Back in May of 2015 I spent a week in Canada reporting from the American Psychiatric Association’s annual meeting.

While strolling the halls of the Metro Toronto Convention Centre at what I recall being one of those inhumanely early hours during which only doctors and shift workers exist, I came across a mob scene that would become a touchstone in my writing of this book. A crowd had gathered to attend the meeting’s yearly Food and the Brain session, featuring a summary of the latest data on how what we eat influences our mental health. I angled my way through the crowd to witness Columbia University psychiatrist Drew Ramsey handing out raw oysters.

Watching a queue of shrinks dribble brine down their beards before dawn was unsettling. The rest of the presentation was anything but. For three hours, Ramsey and psychiatrist Emily Deans expounded on the ways our diet influences our brain health and mental state, and how through millions of years of evolution, what we ate, and how we sourced and prepared our food, was critical in shaping our most fantastic organ. As an MD turned health and science journalist, I knew from years of medical training and neuroscience research that certain dietary patterns were good or bad for brain health, but I’d never really considered them from an evolutionary standpoint. The talk left me curious about the influences that shaped our brain’s development.

Omnivorism was crucial to our survival. A varied diet meant we could adapt to shifting food supplies brought on by climate change. If a warm spell dried up the fruits of the forest, subterranean tubers on the plains would suffice. Some believe that seafood helped save our species—when early Homo sapiens learned to gather and crack open shellfish along the coasts of Africa (hence Ramsey’s oysters). And just about all scholars on human evolution agree that if it weren’t for meat, which supported our cranial expansion, we’d all still be sitting around primitive campfires with much smaller brains than we’re accustomed to. Becoming carnivorous was among the most dramatic plot twists in the human story—it helped balloon our brain size. After learning to scavenge and to hunt gazelles on the African savanna, we eventually harnessed fire to cook our spoils, further steering our mental journey with preserved, more digestible food.

I’m not advocating for eating meat, this is just what happened. What we ate was one of many influences on the human saga. Equally important was how we obtained and processed food; how we hunted, foraged, and made primitive tools for protection and butchery; how shifts in climate affected our diet, lifestyles, and physiques; how we socialized and communicated. All these factors came together through millions of years of Darwinian evolution to help shape our species and our brain.

I remember back in medical school, a few months into anatomy lab, it was time to touch a human brain for the first time. By this point, medical students are typically hardened to formaldehyde’s nasal singe and the existential reckoning that comes from spending months in a room full of dead bodies. It’s part of the deal. So at twenty-two years old, beholding what looked liked three pounds of a very unappetizing Jell-O variety, I was certain it would be part of my future (but as far as the University of Virginia School of Medicine knows, not something I would ever sneak visiting friends into the lab to see). I was struck by the notion that our behaviors, personalities, and conscious existence all come from a wrinkled orb of brownish mush. All of this comes from that.

The story of the human brain is meandering. It starts small, with simple, single-celled microbes and a host of weird sea creatures evolving early cell-to-cell communication, a harbinger of the neurons that would later coalesce into our nervous system. Through a branching evolutionary tree of wormy things, fish, reptiles, mammals, and monkeys, we eventually get to the apes, which branched off from other primates around twenty-five million years ago. Millions of years after that, our ape ancestors split from those of modern chimpanzees, which are, along with bonobos, our closest relatives. In the millennia that followed, many human-like species (any species belonging to the genus Homo) flourished. Yet Homo sapiens is the only one that remains. Through some combination of happenstance and ancestral adaptation, we endured when other humans did not. We weren’t the strongest species on the African plains. Nor the fastest. It was our large, complex brain that kept us alive and, for better or worse, allowed us to influence the fate of the planet like no species ever before.

By analyzing genomes, crafting ancient tools, and studying ape behavior, researchers from a variety of disciplines are illuminating the story of the human psyche. It’s an eonic tale of cognition that begs the question of where our brain is going. Genetic engineering technologies like CRISPR now allow scientists to literally edit our genomes in the interest of deleting detrimental genes and inserting desirable ones. For the first time in history, we will be able to artificially evolve our genetic code with precision. Some believe environmental factors like dietary patterns, chemical exposures, and the influence of technology will alter our genomes through a concept called epigenetics. It’s the idea that environmental circumstances through life can change our chromosomes without actually changing the sequence of our genetic code—and that those changes are passed along to our children. Others feel that such worries are moot; that long before we raise an army of genetically engineered super babies or succumb to a cognitive breakdown from too many Doritos, we’ll have run ourselves into extinction through some combination of conflict, climate change, and artificial intelligence.

As far as we know, the human brain is the most complex collection of matter in the universe. That’s not to say we’re more important or better than any other species.

As British biologists Charles Darwin and Alfred Wallace enlightened us: species arise and evolve as inherited traits influence their ability to survive and reproduce in their environment. Genes allowing an organism to last long enough to pass along their genes live on. By sheer numbers, bacteria win the evolutionary race, with a population of 5 million trillion trillion, or 5,000,000,000,000,000,000,000,000,000,000. Ants also outnumber us, as do krill. And as science through the years has shown us, many cognitive abilities long considered uniquely human exist as rudiments in other animals, especially our ape cousins. Evolution is not a progression toward complexity or intelligence with humans in the lead. It’s all of us doing the best we can in our given situation.

Our brain is exceptional in so many ways both good and bad. It’s benevolent. It’s cruel. It’s the only known entity that can think about, and operate on, itself.

In this book I’m in no way attempting a comprehensive survey of the scientific literature on human brain evolution. I’m instead trying to bring together leading theories from disparate fields, such as neurobiology, evolutionary biology, anthropology, and the budding discipline of nutritional psychiatry, in the story of how the human brain arose and evolved since the dawn of life on Earth and where it might be going as neuroscience and technology barrel ahead through the twenty-first century.

In doing so, I trace our brain back to its origins: to a prehistoric body of water, the appearance of DNA, and a coastal raw bar that may have helped save our species from extinction.

This is the story of our big, awesomely complex brain and how it got here.