Introduction

Life has been on Earth for at least three and a half billion years—an incomprehensibly long period of time. Earth itself is just over four and a half billion years old. The oldest rocks we know are about 4 billion years old—but they are granites, cooled from a molten melt, so they cannot be expected to contain any remains of ancient, fossilized life. The oldest sedimentary rocks—the very kind of rocks that often do have fossils in them— that were formed from grains of silt and sand deposited in ancient seaways are around three and a half billion years old. These most ancient sediments have yielded traces of early bacterial life on earth. If the oldest rocks that could possibly contain fossils do have fossils, we can only assume that life inhabited earth more than three and a half billion years ago—in other words, we would expect to find the chemical and fossil evidence of even older bacteria if we were to get lucky and find even older sediments. Life, we can only conclude, has been an integral part of the earth almost since the world began.

It took nearly one and a half billion years before more complex cells evolved: the eukaryotic cells we have in our own bodies, the sort of cell we share with all other animals, as well as plants, fungi, and single-celled, mostly microscopic protoctistans like amoebae. Then it took another one and a half billion years for multicelled animals (and even later for plants) to evolve. Life's evolutionary history is full of long periods where nothing much seems to happen before the next big evolutionary advance (often an increase in complexity). The evidence is increasingly mounting that innovations throughout the evolutionary history of life have been triggered by major, physical environmental events that disrupted older systems and spurred the development of the newer ones.

Consider the major mass extinctions of the past half billion years or so—the ones that disrupted life so much, driving great groups like the terrestrial dinosaurs to extinction. There have been five of these global mass extinctions, and each one has profoundly altered the course of evolutionary history. If dinosaurs had not died out—victims of the explosive collision between the earth and one or more comets 65 million years ago—mammals would not have begun to evolve into the tremendously diverse array of species we have seen on Earth in the last 60 million years. That means that we, human beings of the species Homo sapiens, members of the mammalian Order Primates, would not be here.

We cannot understand life—what it is and how it got to be the way we find it today— without also understanding how life fits into the physical dynamics of the earth—its waters (hydrosphere), its gaseous envelope (atmosphere), and its rocks and soils (lithosphere). The history of life and the history of our planet are inseparable. Life on earth continues to exist as an integral part of the physical system, which is its home and its source of sustenance.

Now we find that life is confronted by something not seen for 65 million years: the very real threat of a major mass extinction, a loss of species so rapid and so great that it rivals the five preceding global mass extinctions. The Sixth Extinction. Harvard biolo gist E. O. Wilson has estimated that the earth is losing species at the rate of three every hour—30,000 species a year. Though we are not sure exactly how many species exist on the earth right now, there are at least 10,000,000 of them. Though there is no way that humans will end up removing absolutely all of the earth's species, most will surely be gone during the next 1,000 years if this rate of loss continues unchecked.

This Sixth Extinction is also known as the Biodiversity Crisis. Like the mass extinctions of the past, the Sixth Extinction is the result of the abrupt and devastating loss of habitat for species in nearly all the world's ecosystems. Unlike the five mass extinctions of the geological past, however, this one is not being caused by comets crashing into the earth or by climatic change overwhelming the earth's species—its cause can be traced to the actions of a single species: Homo sapiens. We are the ones who are cutting the forests, plowing the prairies, paving the landscape, and building the cities. We are the ones overharvesting the world's fisheries and forests. We are the ones polluting the rivers, lakes, and oceans. We are the ones moving animals, plants, and microbes around the globe—often to the detriment of local species. We are the ones behind this Sixth Extinction—the human equivalents of the comets that came close to destroying life on earth 65 million years ago.

We should ask ourselves: Does it matter? Should we be concerned that we are destroying, faster and faster, so much of the world's remaining wilderness and driving more and more species to extinction? After all, we no longer live within local ecosystems; we have not done so since we invented agriculture 10,000 years ago and took food production into our own hands. So why should we care that we are destroying the rest of the ecosystems and species of the planet?

Well, of course, it does matter. We are living, breathing animals, after all. We need clean air. We need water—nearly a third of the 6 billion of us on the planet right now do not have access to safe drinking water! We need those fisheries in the ocean—and those trees in the forest (though it is high time we think about sustainable harvesting so future generations can eat fish and use wood to build houses). We need oxygen—and the many chemical cycles essential for all life (certainly including our own) that are essential functions provided only by healthy, intact ecosystems. And, many of us increasingly think, we need nature around us because it is where we came from—it is an essential part of us, as we are a part of it. It is beautiful, this natural world. We need it for that reason alone.

That's what this encyclopedia is about. Far more than just another reference on natural history, far more than a great source of information on ecology and evolution, this book tells us about the earth, about life, and about how humans fit into the scheme of things. It tells us, too, how we are destroying the very fabric of life, why we should not destroy it, and what we can do about stemming the tide of the Sixth Extinction.

We begin with four expansive essays exploring the four questions: What is biodioversity? Why is biodiversity important? What are humans doing to cause the loss of so many species? And, finally, what can we do to stop the loss?

Then, in familiar A—Z format, we present incisive entries on a surprisingly wide range of topics. We are talking here of humans on the planet—what our history has been, how we fit in it, how we cause major ecosystem disruption and species loss, and why and how we should correct our course as we continue collectively to sail through life. To provide a reference that will meet such demanding needs, we have assembled entries in anthropology, archeology, economics, and sociology; geology is also presented, as we need to understand the physical structure of the earth as well as its history. Paleontology is here, too, as we need to understand the history of life—how it came to be the way it is—before we can understand its present condition.

And, because biodiversity—all the species represented in all the world's ecosystems—is a double-identity subject, we include entries from the two central subjects: ecology and evolutionary biology. We need to become familiar, if not with each of the 10,000,000 species on earth, at least with the major groupings of life—from bacteria to redwood trees— that evolution has produced. These are the players in the game of life. On the other hand, the actual game of life is played in the world's ecosystems—comprised of a mélange of players drawn from the bacterial, protoctistan, fungal, plant, and animal basic divisions of the evolutionary spectrum of life. You cannot understand biodiversity unless you realize it is two-sided: first, there is a spectrum of living organisms, from bacteria to redwood trees to ourselves, produced by evolution, and second, there is a world in which matter and energy flow between organisms—the world of ecosystems. Biodiversity is not a dry summary of the principles of ecology and evolution (though both are in this volume in great detail!); rather, biodiversity is the interplay between these realms and beyond, encompassing the physical earth systems in which life exists. Given the role that humans are playing on Earth, biodiversity encompasses all that is human (and all we know about who humans are), how we have evolved, and how we fit into the world around us.

Our hope is to awaken curiosity and to inspire younger generations to gain the wisdom and courage necessary to confront the complex issues of the twenty-first century. May this encyclopedia help you on your way to learning about the world in which you live—and to discover ways in which humanity might continue to prosper without destroying the earth from which we came and on which we still so deeply depend.

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