That question has been asked by everyone who has ever had a migraine, experienced childbirth, endured a root canal or even banged a funny bone.

Pittsburgh neurosurgeon Frank T. Vertosick Jr. has some answers. His book "Why We Hurt: The Natural History of Pain" describes the biology of pain, drawn from case studies of his patients, whose ailments range from back problems and cancer to phantom pain in a nerve-dead arm and trigeminal neuralgia, a condition in which "searing, shock-like pain" can be set off by a tear rolling down a cheek.

The book focuses on those uniquely human pains caused by our strange anatomy. A sampler: childbirth pain that stems from our huge heads, disk disease rooted in our upright posture and carpal tunnel syndrome resulting from overdeveloped thumbs whose massive nerve fibers must funnel through underdeveloped wrists.

Although we share some types of pain with animals -- even dinosaurs had blocked colons, Vertosick says -- humans make pain worse because of the added element of suffering. That is, we foresee the consequences of pain.

Vertosick, 45, who studied and trained at the University of Pittsburgh, also is the author of "When the Air Hits Your Brain: Tales of Neurosurgery" (1996), a tongue-in-cheek account of his journey from steelworker to neurosurgeon. He employs touches of that same sense of humor in "Why We Hurt."

When an extruded piece of disk material is absorbed and the nerve decompresses, he notes, the body is performing spinal surgery upon itself "without preapproval from an HMO." He writes vividly of pain: Herniated disk material "resembles lump crabmeat"; an accident patient remembers feeling his left arm "snap tautly, like a thick piece of taffy"; migraine pain is like a long-lasting version of the "ice cream headache or brain freeze" you get from drinking a milkshake too fast.

A migraine sufferer himself, Vertosick said in an interview with Salon that he wrote his latest book for people who have chronic pain -- "that's most adults at some point in their lives," he said -- as well as those of us who enjoy icky yet fascinating medical topics.

What's the short answer to why we hurt? Is pain just the body's way of warning us that something is wrong?

From an evolutionary point of view, pain pathways are so old they are paleo-pathways. They were around well before we had the capacity for abstract thought; they evolved when we were fairly stupid. We needed to be hurt to learn that fire is hot. To some degree, with cognition, pain has become a burden we really don't need but can't get rid of. It's a warning system that we just can't shut off, like the smoke alarm that goes off when you burn the toast.

How much of pain is psychological?

There's some psychic pain involved in all pain. If I twist my knee, immediately there's a certain degree of anguish. But a professional running back may twist his knee and perceive it with more anxiety and suffering. It's one thing to crack a tooth today, it's another to do it the day before your wedding. Uncoupling the physical and psychological perception of pain is a huge area of interest -- very cutting-edge -- in pharmacology today. In the '40s and '50s they made the serendipitous discovery that lobotomy patients no longer perceived pain as bad. It was stunning. That taught us that it's possible to unhook the bad perception of pain from the physical sensation of pain.