The entirety of the human brain’s 4-billion-year story can be summarised as the culmination of five evolutionary breakthroughs, starting from the very first brains, all the way to the modern human brains. Each breakthrough emerged from new sets of brainmodifications, and equipped animals with a new suite of intellectual faculties.These five breakthroughs are the organising map to this book, and they make up our itinerary for our adventure back in time. Each breakthrough also has fascinating corollaries to breakthroughs in AI. Indeed, there will be plenty of such surprises along the way. For instance: the innovation that enabled AI to beat humans in the game of Go – temporal difference reinforcement learning – was an innovation discovered by our fish ancestors over 500 million years ago. The solutions to many of the current mysteries in AI – such as ‘common sense’ – can be found in the tiny brain of a mouse. Where do emotions come from?