One day in 1950, in the midst of lunch with colleagues, Italian/American physicist Enrico Fermi suddenly asked, "Where is everybody?"

Fermi wasn't referring to the lunch crowd. What Fermi was pondering was why-with so many Sun-like stars in our Galaxy, and the potential of so many Earth-like planets-our Galaxy wasn't awash with signs of extraterrestrial life.

This apparent contradiction between the high probability of, but total lack of evidence for, extraterrestrial civilizations became known as Fermi's Paradox.

One possibility is that ET exists, but we just haven't made contact yet. The book considers the Search for Extraterrestrial Intelligence (SETI).

The other possibility is that intelligent life is rarer than we would suppose. But with so many potential sites where it could arise, the question becomes Why hasn't it arisen? Or, perhaps more pertinently, Why did it arise on Earth and not elsewhere?

This book sets out to answer that question: How do you start with rocks, gasses, and water, and end up with humans? The approach is to describe how geology (e.g. plate tectonics) and astronomy (e.g. asteroid impacts) create habitat, and how changes to habitat influence biology. It begins with the formation of the Earth, and the origin of life from non-life. It ends with the evolution of humans from non-humans. In between it follows the trail of our ancestors: single-celled organisms, multicellular organisms, vertebrates, mammals, apes.

When it finally revisits Fermi's question about the absence of extraterrestrial civilizations, it does so by demonstrating that it is, instead, the presence of intelligent life on Earth that we should consider so extraordinary. . .