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One of the most disturbing insights made by practitioners of "Big History" is that the distinction between geologic time and human time has collapsed in our era. The forces that drove geologic time--plate tectonics, the orientation of the Earth's axis relative to the sun, volcanic activity--were distinct from the forces that drove human time--evolution, technological change, population growth. To be sure, they interacted. But the causal arrow always went from geologic change to human change. As Anthony Penna rightly points out in The Human Footprint: A Global Environmental History (Wiley-Blackwell, 2010), the causal arrow now goes in both directions. Not only do we adapt to the environment, but the environment is adapting to us, and mightily. We are ushering in a new geological period sometimes called the Anthropocene--the era defined by human activity.

It's important to point out that this is not the first time biology has shaped geology: we have good evidence, for example, that 2.4 billion years ago cyanobacteria radically altered the Earth's atmosphere by releasing enormous quantities of free oxygen ("The Great Oxygenization Event"). This time, however, it's different. Cyanobacteria are essentially dumb machines. They could not choose whether they would oxygenate the atmosphere or not. In contrast, we are smart machines. We can choose how we want to alter the environment. Penna tells the story of how we have been altering the environment--and choosing to alter the environment--for the past 50,000 years, and with particular vigor in the past several hundred. We are now masters not only of our own fate, but the fate of the Earth and all life on it. We need to wake up to that fact, and we should thank Anthony Penna for helping to stir us from our slumbers.


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