Brains, Eyes, Flight

The Cambrian had filled the seas with bodies. What happened across the next half-billion years filled the seas, and then the land and the sky, with something harder to define and far more interesting: ways of sensing the world, moving through it, and responding to it that crept, step by patient step, toward the thing we would eventually call mind. Part III is the story of how the universe, having learned to build living bodies, began to build minds inside them. It is also where the third of our great questions, what is consciousness, first stirs, though it will be a very long time before it wakes fully, and we should say plainly at the outset that we will reach the end of this Part, and indeed the end of this book, without a final answer to it.

Start with the eye, because it carries a clue we should not miss. Sight is such a good idea that life has invented it, from scratch, on its own, somewhere between forty and sixty-five separate times. The compound eye of a dragonfly, the mirrored eye of a scallop, the pinhole eye of the nautilus, the camera eye of an octopus, and the camera eye in your own skull were not inherited from one seeing ancestor. They were arrived at independently, by different lineages solving the same problem in similar ways, reaching again and again for the same handful of physical answers because the physics of light leaves only so many good options: a light-sensitive patch, then a cupped patch to tell direction, then a narrowed aperture, then a lens. And yet, in one of the most beautiful discoveries in modern biology, when you look at the genetic switch that tells an embryo "build an eye here," it turns out to be very nearly the same master gene in a fly and a mouse and a human, so interchangeable that you can splice the mouse version into a fly and it will still command an eye to grow. The lineages invented the eye's engineering many times over, but they reached back, all of them, into the same ancient genetic toolbox to do it. Hold onto that doubled fact, because it echoes a question from earlier in the book and will keep returning: when the same complex solution shows up over and over by separate roads, it starts to look less like a lucky accident and more like something the universe is disposed to make, given light and time and a reason to see.

Flight tells the same story. Powered flight, one of the hardest tricks in all of biology, requiring a wholesale redesign of the body for lightness and lift, evolved not once but at least four separate times: first in the insects, hundreds of millions of years ago, then in the great flying reptiles called pterosaurs, then in the birds, then, last, in the bats. Four lineages, four utterly independent inventions of the wing, each from a different starting body, each converging on the same aerodynamic solution because the air, like the light, permits only so many ways to master it.

And under all of this sensing and moving lay the real revolution of the era, quieter than eyes or wings but more consequential than either: the nervous system. Life had discovered that it could run on electricity. A nerve is a living wire; a brain is a dense knot of them, hundreds or millions or billions wired together; and the faint electrical language they speak is so fundamental that when Luigi Galvani made a dead frog's leg twitch with a spark in 1780, he had stumbled onto the very current that runs, in the end, through every thought you have ever had and every word you are reading now. From simple loose nets of nerves in the earliest animals, evolution began, in lineage after lineage, to gather and centralize them, to build knots and then clusters and then true brains. That the living world learned to think in electricity is not a small footnote. It will return, much later and transformed, when we come to a different kind of thinking that also runs on patterned electric charge.

And when the asteroid cleared the dinosaurs, as we saw at the close of Part II, it was a group of small, warm-blooded, unusually brainy survivors that inherited the emptied world. Freed from a hundred and thirty-five million years of living in the shadow of the great reptiles, the mammals radiated with astonishing speed into every niche the dinosaurs had left vacant: into the sea as the ancestors of whales, into the air as bats, into the forests and plains and burrows, growing in a few million years from small nocturnal insect-eaters into the whole gallery of beasts we know. And among the things this radiation carried forward was a pronounced trend toward larger brains and richer, more flexible behavior. But here is the part that should genuinely unsettle and delight us, and that brings the consciousness question quietly into the room for the first time. Rich minds did not evolve only once, on the tidy road that leads to us. They evolved several times over, on wholly separate branches of the tree.

Consider the octopus. Its lineage split from ours more than five hundred million years ago, before there were fish, before there were backbones at all; if you wanted to meet an intelligence that had evolved on this planet entirely independently of our own, you would not need a spaceship, only a diving mask. An octopus carries around half a billion neurons, but most of them are not in its head. They are distributed down its eight arms, so that in some real and dizzying sense the creature thinks with its whole body, and an severed arm can reach and grasp on its own. Octopuses solve puzzles, unscrew jars, use coconut shells as portable armor, recognize and take dislikes to individual human keepers, and, apparently, play. On other far-flung branches the same lamp is lit by different hands. The crows and ravens fashion hooked tools, plan for a future they cannot yet see, and hold what look like grudges. The dolphins and whales carry culture down the generations and call to one another with signature whistles that function like names. In 2012 a group of prominent neuroscientists put their signatures to a formal statement, the Cambridge Declaration on Consciousness, acknowledging in careful institutional language what anyone who has held the gaze of an octopus or an elephant already half-knew: that the neural machinery of conscious experience is not ours alone, and that a great many animals are, in some fashion we are only beginning to respect, a someone and not merely a something.

We will not pretend to more than we know, and here is the first appearance of a caution this book will state more sharply near its end. What it is actually like to be an octopus, or whether there is anything it is like at all, we cannot directly reach. The inside of another mind is the single hardest thing in the universe to verify, and we will meet that difficulty head-on much later, when the mind in question is stranger still than an octopus, and closer to home than we might wish. But the pattern is the striking thing, and the pattern we can see plainly. Vision arose dozens of times. Flight arose four times. And something we are willing to call intelligence arose at least a handful of times, on lineages with no recent common ancestor, from a cephalopod on the seafloor to a corvid in the air to a primate in the trees. Whatever mind ultimately is, the universe appears to reach for it, given enough time and the right pressures, the way it reaches for eyes and for wings. That does not prove that mind was written into things from the beginning, and we are not going to claim it does. But it should make us a great deal slower to call ourselves a fluke.

One of those big-brained mammal lineages took to the trees. It was an unremarkable choice at the time, one branch among many. It would end by changing everything.