Lowdown: Knowledge is what sets humanity apart from all other known living beings. The Human brains owes a lot of its reptilian ancestors, but it has constantly evolved and still is evolving.
Here is a popular science book you won't be able to let go of. Sagan manages it by employing a formula that worked for him before: presenting the latest scientific findings (a relative term in the case of this book, as it was written in the seventies), and then adding his own speculations on top. Sometimes those speculations seem to go over the top, other times they are interesting and thought provoking; but the combination works well because it is exactly what scientific thinking is all about: examining the facts, then coming up with theories to explain them, and then doing the best to counter those theories.
This time around the subject of discussion is human intelligence. Sagan claims that it is our brains that set us apart from all other known living things on the planet, as our brains seem capable of holding significantly more information than what simple beings are capable of storing in their genes or in their inferior brains. It is the knowledge this advantage has allowed us to gather that sets us apart, and it is the knowledge we are now gathering in addition to what we have on us that is continuing to set us apart - things like libraries and computers. Our brains are also the source of the sanctity of each one of us, since they are so complicated that no two brains can be identical to, therefore making each person unique; and our brains also give us the gift of self consciousness, which is not necessarily as good as it may sound, for it also allows us to know that we will die.
Sagan progresses the agenda along step by step, showing the evolution of the brain. If ever one required further proof for the theory of evolution, one need only look at the inside of our heads. As a baby develops in its mother's womb, it first develops gill like members and the brain parts we have in common with fish; then come the reptilian parts, to be followed by the neo-cortex which we share with other mammals but which is the biggest with humans. The only way you can read this and not accept the theory of evolution is if you think Sagan is bullshitting you.
But it's not the size that counts, as other humanoid species have had bigger brains than ours (e.g., Neanderthals). Here Sagan speculates that what separated us from the rest is evolution, again: we had the brain bits to allow us to use language, which in turn allowed for sophisticated tools, which in turn allowed us to exterminate all the primates that could rival us.
The book moves on to examine our brains in detail. It provides illuminating descriptions and theories about the battle for dominance taking place in our heads between the territorial / religious instinct of the reptilian parts, the intuitive right hemisphere of the new-cortex, and the overall dominant logic based left hemisphere. Based upon this battle, Sagan speculates on the physiological aspects of mental illness, learning, and the future evolution of our intelligence - which, according to him, lies mostly with computers and their integration with us.
This overview fails to capture the true spirit of this exhilarating book, which - by progressing step by step through our intelligence - provokes us into committing to utilize our brain in the best way possible and do the unthinkable: think.
Overall: Highly illuminating 4.5 stars.