A conversational exploration of the possibility of life beyond Earth. Within a section on how the Earth may have been visited long ago by “voyaging aliens,” perhaps even before humans had evolved, Wall writes, “let’s indulge in some wild speculation, because it’s fun!” Though his byline as a senior writer for includes his doctorate in biology, the speculation throughout his first book is more playful than scientifically rigorous, as befits a book with a bibliography that includes The Hitchhiker’s Guide to the Galaxy, The Physics of Star Trek, and Alien Invasion: How to Defend Earth. Each chapter title poses a question: “Are We All Martians?”; “Do Aliens Have Sex?”; “Will Aliens Kill Us All?”; “Is Time Travel Possible?” In nearly every case, the answer is a qualified maybe. Instead of asking “why?” Wall asks, “why not?” Since the universe is so vast and there are so many possibilities of planets or moons or other bodies that might possibly sustain life, or once could have, isn’t it more likely than not that life exists somewhere other than here? Again, one answer could stand for all: “We just don’t have enough information at the moment to know what’s actually going on.” If there are aliens out there, they might be microbes or they might be more like machines. They might be hostile, in which case, “engaging an alien colony ship over Los Angeles or London, even with humanity’s full complement of 15,000 nuclear weapons, would be comically fruitless, like fending off a charging rhino with a spatula.” Or, perhaps, “advanced aliens may be silently monitoring us from afar like cosmic peeping toms, waiting for us to show that we want to talk.” Somewhere between science fiction and science fact, there’s a wide range of possibility concerning life in the great unknown.