The evolution of large, multicellular, complex metazoans from their simple unicellular animal ancestors comprises a rich history covering many hundreds of millions of years. Pieced together from fossil remains, DNA analysis, and comparative morphological studies of extant animals, biologists have described a staggering array of adaptations that have arisen (and in some cases been lost and arisen again) through the course of animal evolution. Describing the specific evolutionary pathways from unicellular organisms to human is, of course, well beyond the scope of this chapter. Rather, in this chapter, we focus specifically on the general evolutionary pathway leading to the cardiovascular systems of extant animals, especially highlighting the general principles that continue to shape cardiovascular form and function to this day. In doing so, we intend to create a backdrop and context for other chapters in this book that present more specific information on the vascular endothelium in specific groups of animals (e.g., fish, amphibians, reptiles, birds, and mammals).

Before delving into cardiovascular systems and their evolution, we first must address a semantic issue regarding our use of the term endothelium. The invertebrate literature is replete with discussions of “endothelium” – whether it exists, whether it is discontinuous or continuous, what functions it serves, and the like. Much less often, the vascular lining of blood spaces (where it occurs) in invertebrates is described more generically as a “vascular cellular lining.” Of note, the position taken in this book is that “true” endothelium is found only in vertebrates, and it has distinguishing characteristics, such as being derived from a lateral plate mesoderm, possessing caveolae and Weibel-Palade bodies, and more.