On a recent trip to the San Diego Zoo, my two-year-old son, Elliot, made friends with a young chimpanzee. For some 20 minutes, the two squealed at each other, played games of peek-a-boo, and entertained each other in the typical ways kids do. For everyone on the paying side of the glass, it was an amazing event, these two children seeming so similar and yet so different. But what in the tangle of nerve cells inside their heads will make Elliot and his chimpanzee friend take such different paths as they grow? Why will Elliot absorb a culture, learn a language, develop a rich sense of self, and develop a morality based on understanding that others are people with their own inner worlds of experiences and feelings? And why will his chimpanzee friend reach none of these milestones? Ultimately, the answer lies buried somewhere deep inside their brains.

In beginning to answer the central riddles of the human mystery, brain science is focusing on a region of the brain known as the prefrontal cortex, the part behind our high forehead. Although still a matter of controversy, the prefrontal cortex seems to be the brain region most expanded by human evolution. This enlargement has long been noted, but until recently, the prefrontal cortex was mostly a vast uncharted territory. During the last decade, however, brain scientists have begun charting its functions, filling in the human details with the help of noninvasive imaging technologies that allow us to probe the human brain deep in thought.

Today, brain science is focusing on two prefrontal regions critical to human mental life. One area, the dorsolateral prefrontal cortex, plays a key role in analytic thinking. Consider, for example, the challenges involved in deciphering assembly instructions to successfully transform 50 or so assorted pieces of plastic into a working children's toy. All that planning, ordering, and sequencing calls on the dorsolateral region's two components of working memory: temporary short-term storage, crudely reminiscent of RAM in a computer, and executive processes like attention and task management which oversee informational flow.

The connection between our expanded prefrontal cortex and these thinking abilities is a crucial one, no doubt underlying our self-description as the wise species. But as brain scientists uncover more of our prefrontal workings, there is a surprising twist with far-reaching implications. We are increasingly realizing that the prefrontal cortex is not simply an analytic, rational calculator. Instead, the prefrontal region known as the orbitofrontal cortex is intimately involved in our personality, in navigating our social relationships, and in coloring our emotional life. Appreciating the role of the orbitofrontal cortex is prompting a serious reconsideration of the traditional divide between thought and emotion.

To see how, consider your reaction if I were to ask you to recollect a personal loss or other traumatic life event. Chances are you wouldn't just remember it; you would also feel it. Our experiences and recollections aren't drab gray; they are colored with emotional hues. Brain scientists increasingly suspect that the orbitofrontal cortex is where gray, experiential information gets its emotional tone, a sort of emotional working memory.

But it goes beyond just adding color to inner life. In fact, our capacity to make sound decisions -- to think and act right -- in everyday matters depends on utilizing the emotional value that the orbitofrontal cortex supplies to our experiences. This lesson stems from observing some people with damage to the orbitofrontal cortex, a condition known as acquired sociopathy. Stripped of the emotional value of their experiences, literally unable to feel, the result is often profound personality shifts, impaired decision-making, and loss of moral insight.

On a more ominous note, acquired sociopathy mirrors developmental sociopathy, a condition of those living among us who have no emotional value to their experiences, making them unfeeling toward others. The orbitofrontal cortex appears impaired in them, as it does in about one in five prison inmates arrested for violent crime. Our everyday capacity to treat others not as objects but as persons stems from the workings of the orbitofrontal cortex, and so may underlie our capacity for moral reasoning. Treating others as persons in turn requires thinking of oneself as a person, too. Indeed, among the great outstanding questions of brain science is how the prefrontal cortex helps construct our sense of personal identity, an awareness of oneself as an enduring entity across time. This awareness, too, can be stunted with prefrontal damage, damage that strikes at the very core of what makes us who we are.

There was something else deeply puzzling about that day at the zoo. An inch-thick plate of glass separated Elliot from his friend at the zoo, but a smaller margin -- a mere 1.6 percent -- separated them genetically. In other words, Elliot and his chimpanzee friend share 98.4 percent of their genes. This means that there isn't much uniquely human genetic material to build the prefrontal brain structures underlying human mental life. Instead, I think that an important clue to the human mystery lies in the fact that virtually all prefrontal capacities -- working memory, theorizing about other people's minds, our sense of self -- unfold gradually over the first two decades of life. So too, the human prefrontal cortex is distinctive in how long it takes to develop, continuing well into adolescence. I suspect that the key to building a human lies in letting the world help build the prefrontal cortex as it experiences the world. If this is so, then the interaction between brain and world is a far richer one than ever imagined, and crucial to understanding who we are.

Steven Quartz is a theoretical brain scientist at the California Institute of Technology. His research uses neuroscientific methods to address traditional philosophical problems of mind. He is the author, with Terrence Sejnowski, of the forthcoming Who We Are: How Today's Revolutionary Understanding of the Brain is Rewriting Our Deepest Beliefs About Ourselves.