A major event that took place about 2.5 billion years ago was the evolution of photosynthesis: the ability to use the energy of sunlight to power metabolism. All cells must obtain raw materials and energy to fuel their metabolism. Photosynthetic cells take up raw materials from their environment, but the energy they use to metabolize those chemicals comes directly from the sun. Early photosynthetic cells were probably similar to present-day prokaryotes called cyanobacteria (Figure 1.4). The energy-capturing process they used, which we will describe in Chapter 8, is the basis of nearly all life on Earth today.
Oxygen gas (O2) is a by-product of photosynthesis. Once photosynthesis evolved, photosynthetic prokaryotes became so abundant that they released vast quantities of O2 into the atmosphere. The O2 we breathe today would not exist without photosynthesis. When it first appeared in the atmosphere, however, O2 was poisonous to most organisms then living on Earth. Those prokaryotes that evolved a tolerance to O2 were able to successfully colonize environments emptied of other organisms and proliferate in great abundance. For those prokaryotes, the presence of oxygen opened up new avenues of evolution. Metabolic reactions that use O2, called aerobic metabolism, are more efficient than the anaerobic (non-oxygen-using) metabolism that earlier prokaryotes had used. Aerobic metabolism allowed cells to grow larger, and it came to be used by most organisms on Earth.
Over a much longer time frame, the vast quantities of oxygen released by photosynthesis had another effect. Formed from O2, ozone (O3) began to accumulate in the upper atmosphere. The ozone slowly formed a dense layer that acted as a shield, inter-
- First hominids
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.