In one sense, environment has always been on the edge, and always will be. It is just that the shortness of our lives and the narrowness of our perspective on Earth history mean that we are mostly unaware of change, and until now scarcely notice pressures on the environment. Over millions of years, the environment has been on many edges. There have been big hits from space, the changing relationship between the Earth and the Sun, the slow movement of tectonic plates on the Earth's surface, major volcanic eruptions, and not least the influence of life itself. The tightly linked living organisms on the Earth's surface work as a single self-regulating system, tending to create and maintain the environment most favorable to them. Over time, the environment has tipped many ways, sometimes violently, to the detriment of this or that ecosystem. Yet today one small animal species—our own—is tipping the system in ways that cannot be foreseen.
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