Worms and other structurally simple animals have simple, single-chambered hearts. Fish have a two-chambered heart. Reptiles and amphibians generally have three-chambered hearts. Mammals and birds have four-chambered hearts, with the four chambers arranged differently in each group. Explore Evolution presents this arrangement as a mystery, unbridgeable by evolutionary means.

Contrary to the account in this chapter, the evolution of the four-chambered heart is no mystery. Many researchers have addressed elements of the question, but rather than addressing these well-tested models, Explore Evolution makes passing reference to their existence and reliance on specific processes in developmental biology, but never gives enough detail for students to understand those models. Having given an inadequate account of mainstream models of heart evolution, the book then criticizes those models, again without offering enough detail for students to evaluate or even fully understand the critique. Rather than encouraging inquiry, this approach forces students to simply learn by rote that certain things cannot evolve, leaving students with no basis for understanding future advances in the evolution of hearts and other anatomical structures.

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