Sagan begins the book with a broad history of human civilization told as a history of wandering. Humans, for 99.9% of their history, were hunters and foragers, nomads who migrated to avoid bad weather or find food. It is only in the last 10,000 years, after domesticating plants and animals, that humans started to settle in permanent homes. But humans continued to explore, each group discovering continents and peoples that were previously unknown in their part of the world. These discoveries led to new knowledge and perspective regarding their place in the Universe. Sagan shows that every region of the world had its explorers and argues that the desire to explore is universal.

People continue to migrate. They flee war, famine, and oppression. Changing climate will continue to reshape where people will want to live. But for the most part, Earth is settled, and Sagan frames this as a problem: “Victims of their very success, the explorers now pretty much stay home” (xvi). Sagan surmises that this comparatively new, non-transitory lifestyle causes humans to feel unfulfilled, romanticize far-off places, and crave discoveries. He suggests that humans have an evolutionary need to wander.

Sagan draws a parallel between migration on Earth and exploration into the solar system.