Dragons of Eden: Does Our Brain Account for Our Human Nature?

The Dragons of Eden: Speculations on the Evolution of Human Intelligence involves astronomer Carl Sagan stepping outside his expert realm to bring immense insight on the biology of being human. Sagan takes us through the evolution of the human brain, from reptile to mammal, comparing us along the way to our closest relatives, tracking the differences and similarities in our ancestors, and even hypothesizing the future of the human nature. According to his “Cosmic Calendar,” which positions the origin of the universe (whose estimate was 15 billion years ago, when this book was written in 1977) at January 1st and the present day as December 31st, reptiles did not appear until December 23rd, mammals until the 26th, and Homo sapiens until 10:30 PM on December 31st (13-14). Perhaps no other metaphor scales the entire history of the universe to a more fathomable quantity, and no other metaphor so elegantly puts us humans in our place.

Carl Sagan

The first theme of the book is distinguishing the quality of information from the quantity of information. Sagan points out that a rabbit’s brain is 100 times smaller than that of a human, but that does mean a human is 100 times more intelligent than a rabbit, and 100 rabbits are not as smart as a human. Surely humans have a high ratio of brain mass to body mass when compared to other animals, but Sagan focuses more on how our neurons and synapses are arranged in our brain for the most efficient means of information retrieval. Though human DNA contains a huge amount of information, their amount is constrained by mutation rates, where higher amounts of genes leads to higher mutation rates, and thus a balance is formed. Sagan points out that “large and complex organisms, by the mere fact of their existence, have to have substantial resources of extragenetic information. That information is contained, in all higher animals except Man, almost exclusively in the brain” (24-25). To me, these words ring true of cultural evolution, aka the meme. Computers were in their infancy when this book was published, so Sagan’s estimate of the human brain being ten thousand times more densely packed with information than a computer (41) is likely very off. I do not know what a better estimate would be in the present day.

Sagan’s calculations lead to very interesting mathematical findings. The number of mental states can be calculated by 2^n, where is the number of synapses. The human brain is characterized by some 10^13 synapses, so the total possible mental states is 2 raised to that power. This number is actually larger than the total amount of electrons and protons in the entire universe, which is around “only” 2 raised to the power of 10^3. Sagan postulates that “These enormous numbers may also explain something of the unpredictability of human behavior and those moments when we surprise even ourselves by what we do… all possible brain states are by no means occupied; there must be an enormous number of mental configurations that have never been entered or even glimpsed by any human being in the history of mankind. From this perspective, each human being is truly rare and different and the sanctity of individual human lives is a plausible ethical consequence” (38).

In charting the evolution of the brain, Sagan uses an analogy from Plato’s Phaedrusin which Socrates “likens the human soul to a chariot drawn by two horses–one black, one white– pulling in different directions and weakly controlled by a charioteer” (73). He relates one horse to the reptilian (R-)complex, the most basal part of the brain, one horse to the limbic system, the part responsible for basic and vivid emotions, and the careening charioteer as our neocortex, the large part of the brain which accounts for many of our behavioral intricacies, such as morality. Sagan hypothesizes that altruistic behavior has its beginnings in the limbic system, since mammals and birds are the only (with exceptions such as social insects) organisms to “devote substantial attention to the care of their young… Love seems to be an invention of the mammals” (58-59). Interestingly, Sagan goes further in wondering how far such emotions go: “Do horses on occasion have glimmerings of patriotic fervor? Do does feel for humans something akin to religious ecstasy? What other strong or subtle emotions are felt by animals that do not communicate with us?” (60).

In the ethically stimulating chapter on whether other animals, mainly chimps, are capable of abstraction, Sagan makes a strong case for the rights of animals: “If they are ‘only’ animals, if they are beasts which abstract not, then my comparison is a piece of sentimental foolishness. But chimpanzees can abstract. Like other mammals, they are capable of strong emotions. They have certainly committed no crimes. I do not claim to have the answer, but I think it is certainly worthwhile to raise the question: Why, exactly, all over the civilized world, in virtually every major city, are apes in prison?” (113). It is especially interesting to see the abilities of chimps to manipulate language. A great point is made that chimps are not “dumb” because they cannot talk; they cannot talk because their mouths and throat did not evolve for human language. Thus, the great dexterity of apes allows for sign language to be the best mode of chimp to human communication. Sagan captures my thoughts exactly when he postulates, “One of the most intriguing questions is whether a verbally accomplished chimpanzee mother will be able to communicate language to her offspring. It seems very likely that this should be possible and that a community of chimps initially competent in gestural language could pass down the language to subsequent generations” (109). At the time of publication, no studies had been accomplished, due to the danger of keeping adult chimpanzees; all experiments are abandoned when the chimp reaches puberty.


At the conclusion of the book, Sagan surmises about the future of our intelligence. His words ring like prophecy: “Unless we destroy ourselves utterly, the future belongs to those societies that, while not ignoring the reptilian and mammalian parts of our being, enable the characteristically human components of our nature to flourish, to those societies that encourage diversity rather than conformity, to those societies willing to invest resources in a variety of social, political, economic, and cultural experiments, and prepared to sacrifice short-term advantage for long-term benefit; to those societies that treat new ideas as delicate, fragile and immensely valuable pathways to the future” (182). Sagan is right to say that societies did not evolve to be dynamic and progressive; they evolved to be stable and static. But we can break free from the control of our genes, and we know better than to stay still. What is “natural” is not always right.

Like many others, Sagan criticizes the abundance and resurgence of ignorance in modern society: “There is today in the West (but not in the East) a resurgent interest in vague, anecdotal and often demonstrably erroneous doctrines that, if true, would betoken at least a more interesting universe, but that, if false, imply an intellectual carelessness, an absence of tough-mindedness, and a diversion of energies not very promising for our survival… These are by and large, if I may use the phrase, limbic and right-hemisphere doctrines, dream protocols, natural– the word is certainly perfectly appropriate– and human responses to the complexity of the environment we inhabit. But they are also mythical and occult doctrines, devised in such a way that they are not subject to disproof and characteristically impervious to rational discussion” (224). Thus, we as a human race should give up this “intellectual carelessness” if we want to survive. A New Enlightenment has never been more needed.

  • The Dragons of Eden by Carl Sagan can be purchased here from Amazon.com
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