There are many ways of being alone. You can be alone in a room, a house, even a crowd. You can be really alone in a wilderness, or really, really alone in the universe.It’s that last, existential and cosmic loneliness that astronomers and specialists in astrobiology have in mind when they ask “Are we alone?” In his book “The Zoologist’s Guide to the Galaxy,” Arik Kershenbaum, a zoologist and lecturer at Girton College, University of Cambridge, takes a novel and rewarding approach to this question. He is not too concerned about the evidence for or against the existence of extraterrestrial life; rather, he is interested in hypothesizing about what forms it might take, given what we know about conditions on other worlds. Instead of Enrico Fermi’s famous question, But where is everybody? Mr. Kershenbaum asks: What would everybody be like? There are good reasons to think that we may not be alone. Humans and other earthlings exist, so life itself—for all its seemingly unique characteristics—isn’t altogether unimaginable. And there are many other planets, almost certainly in the hundreds of millions, perhaps billions, including a very large number that appear to be roughly similar to Earth. Have we any basis to presume our planetary life forms are so special? In 2017 astronomers were perplexed by an object first spotted by the telescope on Mount Haleakala, Hawaii. It had many traits that appeared to distinguish it from other extragalactic objects that occasionally enter our planetary neighborhood: unusual shape, rotation and speed. It was dubbed “Oumuamua” the Hawaiian word for “scout,” and although most scientists doubt that it was a scout from another civilization, at least one highly regarded astronomer thinks it was. As befits a good biologist, Mr. Kershenbaum presents insights informed by what we know about the process of evolution by natural selection. He argues that, although the details will necessarily vary from one exoplanet to another—whether life might be based, say, on silicon, or whether gravity will be stronger or weaker than on Earth—life most likely will be subject to the basic principles of variation, selective retention and reproduction. Whatever the specific planetary environment, some sort of evolutionary mechanisms could very well be inevitable. If so, there should be interplanetary commonalities when it comes to biology—just as there appear to be shared patterns of chemistry, physics and mathematics that apply to the rest of the universe’s inanimate objects, from subatomic particles to black holes. Photo: WSJ “A zoologist observing a newly discovered continent from afar,” Mr. Kershenbaum writes, “would be buzzing with ideas about what kind of creatures might live there. Those ideas wouldn’t be wild speculations, but keenly reasoned hypotheses based on the huge diversity of animals we already know, and how each animal’s adaptations are well suited for the life they live: how they eat, sleep, find mates and build their dens. The more we know about how animals have adapted to the old world, the better we can speculate about the new.” “Zoologist’s Guide” is a wonderful mix of science-based speculation and entertaining whimsy. Each chapter examines a different aspect of animal behavior on Earth that, according to the author, would likely be replicated on other planets: sociality, cooperation, communication, intelligence, language and so forth. He’s always mindful to anchor his conjectures on what is already known—not just about well-studied terrestrial species but, crucially, about universal (hence universe-wide) evolutionary principles. Mr. Kershenbaum proceeds to argue, persuasively, that “we have enough of a diversity of adaptations here on Earth to give us at least potential mechanisms that seem appropriate solutions even on worlds almost unimaginably different from ours.” That may lead the reader to conclude that extraterrestrial creatures, however exotic, will resemble their Earthbound counterparts in recognizable ways. They may have long, short, flexible or jointed appendages, but nevertheless would have some sort of protuberant structures used for locomotion or manipulation. They might have big, little, single, multiple, round, slitted or geometric eyes, but in any case would need some devices for apprehending what we call visible energy. Recall the justly beloved cantina scene in the first “Star Wars” movie, in which the diverse denizens were all, in some way, “animal.” Mr. Kershenbaum doesn’t go that far, sidestepping the temptation to make assumptions by concerning himself with how aliens would behave rather than how they would appear; that is, their functions rather than their forms. Thus, when he hypothesizes about alien language, he focuses on the presumed universal payoff of communication, without speculating about, say, dialects of Klingon, à la “Star Trek.” “If alien animals use sound for their alarm calls, their screams will probably be very much like ours,” he writes. “Don’t believe it if they say ‘no one can hear you scream’—screams evolved to be heard, and to be disturbing. Even if aliens don’t use sound, it’s likely that alien alarm calls will be similarly chaotic in whatever medium they do use. They will have whatever properties are characteristic of the alien signal-production organ when you jump out from behind a rock and give the alien a fright. ‘Scary’ is going to be similar on every planet.” A skeptic—even if one accepts the prospect of complex extraterrestrial life—might object that “Zoologist’s Guide” is too steeped in Earth-centrism. But one benefit of Mr. Kershenbaum’s method is that, while imbibing credible theories about the possible nature of extraterrestrial life, the reader will learn much about the actual nature of life on Earth. The book’s title is meant to be suggestively similar to Douglas Adams’s marvelous sci-fi fantasy “The Hitchhiker’s Guide to the Galaxy.” Readers won’t discover “Hitchhiker’s”whimsical answer to the ultimate question of life, the universe and everything (spoiler: it’s 42). But Mr. Kershenbaum’s answers to the questions he raises are every bit as original and have the added advantage that they could well be true. Mr. Barash is a professor of psychology emeritus at the University of Washington. His most recent book is “Threats: Intimidation and Its Discontents.”