Around 3000 BC, in the area now known as Iraq, human beings began to store information by leaving marks on pieces of moist clay. What they downloaded, mostly, was accounting data -- "Ubak owes Syrix three goats," that sort of thing. No doubt you, the 21st century reader, take writing for granted. But just think for a second: Recognizably human creatures have been wandering the planet for well more than a million years. On that scale, 5,000 years is not long at all. Yet look what we've accomplished, almost overnight. Writing has transformed a band of upright apes into the dominant species on the planet. So much so that we now have the power to destroy all life, ourselves included, several times over. Progress!

In The Midnight Disease, Alice Flaherty, a neurologist at Massachusetts General Hospital, confronts the biological strangeness of writing -- not from the evolutionary perspective, but at the no less mystifying level of how our brains are wired. Her interest is not academic. Following each of her two pregnancies, she underwent severe depression accompanied by long bursts of hypergraphia, or compulsive writing, much of it, by her account, incomprehensible to others.

With her neurochemical tempest now under control, Flaherty wants to follow up the intuition that there might be a link between the sort of physical disorder that induced her hypergraphia and the opposite state of writer's block, in which putting words on paper becomes almost impossible. Or, at any rate, unbearable. (Such is my own experience of the condition: I will spend hours crafting and revising three sentences, only to find that they are atrociously inadequate, begging for the wastepaper basket.)

Anyone who writes very much tends to develop theories, or at least a few durable superstitions, about the process. What sets Flaherty apart is that she has a solid body of medical expertise behind her. She is also widely read in literature. She writes that "the lines that used to appear to meet at right angles in my head -- science and literature, mind and body, health and disease, creativity and psychosis, truth and falsehood -- are now notably wavy. ... " Her prose is, for the most part, sharp and lucid, but the fusion of science and the humanities does cause strange patterns to form in the reader's mind.

A layperson might well guess that some particular section of the brain controls writing and that it is not too distant from whatever patch of gray matter permits us to speak, read and otherwise use language. The layperson who is slightly better informed than average would suppose that it can be found in the neocortex, a relatively thin layer of the brain that sits atop the parts we share with reptiles and lower mammals. The neocortex is a recent development, evolution-wise. (Hence the "neo.") So it stands to reason that the distinctly human power of writing -- or, perhaps, of procrastinating about writing -- operates thanks to the upgrade in our biological hardware.

None of this guesswork, it seems, is quite accurate. Drawing on the medical literature -- including material about the effects of various kinds of brain damage on linguistic skills -- Flaherty shows that the neurological basis of writing is hard to pin down. Obviously, it incorporates visual, auditory and motor abilities. But these are located in different parts of the brain.

While the physiological components governing the act of writing are dispersed across the "higher" levels of the brain, the forces that drive the process come from elsewhere -- namely, the limbic system, which is (quite literally) a deeper part of our neurological equipment, buried inside the cortex. The limbic system, as Flaherty puts it, "controls the four F's (fear, food, fighting and ... sex), but also more upscale functions, such as social bonding, learning and memory, that are important for writing." The rumblings of the limbic system are what we recognize as emotion.

Sometimes the flow of communication among different sectors of the brain spills over, very spontaneously, into writing. People otherwise uninterested in poetry often find themselves composing it when they fall in love, for example (which is not necessarily good for literature, however useful it may be to the limbic system).

But any number of things can go wrong, creating endless discord in the moist structure between our ears. That inner chaos may lead (paradoxically enough) to new kinds of order, as we use writing in an effort to sort things out. And then other people absorb those written words into their own nervous systems -- a process that can change how they understand the whole world.

In 5,000 years, we have learned to carry part of our brains outside of our bodies. A very strange thought! But ever since reading Flaherty, I've noticed that things don't quite look the same. For example, my bookshelves.