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'A Brief History of Intelligence' is a synthesis of the work of many others. At its heart, it is merely an attempt to put together the pieces that were already there. I have done my best to give due credit throughout the book, always aiming to celebrate those scientists who did the actual research. Anywhere I have failed to do so is unintentional. Admittedly, I couldn't resist sprinkling in a few speculations of my own, but I will aim to be clear when I step into such territory.

It is perhaps fitting that the origin of this book, like the origin of the brain itself, came not from prior planning but from a chaotic process of false starts and wrong turns, from chance, iteration, and lucky circumstance.

A Final Point (About Ladders and Chauvinism)

I have one final point to make before we begin our journey back in time. There is a misinterpretation that will loom dangerously between the lines of this entire story.

This book will draw many comparisons between the abilities of humans and those of other animals alive today, but this is always done by picking specifically those animals that are believed to be most similar to our ancestors. This entire book—the five-breakthroughs framework itself—is solely the story of the 'human' lineage, the story of how 'our' brains came to be; one could just as easily construct a story of how the octopus or honeybee brain came to be, and it would have its own twists and turns and its own breakthroughs.

Just because our brains wield more intellectual abilities than those of our ancestors does not mean that the modern human brain is strictly intellectually superior to those of other modern animals.

Evolution independently converges on common solutions all the time. The innovation of wings independently evolved in insects, bats, and birds; the common ancestor of these creatures did not have wings. Eyes are also believed to have independently evolved many times. Thus, when I argue that an intellectual ability, such as episodic memory, evolved in early mammals, this does 'not' mean that today 'only' mammals have episodic memory. Like with wings and eyes, other lineages of life may have independently evolved episodic memory. Indeed, many of the intellectual faculties that we will chronicle in this book are not unique to our lineage, but have independently sprouted along numerous branches of earth's evolutionary tree.

Since the days of Aristotle, scientists and philosophers have constructed what modern biologists refer to as a "scale of nature" (or, since scientists like using Latin terms, 'scala naturae'). Aristotle created a hierarchy of all life- forms with humans being superior to other mammals, who were in turn superior to reptiles and fish, who were in turn superior to insects, who were in turn superior to plants.

Even after the discovery of evolution, the idea of a scale of nature continues to persist. This idea that there is a hierarchy of species is dead wrong. All species alive today are, well, 'alive'; their ancestors survived the last 3.5 billion years of evolution. And thus, in that sense—the only sense that evolution cares about—all life-forms alive today are tied for first place.

Species fall into different survival niches, each of which optimizes for different things. Many niches—in fact, 'most' niches—are better served by 'smaller' and 'simpler' brains (or no brains at all). Big-brained apes are the result of a different survival strategy than that of worms, bacteria, or butterflies. But none are "better." In the eyes of evolution, the hierarchy has only two rungs: on one, there are those that survived, and on the other, those that did not.

Perhaps instead, one wants to define 'better' by some specific feature of intelligence. But here still, the ranking will entirely depend on what specific intellectual skill we are measuring. An octopus has an independent brain in each of its tentacles and can blow a human away at multitasking. Pigeons, chipmunks, tuna, and even 'iguanas' can process visual information faster than a human. Fish have incredibly accurate real-time processing; have you ever seen how fast a fish whips through a maze of rocks if you try to grab it? A human would surely crash if he or she tried to move so quickly through an obstacle course.

My appeal: As we trace our story, we must avoid thinking that the complexification from past to future suggests that modern humans are strictly superior to modern animals. We must avoid the accidental construction of a 'scala naturae'. All animals alive today have been undergoing evolution for the same amount of time.

However, there are, of course, things that make us humans unique, and because 'we are human', it makes sense that we hold a special interest in understanding ourselves, and it makes sense that we strive to make artificial 'humanlike' intelligences. So I hope we can engage in a human-centered story without devolving into human chauvinism. There is an equally valid story to be told for any other animal, from honeybees to parrots to octopuses, with which we share our planet. But we will not tell these stories here. This book tells the story of only 'one' of these intelligences: it tells the story of us.