THIS BOOK IS ABOUT the history of life on Earth--from its birth, shrouded in the depths of the past, to the variegated pageantry of living beings that cover our planet today. It is the most extraordinary adventure in the known universe, an adventure that has produced a species capable of influencing in decisive fashion the future unfolding of the natural process by which it was born.
The history of life is marked by a series of innovations, each introducing a new level of complexity, each to be accounted for in terms of the natural laws of physics and chemistry. Before we embark on this voyage of discovery, I shall define a few general notions that will be with us all along the way.
Life is one. This fact, implicitly recognized by the use of a single word to encompass objects as different as trees, mushrooms, fish, and humans, has now been established beyond doubt. Each advance in the resolving power of our tools, from the hesitant beginnings of microscopy little more than three centuries ago to the incisive techniques of molecular biology, has further strengthened the view that all extant living organisms are constructed of the same materials, function according to the same principles, and, indeed, are actually related. All are descendants of a single ancestral form of life.
This fact is now established thanks to the comparative sequencing of proteins and nucleic acids. These two groups of substances, which are the most important constituents of all forms of life, are entirely different chemically but are both long chains made by the stringing together of a large number of molecular units--up to several hundred for the proteins, often considerably more for the nucleic acids. Think of strings of beads of different colors, of trains made of different kinds of cars hooked end to end, or, more appropriately, of very long words assembled with