Lesson 2: Earth Planet of Life 2.0 Introduction and Overview The third planet from the Sun is our home. We call it "Earth" even though it is mainly covered by water. Planet "Ocean" might have been more appropriate, but we are creatures of the land and rarely spend time in or on the water. In fact, etymologically "Earth" means the "home farm" (cf. arable land), and we should think of it as the "home planet." Big Sur Courtesy of The single most important thing about Earth is that it has Life, and the single most important thing about Life is that all its forms are interrelated, which means, in a sense, that there is only one kind of life. Thus, not only is Earth (so far) our only documented example of a planet with life, but also Life on Earth only gives us one type of life form to contemplate, one that has cells and uses DNA for instructions for making more cells. However, except for this basic similarity in building plan, Life on Earth is extremely diverse in its manifestations. Among the larger organisms, it includes such different forms as trees, whales, and giant fungi. Among the smallest, there are an incredible variety of single-celled organisms that pervade practically all available habitats on the surface of Earth, and to at least three and a half kilometer below Earth's solid surface. The presence of Life on Earth is evident in the composition of the rocks making up the crust of the planet, the composition of the water of the ocean, and the atmosphere. Life is intimately involved in the processes shaping the face of the planet, especially in weathering and soil formation and in the making of sedimentary rocks. It is also intimately involved in producing the climate on Earth, through control of greenhouse gases. Thus, the fact that Earth has Life makes it into a planet that is quite different, in its physical state and appearance, from what it would be without Life. Earth is the planet that Life shaped. This insight is not the same as ascribing life to Earth itself, as in the animistic religions and their derivations. In many, perhaps all, such religions Earth is seen as a female being bearing offspring (sometimes with the sky, or thunderstorm, as the male counterpart). The female principle of fertility is commonly personalized, and represented as a goddess. Such goddesses are "Erda" (Germanic) and "Gaia" (Greek) whose names we recognize in "Earth" and "Geology". The insight that Life shapes Earth is relatively recent, and goes back to the German ecologist and geologist C. G. Ehrenberg, the Russian geochemist V. I. Vernadsky, and the American ecologist G. Evelyn Hutchinson. Ehrenberg (1795-1876), who studied the fossil remains of microscopic organisms, realized that entire mountains are made of such remains: limestone is a result of life processes. Vernadsky (1863-1945) explored the geochemical cycling for the major elements on the surface of Earth. He realized that life processes are intimately involved in determining the nature of these cycles. Hutchinson (1903-1991) studied the physical and biological in lakes, and had a special interest in the phosphorus cycle. He emphasized that the geochemistry of elements used by organisms (carbon, oxygen, nitrogen, phosphorus) depends entirely on life processes. Source: Johnson Spaceflight Center/NASA In particular, he stated that the atmosphere (both oxygen and nitrogen) is the product of life processes involving photosynthesis, the fixing of nitrogen into organic matter, and the release of nitrogen in the bacterial decay of organic matter.These various ideas are These various ideas are fundamental to the scientific discipline of "biogeochemistry." The "Gaia hypothesis" proposed by the British engineer and science writer James Lovelock (born 1919) builds on these ideas. The hypothesis stipulates that life processes help maintain an environment on the surface of the planet that is favorable to the continued existence of life. Given that life processes are important in governing geochemistry and climate, and given that life has persisted and flourished through billions of years, the hypothesis seems secure from being ever disproved.