Much later, multicelled gelatinous animals evolved neurons. Nerves underneath coordinated cells’ “tiny contractions, contortions and twitches” into propulsive pulsing. Other nerves wired light-sensing organs above to coordinate day-night rhythms. The motion-controlling system may have eventually entangled the light sensors, whence light sensors aided motion guidance. Thus neurons convened into a “chemo-electrical storm of repurposed signaling” — brains. (Deadly evidence: Box jellyfish, some of whose two dozen eyes have lenses and retinas like ours, can navigate by watching landmarks on the shore as they pulse along at three knots.)

Then, a half-billion years ago, Cambrian animals first watched, seized and fled other animals. Senses, nervous systems and behaviors escalated an arms race against the senses and behaviors of others. If a yardlong cockroach-looking appetite with two graspers on its head is swimming rapidly at you, “it’s a very good thing to know, somehow, that this is happening, and to take evasive action.” With better sensory processing and a need for decisions (fight or flee), the Cambrian delivered Earth’s first information revolution. “From this point on,” Godfrey-Smith emphasizes, “the mind evolved in response to other minds.”

Amid explosive evolution, you’d assume that speedy, grasping creatures evolved often. Surprisingly, of about 34 basic animal body plans (phyla), only arthropods (insects, crabs), vertebrates and one subgroup among mollusks — cephalopods — evolved “complex active bodies.” Only vertebrates and cephalopods developed large, complex nervous systems.

Contrary to some philosophers’ assumptions, consciousness doesn’t just project out; it is a relationship in traffic with the outer world. Consciousness did not “suddenly irrupt into the universe fully formed,” Godfrey-Smith says. “Perception, action, memory — all those things creep into existence from precursors and partial cases.” Asking whether bacteria perceive or bees remember “are not questions that have good yes-or-no answers.” From minimal to elaborate sensing there’s a continuum, “and no reason to think in terms of sharp divides.”

How then did feeling begin? Rudimentarily, Godfrey-Smith asks: “Does damage feel like anything to a squid?” Does injury feel bad to a lobster or a bee? Well, insects don’t groom or protect injured parts of their bodies. But injured crabs, shrimp and octopuses do. Injected with a chemical thought to spark pain, zebra fish prefer water with a dissolved painkiller; so yes, fish feel pain. Godfrey-Smith says pain, hunger, thirst and other “primordial emotions” do not require worldviews. If they are felt, this by definition is sentience.

If this is philosophy, it works, because Godfrey-Smith is a rare philosopher who searches the world for clues. Knowledgeable and curious, he examines, he admires. His explorations are good-natured. He is never dogmatic, yet startlingly incisive. His refreshing guidance invites us, allowing breathing room, to consider, occasionally to respectfully disagree.

Nervous systems compose “a symphony of tiny cellular fits, mediated by sprays of chemicals across the gaps where one cell reaches out to another.” Most basically, brains coordinate muscles into motion. In separate elaborations they process and integrate sensory inputs and unify worldly orientation, managing biorhythms and hormones. When surviving requires decision making, brains have developed awareness. “Sentience,” Godfrey-Smith writes, “has some point to it.”