At first, Dr Asimov establishes what sort of intelligence this book will be discussing, as well as why we cannot find it on Earth (aside from humans). These questions are related to both historical accounts and fiction that reflects upon the questions. The second world that we explore is The Moon, and how it played a role in encouraging speculation about life on other planets.
Following the scientific lunar investigations, Dr Asimov establishes that for intelligent life as we know it, water is a requirement and an atmosphere is required for liquid water. The inner planets are assessed for their likelihood of possessing the requirements for life, and declared lifeless, so he goes further. In analyzing the inner planets, Dr Asimov discusses organic molecules, and explains why they form the basis of known life. These molecules are contrasted with hydrocarbons (like methane) and other volatile substances. With these established, where in the outer planets could life form?
With the solar system having strong evidence for a lack of intelligent life (aside from humans), Dr Asimov takes the investigation to other star systems, and introduces a new requirement for life/civilization; energy. From the Milky Way to other galaxies, he speculates and explains the formation of planets and their associated stars based on then-current astronomical understanding.
At this point, though, he's determined that there should be an estimated 390 million civilizations that are technologically more advanced than us. So... "Where is everybody?" (page 171). This leads into the question of where his calculations were in error; is the habitable zone simply that small, is the moon more important for the formation of life than otherwise assumed, or is intelligence not a natural product of evolution? Dr Asimov proceeds to analyze various factors of pressure upon intelligence, giving evidence for why sentience eventually becomes sapience and discussing great filters against widespread civilizations, but still coming to the conclusion that extraterrestrial civilizations should be out there, somewhere.
What follows after this is theory-crafting and encouragements. The "Space Age" is upon us; we've been to the moon, and there are several more targets we can reach for colonizing. How will we send humanity to our neighboring stars? How can we communicate with other civilizations that exist now or will exist at those distances? Our goal should be to inherit the universe, not merely squabble over a pale blue dot.
Extraterrestrial Civilizations provides examples of:
- Lunarians: (Discussed Trope) Dr Asimov cites a number of fictional works that make use of a civilized Moon.
- The Man in the Moon: (Discussed Trope) Dr Asimov credits the illusion of a man (or a woman, or a rabbit, and so on) in The Moon for inspiring humanity's search for civilizations from alien worlds.
- No Fourth Wall: Dr Asimov writes this Non-Fiction with very little reference to himself as narrator or you as audience, but occasionally addresses the reader directly. This results in many tropes being Discussed based on their real-life inspiration.
- (Discussed Trope) Life on Earth was begun by spores traveling through the universe is the next theory advanced after spontaneous generation of life is discarded. It,in turn, is also discarded, because of the complications required for such microscopic life to both survive the trip to a second planet in a new solar system and the fact that it merely displaces "how does life originate?" to a different planet.
- A footnote describes how Fred Hoyle has advanced the theory that comets, approaching close enough to Earth's orbit, are the source of viral pandemics.
- Talking Animal: (Discussed Trope) When Dr Asimov explores the concept of human-level intelligence in animals, the inability of animals to communicate is a reason why they are excluded. Several works of fiction are used to illustrate humanity's tendency to anthropomorphize animal intelligence.
- Science Marches On: Dr Asimov lampshades the progress of astronomical science, pointing out that Pluto was considered larger than 2,900 kilometres (1,800 miles) until 1978, when Charon was observed and Pluto was discovered to be much smaller than previous estimations.
- 2001: A Space Odyssey is referenced because of its motif of aliens observing Earth's evolution from hominids to space travel.
- Aesop's Fables is used to reference the Talking Animal trope.
- Isaac Asimov:
- The Collapsing Universe is mentioned several times as a resource readers might be interested in for details on stars at extreme ends of size/mass, like black holes.
- Lucky Starr and the Oceans of Venus is mentioned as an example of the scientific assumption that Venus Is Wet affecting Science Fiction until 1956.
- The Bible:
- Genesis 3:1 is used to illustrate how humans attribute human-level intelligence in animals.
- Numbers 15 is referenced for the Fanon that arose during medieval times that the man gathering sticks during Sabbath was punished by being sent to the moon, becoming The Man in the Moon.
- The creation story is referenced in chapter nine at length, providing the basis for the early scientific theory of life being created through spontaneous generation.
- Classical Mythology is used as an example of early anthropomorphism, giving human intelligence to aspects of nature.
- William Congreve is referenced for his opinion that monkeys fall into the Uncanny Valley.
- Charles Darwin is referenced for both On The Origin Of Species and The Descent Of Man.
- Dr Dolittle, by Hugh Lofting, is mentioned as a story with Talking Animal.
- Human Destiny, by Pierre Lecomte du Nouy, is cited as a work that argues against the spontaneous generation of life theory on the basis that the molecules important to life are too complex to arise from random chance. Dr Asimov then relates a series of experiments by Stanley Lloyd Miller who proved the construction was due to reliable chemical interactions and not random chance.
- The Man In The Moone, by Francis Godwin, is referenced for being a story where characters travel to the moon. It is also contrasted against a Non-Fiction book published in the same year, The Discovery Of A World In The Moone by John Wilkins.
- The titular Mickey Mouse is mentioned as an example of how human characteristics are seen in nonhuman animals.
- A Midsummer Night's Dream, by William Shakespeare, is credited for referencing the Christian Fanon of the origin on The Man in the Moon.
- Odyssey is used as an example of spirits initially inhabiting Earth, before the concept of "afterlife" moving to an underground/alternate world.
- The Origin Of Life, by Aleksandr Ivanovich Oparin, is used to illustrate the scientific theory that the primordial atmosphere of Earth was vastly different from today's atmosphere (Dr Asimov explains that our world has had three substantially different atmospheres).
- Orlando Furioso, by Ludovico Ariosto, is referenced as a story where a character travels to the moon and finds it populated by a human civilization.
- Reynard the Fox is used to illustrate how humans attribute human-level intelligence in animals.
- The Roots Of Civilization, by Alexander Marshak, is referenced as a claim for humans keeping track of lunar activity.
- Somnium, by Johannes Kepler, is referenced for being a story where a character travels to the moon (in a dream) and for providing an accurate account of a lunar day length.
- Stonehenge Decoded, by Gerald Hawkins, claims Stonehenge was used as a prehistoric astronomical calendar.
- True History, by Lucian Of Samosata, is referenced as a story wherein a character travels to the moon and finds other worlds with civilizations on each of them.
- Uncle Remus, by Joel Chandler Harris, is mentioned as a story with Talking Animal.
- Voyages To The Moon And The Sun, by Cyrano De Bergerac, considered several ways to travel into space, although the main character eventually used a method that would prove unworkable in real life.
- H. G. Wells:
- The First Men On The Moon is referenced as a story where the protagonists travel to The Moon and discover Insectoid Aliens living underground.
- War of the Worlds is referenced for being based partially on the astrometric reports OF Mars by Percival Lowell and partially on how the European nations were dividing the continent of Africa. It is also recognized for popularizing the idea that First Contact would mean the potential eradication of humanity.
- Worlds In The Making, by Svante August Arrhenius, is referenced for being the first suggestion of Panspermia.
- Uncanny Valley: (Discussed Trope) When comparing humans to other primates, Dr Asimov quotes William Congreve for his 1695 statement on finding disturbing similarities between monkeys and humans."I could never look long upon a monkey, without very mortifying reflections." — William Congreve