The bottom line

This chapter summarizes the most important aspects of the entire book. Writing an abstract of a summary can result in a ‘bouillon cube’ of information that is nearly incomprehensible, so this sticks to the most far-reaching observations and conclusions. The structure–function unit referred to here as the respiratory faculty did not just suddenly appear, but rather bits and pieces of it are recognizable even in most basally branching metazoan lineages. The use of mitochondria in an aerobic atmosphere to produce large amounts of energy-carrying molecules precipitated a kind of arms race, whereby the individuals that could compete better for food sources or become predatory could become part of an evolutionary cascade. These new animals moved into another realm, but the old ones did not necessarily disappear: they just did what they always did, maybe a little better. In the most diverse lineages of invertebrates and craniotes we see similar changes appearing: gills with counter-current exchange, highly specialized oxygen-carrying proteins, a partly or completely closed circulatory system that includes the gas exchange organs, lungs. The more extreme the grounds for specialization, the more similar are these structures and functions. Often the functional result remains unchanged or becomes improved while the anatomical cause changes dramatically, but just as often structures change little but minor functions become major ones: a phenomenon called exaptation. This book has looked at most major animal groups and these principles turn up everywhere. It talks about multidimensional forces at work in a multidimensional world, and respiration is the keystone to it all.