The focus of this book is the evolution of cardiovascular and respiratory control in vertebrates. Life originated in water, which has constantly changing temperatures and O₂ levels. Fish gills can extract up to 80% of their inspired O₂, because they have a countercurrent bloodstream. Oxygen sensors have been found within the gill arches of ray-finned fish such as carp and trout, and these O₂ sensors screen the inspired water and the capillary blood.

Very likely, land vertebrates and the lungfish arose as a sister group, and both possess real lungs. Lungfish include 6 species, inhabiting shallow lakes or rivers, whereas the second ramification includes all the land vertebrates. A possible ancestor to the lungfish and land vertebrates has been discovered in China, and this fossil (Styloichthys) bridges a gap. Living 417 million years ago, it could represent one of the last ramifications before the common ancestor to the lungfish and land vertebrates. In addition, rather constant atmospheric O₂ levels permit a joint acid-base regulation by the lung and the kidney. Likewise, lungfish and land vertebrates share a central control of pulmonary ventilation, while the peripheral receptor contribution to acid-base regulation is minor.