Around 370 million years ago, a distant relative of a modern lungfish began the most exciting adventure the world had ever seen: it emerged from the water and laid claim to the land. Over the next 70 million years, this tentative beachhead became a worldwide colonization by an ever-increasing variety of four-limbed life. These first "tetrapods" are the ancestors of all vertebrate life on land. Gaining Ground tells the rich and complex story of their emergence and evolution. Beginning with their closest relatives, the lobefin fishes such as lungfishes and coelacanths, Jennifer A. Clack defines the characteristics of tetrapods, describing their anatomy and explaining how they are related to other vertebrates.

Clack looks at the Devonian environment in which tetrapods evolved, describes the known species, and explores the order and timing of anatomical changes that occurred during the fish-to-tetrapod transition. She reports that older ideas about the transition are being overturned by recent discoveries and new ideas about evolutionary change. Following the story through the Carboniferous period, she shows how the evolution of terrestrial characters occurred several times, convergently, among different groups.