In presenting [the theory of the triune brain in The Dragons of Eden], Sagan encourages the reader to examine human intelligence and behavior in terms of the elements they have in common with other living animals. While this is, in itself, appropriate, the association of our behavior with that of other living animals distorts the modern concept of evolution. Humans did not evolve from contemporary snakes (nor even from extinct dinosaurs). Rather, all currently living beasts had, in the unfathomable past, common ancestors who gave rise to many different evolutionary lines. Reptiles—extinct or alive—are not our evolutionary precursors. If anything, they are evolutionary siblings that have grown in a different environment. (pp. 319-20)

These points are not made clearly, if at all, by Sagan. His presentation is reminiscent of a simplistic and largely discounted 19th-century theory that stated, "ontogeny recapitulates phylogeny": in other words, in the course of embryological development, each animal goes through stages that resemble evolutionary precursors, and characteristics are sometimes maintained. So, humans would have retained the R-complex [a section of the triune brain] and reptilian behavior from the "lower" reptiles and elaborated upon them. Evolution is not that simple, and man most likely evolved from something very different from a reptile. One could as easily—and as erroneously—state that living reptiles have, in part, humanoid brains and behaviors.

To balance this criticism, I highly recommend Sagan's fifth chapter, entitled "The Abstractions of Beasts." This chapter proves quite simply that chimpanzees can "talk" to their human trainers using American Sign Language. In a clear, fascinating report of work being done at several institutions, Sagan points out that nonhuman primates are "close to the edge of language" and can communicate when taught the "language."

Sagan uses the chimpanzee-learning-language story to indicate that these primates are intelligent. But surely communication in human terms is not the only, or primary, sign of animal intelligence. Rather, one must look for "intelligence" in an animal by examining the way it copes with its own environment, not in the way it relates to aspects of a human environment….

A concluding chapter of this entertaining and thought-provoking book considers extraterrestrial intelligence and is only a taste of the subject on which Sagan is perhaps the authority. (p. 320)