PRO-3

For most of us, one star, our own sun, is far more important than all the others, despite its quite ordinary standing among the billions of stars in the universe and its somewhat remote location on the outer edge of the Milky Way galaxy. Circling that star is a series of planets, formed of leftover materials from the sun’s birth. One of those planets, the third from the sun and the fifth largest, is home to all of us. Human history—our history—takes place not only on the earth but also as part of the planet’s history.

That history began with the emergence of the entire solar system about two-thirds of the way through the history of the universe, some 4.7 billion years ago, or early September on the cosmic calendar. Geologists have learned a great deal about the history of the earth: the formation of its rocks and atmosphere; the movement of its continents; the collision of the tectonic plates that make up its crust; and the constant changes of its landscape as mountains formed, volcanoes erupted, and erosion transformed the surface of the planet. All of this has been happening for more than 4 billion years and continues still.

The most remarkable feature of the earth’s history—and so far as we know unrepeated elsewhere—was the emergence of life from the chemical soup of the early planet. It happened rather quickly, only about 600 million years after the earth itself took shape, or late September on the cosmic calendar. Then for some 3 billion years, life remained at the level of microscopic single-celled organisms. According to biologists, the many species of larger multicelled creatures—all of the flowers, shrubs, and trees as well as all of the animals of land, sea, and air—have evolved in an explosive proliferation of life-forms over the past 600 million years, or since mid-December on the cosmic calendar. The history of life on earth has, however, been periodically punctuated by massive die-offs, at least five of them, in which very large numbers of animal or plant species have perished. The most widespread of these “extinction events,” known to scholars as the Permian mass extinction, occurred around 250 million years ago and eliminated some 96 percent of living species on the planet. That catastrophic diminution of life-forms on the planet has been associated with massive volcanic eruptions, the release of huge quantities of carbon dioxide and methane into the atmosphere, and a degree of global warming that came close to extinguishing all life on the planet. Much later, around 65 million years ago, another such extinction event decimated about 75 percent of plant and animal species, including what was left of the dinosaurs. It too, some scientists believe, involved another wave of volcanic eruptions and drastic climate change, exacerbated this time by the impact of a huge asteroid, perhaps six miles in diameter, which landed near the Yucatán peninsula off the coast of southern Mexico. Many scholars believe we are currently in the midst of a sixth extinction event, driven, like the others, by major climate change, but which, unlike the others, is the product of human actions.

PRO-4

So life on earth has been and remains both fragile and resilient. Within these conditions, every species has had a history as its members struggled to find resources, cope with changing environments, and deal with competitors. Egocentric creatures that we are, however, human beings have usually focused their history books and history courses entirely on a single species—our own, Homo sapiens, humankind. On the cosmic calendar, Homo sapiens is an upstart primate whose entire history occurred in the last few minutes of December 31. Almost all of what we normally study in history courses—agriculture, writing, civilizations, empires, industrialization—took place in the very last minute of that cosmic year. The entire history of the United States occurred in the last second.

Yet during that very brief time, humankind has had a career more remarkable and arguably more consequential for the planet than any other species. At the heart of human uniqueness lies our amazing capacity for accumulating knowledge and skills. Other animals learn, of course, but for the most part they learn the same things over and over again. Twenty-first-century chimpanzees in the wild master the same skills as their ancestors did a million years ago. But the exceptional communication abilities provided by human language allow us to learn from one another, to express that learning in abstract symbols, and then to pass it on, cumulatively, to future generations. Thus we have moved from stone axes to lasers, from spears to nuclear weapons, from “talking drums” to the Internet, from grass huts to the pyramids of Egypt, the Taj Mahal of India, and the skyscrapers of modern cities.

This extraordinary ability has translated into a human impact on the earth that is unprecedented among all living species.4 Human populations have multiplied far more extensively and have come to occupy a far greater range of environments than has any other large animal. Through our ingenious technologies, we have appropriated for ourselves, according to recent calculations, some 25 to 40 percent of the solar energy that enters the food chain. We have recently gained access to the stored solar energy of coal, gas, and oil, all of which have been many millions of years in the making, and we have the capacity to deplete these resources in a few hundred or a few thousand years. Other forms of life have felt the impact of human activity, as numerous extinct or threatened species testify. Human beings have even affected the atmosphere and the oceans as carbon dioxide and other emissions of the industrial age have warmed the climate of the planet in ways that broadly resemble the conditions that triggered earlier extinction events. Thus human history has been, and remains, of great significance, not for ourselves alone, but also for the earth itself and for the many other living creatures with which we share it.