Every person on this planet began life as one cell, about the same size as this period. Each one of us remained like this for about 24 hours before dividing in two—the first step toward creating the complex, multicellular organism that humans are today. It is an incredible and fascinating thought, that a human could have been contained in a single cell, and that, perhaps more remarkable still, that one basic unit of life knew what to do to next. To understand just how important cells are, consider the following. The total number of living things currently inhabiting our planet is unimaginably large (there are an estimated 8.7 million unique species, most of them numbering millions, billions, or trillions of individuals), and every last one of them, without exception, is made of one or more cells. Next, consider the incredible variety of processes and materials that occur in the natural world. The glow of a firefly, a plant bending toward the light, cancer, a 100-meter sprint, wood, mucus, elephant dung, a blue whale’s skeleton, body odor, the memory of the smell of ratatouille, the call of a howler monkey, houseplants, a hawk’s beak, a snake’s venom … all of these are the result of activity in cells. What is this life?
The difference between a living and a nonliving thing has always been difficult to define. Biologists generally agree that for something to be considered alive it must satisfy a set of criteria, including the use of energy to build complex molecules and organize its internal systems, and the ability to respond to its surroundings and to reproduce. Rhododendrons and ants satisfy all of these criteria—but only because they are made of cells, life’s building blocks. Cells are life, and to understand their behavior, their structure and their remarkable microscopic and submicroscopic machinery is to understand life itself. Chapter one outlines the history of that understanding (so far), and examines some of the tools and techniques that have nurtured it. A cell is just a mixture of molecules, a cocktail of chemicals, inside a little bag. Despite the unambitious simplicity of that description…