CHAPTER 1Cells, Genomes, and the Diversity of Life

The surface of our planet is populated by living things—organisms—curious, intricately organized chemical factories that take in matter from their surroundings and use these raw materials to generate copies of themselves. These organisms appear extraordinarily diverse. What could be more different than a tiger and a piece of seaweed or a butterfly and a tree? Yet our ancestors, knowing nothing of cells or DNA, saw that all these things had something in common. They called that something “life,” marveled at it, struggled to define it, and despaired of explaining what it was or how it worked in terms that relate to nonliving matter.

The remarkable discoveries of the past 100 years or so have not diminished the marvel—quite the contrary. But they have removed the central mystery regarding the nature of life. We can now see that all living things are made of cells: small, membrane-enclosed units filled with a concentrated aqueous solution of chemicals and endowed with the extraordinary ability to create copies of themselves by growing and then dividing in two.

Because cells are the fundamental units of life, it is to cell biology—the study of the structure, function, and behavior of cells—that we must look for answers to the questions of what life is and how it works. With a deeper understanding of cells and their evolution, we can begin to tackle the grand historical problems of life on Earth: its mysterious origins, its stunning diversity, and its invasion of every conceivable habitat. Indeed, as emphasized long ago by the pioneering cell biologist E. B. Wilson, “the key to every biological problem must finally be sought in the cell; for every living organism is, or at some time has been, a cell.”

Despite their apparent diversity, living things are fundamentally similar inside. The whole of biology is thus a counterpoint between two themes: astonishing variety in individual particulars and astonishing constancy in fundamental mechanisms. In this chapter, we begin by outlining the universal features common to all life on our planet, along with some of the fundamental properties of their cells. We then discuss how an analysis of DNA genomes allows scientists to position the wide variety of organisms in an evolutionary “tree of life.” This approach, which quantifies how closely organisms are related to one another, allows us to identify the three major branches of life on Earth, eukaryotes, bacteria, and archaea—each with unique qualities. We shall see that the familiar world of plants and animals—the focus of scientists for many centuries—makes up only a small slice of the complete diversity of life, the vast majority of which is invisible to the unaided human eye.

After exploring some of the ways that genomes change over evolutionary times, we highlight the handful of model organisms that biologists have chosen to focus on to dissect the molecular mechanisms underlying life. A few specific viruses, including SARS-CoV-2, pose grave threats to humans, so they too have become objects of intensive study. For this reason, this section also includes an introduction to viruses, the ubiquitous parasites that have evolved to feed on cells. Viruses are now recognized to be the most abundant biological entities on the planet.