One of the late Carl Sagan’s hallmark qualities was to engage in speculation to a degree that was unusual for a rigorous scientist. While this sometimes resulted in largely unnecessary scorn and mockery from his fellow scientists, his honest skepticism combined with his open-mindedness also led to some of the most memorable popular science writing of our times. These qualities are on full display in this fascinating book, written in 1977 . Sagan tackles a topic that is far from his expertise – the evolution of human intelligence – and largely succeeds in presenting highly thought-provoking theses for us to ponder. Much of the book discusses what was then frontier research in neuroscience, but what makes it different are Sagan’s regular speculations.

The book tries to make sense of two important facts about the human brain: our strikingly different cognitive abilities relative to other animals and the interplay between emotion and reason. Sagan is quite upbeat about chimp intelligence and he spends a sizable part of the book talking about experiments that reveal chimps’ prowess in using sign language. He also talks about the mysterious communication used by whales and dolphins that still defies comprehension. Clearly apes can come quite close to using the kind of simple vocabulary that humans do, so why are humans the only ones which actually crossed the language barrier, profiting from breakthrough linguistic inventions like recursive embedding and complex sentence construction? Sagan advances a chilling and all too likely hypothesis, that humans killed off apes who they thought came dangerously close to mimicking their linguistic capabilities. Given how closely language is tied to human intelligence, it then ensured that humans would be the dominant species on the planet; chimps, gorillas and orangutans were presumably saved because they lived deep inside the inaccessible jungle. Sagan’s discussion of animal intelligence hems uncomfortably close to ethical discussions about the killing of animals that are still so pertinent; what gives us the right to clearly assign personhood to a one-month-old fetus but not to a two-year-old chimpanzee, to have serious qualms about terminating the life of the former while cheerfully ending the life of the latter? Coming on the heels of this comparison is Sagan’s commonsense (in my opinion) take on abortion: he tries to reach a compromise by arguing that it should be unethical to kill a human fetus after it develops the first rudiments of a cerebral cortex, presumably the one thing that distinguished humanity from other species. Later work would probably cast some doubt on this assertion, since reptiles have also now been found to possess cortical cells.

This part provides a good segue into the even more interesting part of the book which deals with some fascinating speculations on the reptilian origins of human intelligence. Sagan’s fulcrum for this discussion is a theory by psychiatrist Paul McLean who divided the brain into three parts (the “triune brain”). At the top is the uniquely human cerebral cortex which controls thought, reason and language. The second layer is the limbic system containing structures like the amygdala which modulate emotions like anxiety. The limbic system also includes the basal ganglia and the R complex, an ancient, inherited assembly responsible for instinctive behavior, including responding to reward and punishment. Finally you have the “neural chassis” which just like a car’s chassis includes structures like the brain stem responsible for basic and primitive functions: breathing, blood flow and balance for instance.

Sagan’s focus is the R complex, part of the “reptilian brain”. It is quite clear that parts of this brain structure are found in reptiles. Reptiles and mammals have an ancient relationship; reptiles originated 500 million years before human beings, so we came into a world that was full of hissing, crawling, terrestrial, arboreal and aquatic reptiles. As Sagan describes, it’s no surprise that many of the world’s foremost civilizations and religions used reptiles as key symbols; from the snake in Eden to the worship of snakes in ancient Egypt to snake symbolism in modern day India, reptiles and human have shared an indelible bond. Reptiles have also often featured as omens in dreams dictating the fates of empires and societies. Some of our reptilian connections raise mundane but fascinating questions; for instance, Sagan wonders whether the shushing sound we make for communicating silence or disapproval is a leftover of the hissing sound of reptiles.

But how does this relationship contribute to our behavior? It is here that the book takes off from firm ground and starts gently gliding on speculation.
Sagan’s main springboard for investigating the R complex is Roger Sperry’s seminal work in delineating the separate roles of the left and right hemispheres of the brain. As Sperry demonstrated in amazing split-brain studies, the left brain is more logical and analytical while the right is more synthetic imaginative. Sagan’s contention is that the right brain is really the essence of our reptilian origins, helping us fantasize and imagine, and it’s also a key part of what makes us creative human beings. This is most prominent when we are dreaming. Notice that dreams almost never include details of problem solving, instead they feature highly imaginative scenarios, part familiar and part alien that seem to be largely driven by our fears and hopes: are we partly seeing the world through our ancient reptilian neuroanatomy when we are dreaming, then? Are dreams holdovers from a prehistoric world where, because of inadequate shelter and protection, we had to stay alert and awake during the night to engage with snakes and crocodiles on their own terms? And in the ensuing history of civilization, did reptilian anatomy contribute to our achievements in art and music? Sagan believes that we should encourage the operation of our reptilian brain, constantly tempering its excesses with the logical constraints of the left hemisphere. This distinction between right and left brain behavior also raises very interesting questions regarding whether we can suppress one or another temporarily using drugs and surgery. In fact, it’s likely that that is partly what hallucinogens like LSD do. Here we see Sagan the Renaissance Man, trying to bridge hard scientific thinking with artistic intuition.

With its bold style and engaging language, “The Dragons of Eden” won a Pulitzer Prize. While I was aware of it, I always thought it would be too dated. I now realize that I was wrong and am glad I read it; it has given me plenty of fodder to think about and has prompted me to seek out new research on the topic. The book asks fascinating questions about our kinship with other creatures and about the evolution of our brain, topics that will be of perpetual and consummate interest as long as our species is around.

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The Dragons of Eden: Speculations on the Evolution of Human Intelligence