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Let’s imagine that we could tag a carbon atom at its moment of origin and then follow its odyssey through time and space. Formed in a nuclear blast furnace deep within an ancient star and then ejected into space as the star died, our atom was eventually swept up with other materials to form the Earth, a small planet orbiting the newer star we now call the sun. Volcanoes introduced our carbon atom into the early atmosphere as carbon dioxide (CO₂), and slowly, over millions of years, this CO₂ reacted chemically with rocks, transferring the carbon to limestone that accumulated on the sea floor. Here, our atom sat for many millions of years, until earthquakes, erosion, or other geologic activities returned it to the atmosphere as, once again, CO₂. Slowly but surely, geologic processes on the early Earth cycled carbon from atmosphere to rocks, and back again.

Sometime between 4 and 3.5 billion years ago, our carbon atom began to cycle more rapidly—much more rapidly—as the carbon cycle gained new players. Photosynthetic microorganisms converted CO₂ into organic molecules, and respiring microorganisms returned CO₂ to the environment, completing a rapid biological carbon cycle. To this day, the biological carbon cycle endlessly propels our atomic wayfarer from atmosphere and oceans to cells and back again, while the slow burial of organic matter and limestone in sedimentary rocks continues to bring carbon atoms to the solid Earth, returning it to the atmosphere only slowly, on geologic timescales.

In Part 1, we explored the molecular and cellular basis of life, culminating in a discussion of how genetic variation and natural selection shape evolution. Part 2 examines the products of evolution as it has played out through Earth history: our emerging sense of the immense biological diversity of Bacteria and Archaea; the myriad microscopic eukaryotes found in oceans, lakes and soils; and the millions of plant, fungal, and animal species that define our biological landscape. As we will see, function and diversity underpin the ecological interactions that shape communities, ecosystems, and biomes across our planet.

We begin with an introduction to the carbon cycle, the intricately linked network of biological and physical processes that shuttles carbon among rocks, soil, oceans, air, and organisms. Why start here? Because the carbon cycle provides a fundamental organizing principle for understanding life on Earth. Indeed, it lies at the heart of everything else we discuss in Part 2. The chemistry of life is, to a first approximation, the chemistry of carbon. How organisms move carbon from one species to another, and between organisms and their surrounding environment, underpins both the efficient functioning of ecosystems and their persistence over an immense span of time. In no small part, biological diversity can be understood in terms of the varied ways that organisms obtain the carbon needed for growth and reproduction. Much of ecology, in turn, concerns the ways that organisms interact to cycle carbon and transfer the energy stored in organic molecules. The carbon cycle focuses our attention on the ways that physical and biological processes together determine the properties of environments. And it provides a basis for assessing the role that humans play in our environmental present and future.