There are still a few places in the world where one can sense what Earth was like before the advent of humans. In the aisles of a tropical rainforest, such as the one that flourishes by the Rio Napo in Peru, there are so many species of trees that often one has to walk some distance before finding the same one twice, and the variety of iridescent butterflies, mantises and other insects is incredible. In a cave under the coastal cliffs of Oregon, open to the breakers of the seemingly changeless ocean, the great sea lions bark clouds of steam above pools where mussels and anemones cling amid a constantly moving throng of crustaceans. At evening in springtime around a desert water hole in Organ Pipe Cactus National Monument, Arizona, bright flowers spice the air as bighorn sheep lower their heads and bats dive to the surface, drinking on the wing. Earth before mankind was a place of abundant biodiversity and of dynamic balance among species and elements.
An environmental history has to begin with the environment. There is a history of the environment before the human species evolved into its present form. Indeed, the appearance of Homo sapiens came only recently in the long story of the Earth’s geology and biology, a story to which many scientists apply the term “environmental history” in a broader sense than it is used in this book.
What is the natural state of Earth? This is a question that must be answered before it is possible to understand how the human species relates to, and changes, the natural world. Some early ecologists argued that selected areas ought to be preserved from virtually all human disturbance to show how natural living systems operate when compared with areas that have suffered from various kinds of interference. 1 While it is even more important today to preserve habitats for animal and plant species, it is also increasingly apparent that no place on Earth is really unaffected by human activity; none has escaped such widespread effects as air pollution, intensification in the acidity of precipitation, radioactive fallout, and the penetration of ultraviolet radiation due to the depletion of the ozone layer in the high atmosphere. This means that historians must look to evidence from the deep past to find out how nature operated without humankind, and use that as a baseline or control against which to judge the changes brought about since the beginning of human history.
Contemplating the immense age that Earth had reached before humans appeared may provide perspective. The planet condensed into its nearly spherical shape, seas and continents formed, the phyla of the animal and vegetable kingdoms evolved, and living species evolved ways of interacting with the physical matrix and with each other over hundreds of millions of years. The result was an ecological balance that sustained the conditions for life. Natural laws may, according to the new views of cosmological physics, change as the universe unfolds, but they do not apparently make exceptions for individuals or species. Humans, whose written history has spanned only the last few thousand years, must live