The story of life has a cast of millions, with humans playing only one of many roles in an epic 4 billion years in the making. Our own species, Homo sapiens, has existed for only the most recent 1/200 of 1% of life’s history, yet there are compelling reasons to pay special attention to ourselves. We want to understand how our own bodies work and how humans came to be: Curiosity about ourselves is after all a deeply human trait.

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We also want to understand how biology can help us conquer disease and improve human welfare. Epidemics have decimated human populations throughout history. The Black Death—bubonic plague caused by the bacterium Yersinia pestis—is estimated to have killed half the population of Europe in the fourteenth century. Casualties from the flu pandemic of 1918 exceeded those of World War I. Even King Tut, we now know, suffered from malaria in his Egyptian palace approximately 3300 years ago. Throughout this book, we discuss how basic biological principles are helping scientists to prevent and cure the great diseases that have persisted since antiquity, as well as modern ones such as AIDS and Ebola.

Furthermore, we need to understand the evolutionary and ecological consequences of a human population that now exceeds 7 billion. All species affect the world around them. However, in the 21st century human activities have taken on special importance because our numbers and technological abilities make our footprint on Earth’s ecology so large. Human activities now emit more carbon dioxide than do volcanoes, through industrial processes we convert more atmospheric nitrogen to ammonia than nature does, and we commandeer, either directly or indirectly, as much as 25% of all photosynthetic production on land. To chart our environmental future, we need to understand our role in the Earth system as a whole.

And, as we have become major players in ecology, humans have become important agents of evolution. As our population has expanded, some species have expanded along with us. We’ve seen how agriculture has sharply increased the abundance and distribution of grapes, and the same is true for corn, cows, and apple trees. At the same time, we have inadvertently helped other species to expand—the crowded and not always clean environments of cities provide excellent habitats for cockroaches and rats (Fig. 1.21).

Other species, however, are in decline, their populations reduced by hunting and fishing, changes in land use, and other human activities. When Europeans first arrived on the Indian Ocean island of Mauritius, large flightless birds called dodos were plentiful (Fig. 1.22). Within a century, the dodo was extinct. Early Europeans in North America were greeted by vast populations of passenger pigeons, more than a million in a single flock. By the early twentieth century, the species was extinct, a victim of hunting and habitat change through expanding agriculture. Other organisms both great (the Bali tiger) and small (the dusky seaside sparrow) have become extinct in recent decades. Still others are imperiled by human activities; the magnificent white rhinoceros of Africa is threatened by both habitat destruction and poaching. Whether any rhinos will exist at the end of this century will depend almost entirely on decisions we make today.

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Throughout this book, we return to the practical issues of life science. How can we use the principles of biology to improve human welfare, and how can we live our lives in ways that control our impact on the world around us? The answers to the questions critically depend on understanding biology in an integrated fashion. While it is tempting to consider molecules, cells, organisms, and ecosystems as separate entities, they are inseparable in nature. To tackle biological problems, whether building an artificial cell, stopping the spread of infectious diseases such as HIV or malaria, feeding a growing population, or preserving endangered habitats and species, we need an integrated perspective. In decisively important ways, our future welfare depends on improving our knowledge of how life works.