Life undersea has a mesmerizing strangeness, from glass sponges—lacy matrices draped with cellular nets—to rococo sea dragons and soft corals like trees in a slow wind. It’s the stuff of a thousand documentaries, but for Peter Godfrey-Smith the spectacle is a curtain-raiser to a profound scientific drama, in which the lives of quite un-human creatures illuminate deep mysteries about the nature of sentience, and what it means to possess a mind. In “Metazoa,” the scuba-diving historian and philosopher of science tackles these questions with eloquent boldness, reminding us that “life and mind began in water.” Mr. Godfrey-Smith continues the journey he began in “Other Minds” (2016), which focused on the octopus, the closest we have to an “intelligent alien”: an invertebrate with a big, complex nervous system and capacities for play and adaptation. Now he expands the exploration to multicellular animals as a group—the Metazoa of the title—homing in on those marking key transitions in the evolution of mind. As a biological materialist, Mr. Godfrey-Smith sees consciousness as an evolutionary product emerging from the organization of a “universe of processes that are not themselves mental.” He makes no claim to having cracked the conundrum of how meat gives rise to mind. Instead, to get under the skins of his slithering, bobbing subjects, he builds evidence from the evolutionary record to create a picture of the “different forms of subjectivity around us now.” “Metazoa” sweeps readers from Aristotle through the Darwinian revolution and on to current research into the origins of life, spider cognition, the evolution of warm-bloodedness and beyond. He also revisits philosopher Thomas Nagel’s “What Is It Like to Be a Bat?,” the 1974 essay that famously probed the primal difficulties of understanding subjective experience in other organisms. As in “Other Minds,” Mr. Godfrey-Smith recounts close encounters with marine fauna, gleaned from years of diving off the Australian coast. These have an electric immediacy: He’s often as much observed as observing. At one point he is hovering over a field of starfish when a seal comes in fast, then shoots away “curling and surging through space” to leave him “down there among the stars.” At another, a red-and-white-striped banded shrimp, one limb raised like “a tiny maestro,” looks him full in the face. These immersive moments become springboards into investigations of metazoans over some 800 million years of evolution, using available records and animals now alive as guides. The tree of life (first theorized by Darwin, now more tangled) punctuates the text to clarify ancestry and descent and, importantly, show how all organisms alive now, from ants to us, sit at its top. (The author is careful to point out that the scala naturae—the representation that puts humans on the uppermost rung of a ladder, above other animals—is long defunct.)