Evolution

O far, you have learned that Earth’s changes

Have been carefully recorded in stratigraphic layers and that we can use correlation techniques to decipher the “chapters” and “pages” of Earth’s long saga. That narrative, however, is written not just in rock layers but also partly in biologic organisms.

Life forms are the source of two important types of information about Earth history. First, they are the most obvious outcome of biologic evolution over geologic time spans. Second, life forms carry a superb repository of evolutionary information—their genetics. The evolutionary history of every living organism is succinctly recorded in its genetic code. This code is contained in DNA molecules such as the one shown here. Researchers can use that information to determine species relationships and even to estimate when organisms diverged from each other.

In this chapter, you will learn how biologic evolution has left its mark in the great diversity of fossil organisms through geologic time. You will also learn about two factors that govern how evolution proceeds and that influence the morphologic outcome: natural selection and inheritance. Understanding these central concepts of evolutionary biology provides the metaphorical “character development” that is essential to understanding the story written in Earth’s stratigraphic “pages.”