The main hypothesis of the book is that civilization as we know it would not have been possible without domestication of animals. Each chapter of the book is dedicated to a different animal or group of animals, from animals used in agriculture (as source of food and materials or as source of labor power) to animals used as companions. While initally, Caras still (if superficially) touches upon the question how the animals impacted the development of societies and civilization as a whole, he increasingly seems to lose sight of it throughout the book. As a result, the chapters in the second half of the book are mostly overviews of where and when certain animals were domesticated or used in some way. While these overviews certainly have some value and no doubt contain some interesting facts (e.g. that the Romans kept dormice in jars and ate them, or that, again, the Romans kept mongooses as pets to deal with snakes and rodents), they cannot serve as proof of Caras' initial hypothesis.
Where he does discuss that hypothesis in more detail, he does so by bringing up more hypotheses that themselves would need to be proven first: without the domestication of animals, humans can not have a healthy diet and therefore end up suffering from diseases or deficiencies; without domestication of animals, societies would have remained isolated and therefore wouldn't have developed new ideas or new technologies; without companion animals, humans prone to stress and high blood pressure tend to die early and not have many offspring (!).
Somewhat unrelated, throughout the book, the author speaks of "man" and "his/him" when referring to humans, only sometimes mentioning women explicitly - to improve readability, as he points out in the beginning.
Finally, the title for the book is chosen poorly, as Caras himself mentions repeatedly how badly humans treat animals.