currently, Earth life is the only known example of life; therefore, understanding life on Earth is essential to our search for life elsewhere in the Universe. Life can take many shapes and forms, and exist in a variety of environments. While most life on Earth lives comfortably with enough energy, water and temperatures of around 37C, some unique organisms have adapted to live in environments in which these conditions are very different, extending the limits of life to the extremes. We call these extremophiles, and these species teach about the the limits at which life can exist. In this chapter, we will discuss what we know about life and its limits on Earth, and the implications for the search for life elsewhere.

Scientists often describe the history or life on Earth and the evolutionary relationships between organisms using what is known as the Tree of Life. Here we begin with an introduction to the tree of life and briefly discuss our understanding of the history of life on Earth. Remarkably, all life on Earth uses essentially the same the elemental make-up and molecular building blocks. These important elements and molecules are introduced and their roles are discussed. Additionally, all life on Earth requires a source of energy to perform chemical reactions, build biomass and reproduce. Together the chemical transformations required to sustain life are known as metabolism. The energy required for metabolism comes either from light or chemical sources and depends on redox gradients. The types of metabolism and ultimate planetary sources of energy are discussed in this chapter. The final section of this chapter discusses the known limits of life on Earth by examining extremophiles. Extremophiles are organisms that live in extreme environments. Examples of organisms from extreme high and low temperatures and pressures, high salt, high and low pH, high radiation and low energy environments are discussed along with their relevance as analogs to environments elsewhere in the Universe.