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path forward from that underwater split leads to our branch of the tree. It leads to vertebrates, among others, and within the vertebrates, to mammals and eventually humans. The other path leads to a great range of invertebrate species, including crabs and bees and their relatives, many kinds of worms, and also the mollusks, the group that includes clams, oysters, and snails. This branch does not contain all the animals commonly known as “invertebrates,” but it does include most of the familiar ones: spiders, centipedes, scallops, moths.

      In this branch most of the animals are fairly small, with exceptions, and they also have small nervous systems. Some insects and spiders engage in very complex behavior, especially social behavior, but they still have small nervous systems. That’s how things go in this branch – except for the cephalopods. These are a subgroup within the mollusks, so they are related to clams and snails, but they evolved large nervous systems, and the ability to behave in ways very different from other invertebrates. They did this on an entirely separate evolutionary path from ours.

      Cephalopods are an island of mental complexity in the sea of invertebrate animals. Because our most recent common ancestor was so simple and lies so far back, cephalopods are an independent experiment in the evolution of large brains and complex behavior. If we can make contact with cephalopods as sentient beings, it is not because of a shared history, not because of kinship, but because evolution built minds twice over. This is probably the closest we will come to meeting an intelligent alien.

      ~ Outlines

      One of the classic problems of my discipline – philosophy – is the relation between mind and matter. How do sentience, intelligence, and consciousness fit into the physical world? I want to make progress on that problem, vast as it is, in this book. I approach the problem by following an evolutionary road; I want to know how consciousness arose from the raw materials found in living beings. Aeons ago, animals were just one of various unruly clumps of cells that started living together as units in the sea. From there, though, some of them took on a particular lifestyle. They went down a road of mobility and activity, sprouting eyes, antennae, and means to manipulate objects around them. They evolved the creeping of worms, the buzzing of gnats, the global voyages of whales. As part of all this, at some unknown stage, came the evolution of subjective experience. For some animals, there’s something it feels like to be such an animal. There is a self, of some kind, that experiences what goes on.

      I am interested in how experience of all kinds evolved, but cephalopods will have special importance in this book. This is firstly because they are such remarkable creatures. If they could talk, they could tell us so much. That is not the only reason they clamber and swim through the book, though. These animals shaped my path through the philosophical problems; following them through the sea, trying to work out what they’re doing, became an important part of my route in. In approaching questions about animal minds, it is easy to be influenced too much by our own case. When we imagine the lives and experiences of simpler animals, we often wind up visualizing scaled-down versions of ourselves. Cephalopods bring us into contact with something very different. How does the world look to them? An octopus’s eye is similar to ours. It is formed like a camera, with an adjustable lens that focuses an image on a retina. The eyes are similar but the brains behind them are different on almost every scale. If we want to understand other minds, the minds of cephalopods are the most other of all.

      Philosophy is among the least corporeal of callings. It is, or can be, a purely mental sort of life. It has no equipment that needs managing, no sites or field stations. There’s nothing wrong with that – the same is true of mathematics and poetry. But the bodily side of this project has been an important side. I came across the cephalopods by chance, by spending time in the water. I began following them around, and eventually started thinking about their lives. This project has been much affected by their physical presence and unpredictability. It has also been affected by the myriad practicalities of being underwater – the demands of gear and gases and water pressure, the easing of gravity in the green-blue light. The efforts a human must make to cope with these things reflect differences between life on land and in water, and the sea is the original home of the mind, or at least of its first faint forms.

      At the start of this book I placed an epigraph from the philosopher and psychologist William James, writing at the end of the nineteenth century. James wanted to understand how consciousness came to inhabit the universe. He had an evolutionary orientation to the issue, in a broad sense that included not just biological evolution but the evolution of the cosmos as a whole. He thought that we need a theory based on continuities and comprehensible transitions; no sudden entrances or jumps.

      Like James, I want to understand the relation between mind and matter, and I assume that a story of gradual development is the story that has to be told. At this point, some might say that we already know the outlines of the story: brains evolve, more neurons are added, some animals become smarter than others, and that’s it. To say that, though, is to refuse to engage with some of the most puzzling questions. What are the earliest and simplest animals that had subjective experience of some kind? Which animals were the first to feel damage, feel it as pain, for example? Does it feel like something to be one of the large-brained cephalopods, or are they just biochemical machines for which all is dark inside? There are two sides to the world that have to fit together somehow, but do not seem to fit together in a way that we presently understand. One is the existence of sensations and other mental processes that are felt by an agent; the other is the world of biology, chemistry, and physics.

      Those problems won’t be entirely resolved in this book, but it’s possible to make progress on them by charting the evolution of the senses, bodies, and behavior. Somewhere in that process lies the evolution of the mind. So this is a philosophy book, as well as a book about animals and evolution. That it’s a philosophy book does not place it in some arcane and inaccessible realm. Doing philosophy is largely a matter of trying to put things together, trying to get the pieces of very large puzzles to make some sense. Good philosophy is opportunistic; it uses whatever information and whatever tools look useful. I hope that as the book goes along, it will move in and out of philosophy through seams that you won’t much notice.

      The book aims, then, to treat the mind and its evolution, and to do so with some breadth and depth. The breadth involves thinking about different sorts of animals. The depth is depth in time, as the book embraces the long spans and successive regimes in the history of life.

      The anthropologist Roland Dixon attributed to the Hawaiians the evolutionary tale I used as my second epigraph: “At first the lowly zoophytes and corals come into being, and these are followed by worms and shellfish, each type being declared to conquer and destroy its predecessor …” The story of successive conquests that Dixon outlines is not how the history really went, and the octopus is not the “lone survivor of an earlier world.” But the octopus does have a special relation to the history of the mind. It is not a survivor but a second expression of what was present before. The octopus is not Ishmael from Moby-Dick, who escaped alone to tell the tale, but a distant relative who came down another line, and who has, consequently, a different tale to tell.

      2

       A HISTORY OF ANIMALS

      Beginnings

      The Earth is about 4.5 billion years old, and life itself began perhaps 3.8 billion years ago or so. Animals arrived much later – perhaps a billion years ago, but probably some time after that. For most of the Earth’s history, then, there was life, but no animals. What we had, over vast stretches of time, was a world of single-celled organisms in the sea. Much of life today goes on in exactly that form.

      When picturing this long era before animals, one might start by visualizing single-celled organisms as solitary beings: countless tiny islands, doing nothing more than floating about, taking in food (somehow), and dividing into two. But single-celled life is, and probably was, far more entangled than that; many of these organisms live in association with others, sometimes in mere truce and coexistence, sometimes in genuine

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