SNOW STILL COVERED the peaks and troughs of Yosemite the day I first stepped into a rock climbing harness. Rubbing my hands in chalk dust, I looked around and suddenly realized that "real" rock climbing means climbing UP something. A novice, I had been gently coddled along in leisurely preparation for this climb. For months, I had practiced the finger-clenching holds and balletic foot switches on a low concrete bridge on which some enthusiast had super- glued rock jags, most no thicker than a thumb. But that low bridge hid the true horror of rock climbing. Even at the bridge's highest point, I had never hung more than three feet off the dirt ground.

The wall I faced in Yosemite, ready to climb, was thirty times that height.

My instructor was a friend named Ken, who, if he ever felt fear, never showed it. That day he stood in front of one of Yosemite's many slabs of rock. It was one, he said without cracking a smile, that he usually climbed without ropes, relying on muscle and mind to protect him from the sheer drop to the granite valley floor. The grey wall leaned inward here and I had to strain my neck to see the top. Ken pointed out a route--a toehold on the right, a fist there, a curve in the granite above that.

I felt panicky just looking at my intended route. The panic started, as always, with a knot of dread in my stomach that ignored any soothing or logical words from my mind. Blood rushed to my skin's surface and quickly cooled. My mind, to reassure itself, tried its old tricks: Look at the safety equipment, the rope, the harness. Look at my friend, who will help me. But it was no use. I was terrified.

Ken had jumped onto the wall like a fly, after quickly explaining how to tie the pink and blue rope into a double figure eight knot ("It's critical to get this right"). Easily, he moved straight up, connected to me by the rope that served as his lifeline. If he fell, he had told me before he began, I was to grab the rope, which was strung through my harness, and sit on it to stop his fall. As he climbed he drove barbecue-skewer-like chocks into the rock and strung the rope through them. We had each tied an end of the rope to our harnesses. If he slipped and fell, and I held the rope tight, he would fall only a short distance, then hang from the highest chock.

Forty feet above the ground he looked small and hesitant as he felt out his next grip. He groped blindly until he found enough of a shape to wrap his hand around, dig a toe into, or lean on as he moved relentlessly upward.

Now sixty feet below him, I could barely see his retreating feet twisting and balancing as he stood now on one leg, now on two, pirouetting gracefully and composed, as if freed from gravity's demanding clutches.

Then he slipped.

Perhaps his sweaty fingertips had slid away from the smooth rock. All I knew was that the slack rope suddenly hissed through my fingers, scorching them, then jerked tight around my waist. He fell about twenty feet before the rope tightened and caught him at the last chock he had put in. He floated far above me, perfectly still for a moment, hanging from the rope.

Then he fell again. With a slight tinkling sound the chock tore free of the rock, and Ken dropped another twenty feet. Numb, I watched with glazed eyes, my brain as blank as the snow around me, watching him fall, watching the rope slipping away from me. It seemed he fell and fell, so slowly through the air. What if my hands won't move quickly enough? my brain ruminated. He is going to die, I thought.

I sat down. The rope slowed under my weight, then stopped as I clung to it with the death grip my sister had tortured me with as a child. Smiling, she would squeeze my knuckles with both of her hands until I pleaded for her mercy.

Ken stopped falling. He swung toward the rock's face like a broken pendulum, completing only half an arc and then kicking off the rock toward the valley, his eyes skimming the tops of trees. He bounced once, twice.

Then he gripped the rock again and the rope slackened, falling in coils around my feet. My hands still fiercely gripped the rope, loath to have to tame the wild slithering snake once more. My thoughts raced to catch up with my body in an effort to persuade it to relax, logically insinuating that the danger was over. But my heart continued to pound and my unblinking eyes remained riveted on Ken. I was standing stock-still, but I felt as though I were sprinting. I was panting, my tongue as dry as my chapped hands.

In recollection, this sense of danger seems to have belonged to my body, not to my brain. And maybe that's just as well. If my brain had crowded its way into those few seconds, I might not have caught the rope at all. Fears, it seems, can enter the body directly, essentially avoiding the brain, and initiate a kind of coup d'etat when we sense danger. Though thoughtfulness has its place, the "fight or flight" instinct has no time for careful consideration. A few seconds waiting for the brain to process the information "Ken is falling" would have been too long.

Although we humans pride ourselves on the evolution of the primate's big brain and especially the frontal lobe of the cerebral cortex, which serves as the seat of logic and reason, in times when our lives are threatened the frontal lobe stands as useless as an electric saw with no outlet.

Sounds signifying danger flow right through our ears and into the most primitive part of the brain--a little clump of nerve cells deep in the brain near the stem. This clump is known as the "reptilian" brain--technically, the amygdala. The amygdala receives messages straight from the ears--and possibly from the other senses as well--then sends direct messages back out to the right places in the body to boost the body's ability to react.

Since the mid-nineteenth century, at least, scientists have recognized fear and anger to be the most primitive emotions. But they have only recently begun to understand the biochemical cascade that produces the classic "fight or flight" response. Fear readies the body for fighting or fleeing by releasing the stress hormones. Those hormones, in turn, cause blood to be pumped to the muscles, energy-giving sugar to be released into the blood, and breathing to quicken.

When Ken fell, I needed every bit of this biochemical help. But is all this commotion always necessary? In his 1896 book The Expression of the Emotions in Man and Animals, Charles Darwin writes, for example, that the fear response of hair standing on end is "retained through inheritance, a relic of [our animal past], now become useless." In today's world, perhaps other dimensions of fear may be similarly vestigial, fear that in some cases can grow into paralyzing phobias.

Finally, Ken sat safely at the top, with his end of the rope anchored at the flat peak. It was my turn to begin climbing. My body resisted even before I began.

"I'm tired," I yelled up to him.

He sat there, tiny against the deep blue sky, watching me silently.

My throat parched, my heart beating a conga rhythm, I grabbed onto the rock. I stuck my fingers into the thin crack and formed a fist. It hurt. A few feet off the ground, I got dizzy with fear.

"I'm not this brave," I thought. "I can't make it."

But Ken pulled the rope tight, so that every move I made was exaggerated by the rope's pull on me. He kept pulling and calling to me, feigning patience: "C'mon, c'mon."

I looked straight at the rock, sneaking peeks down past my feet at the horror beneath me--open space and then, far below, the ground. It seemed I had no choice, I had to try this or be angry with myself for being a coward. "You're afraid to even try," I scolded myself. "We've come all this way. I can't give up," I whispered to myself. But what if I just couldn't do it? I wondered.

"I think I have to go back down," I yelled when I was almost halfway up.

"Go ahead and rest," he yelled back, holding the rope tight.

"Go ahead and rest," I mimicked under my breath, and rolled my eyes.

I let my hands go first, testing the rope. It held. Then I let my feet float beneath me, hanging in the harness, unsure whether I could really be here, floating far above the ground. I was forced to laugh at myself--even in the air I was berating myself for my flaws. But each time I glanced down, my panic returned. I didn't know which was worse--my fear of heights or my fear that I would fail. Would Ken tease me for days if I didn't keep going? Probably.

Slowly, achingly, I moved up the rock. Most of the time I felt like crying. Then, when I thought about that, I fumed at myself. This was supposed to be fun. Every few minutes I rested and let the rope take my weight. Somehow, between that rope and my muscles, and without much help from that brain of mine, I made it to the top.

Now I realize that what got me up the cliff was less overcoming my physical fear than it was succumbing to my social fears. I am still as afraid of heights as ever. Likewise, I am still afraid of failing, of being teased, of disappointing myself.

The social fears are all mixed up for me--which ones have their root in a real threat and which are just exaggerated cultural myths? There are spiders, mice, and ants. Dark alleys. Being alone in the house. Being in a crowded room. Eating in front of strangers. The sound of footsteps behind you on a city street. Breaking appointments. Telling your lover the absolute truth. Being ridiculed. Saying hello.

A friend of mine in law school can't go to a concert without a wave of panic overwhelming her. There are people, like my stepmother, who can't ride in elevators. My housemate hates going over bridges. Airplane rides reduce my boyfriend to tears, panic, a lineup of small bottles of gin. Yet, he doesn't mind driving a car like a speedway racer.

These are the kinds of fears that make me wonder if the body has not been completely overrun by the mind's whimsy. These are fears whose functional purpose is questionable. Perhaps they are comprehensible in a cultural setting--but does being afraid to speak in public have any evolutionary significance? Why would people be afraid of failure and also of success?

It is as if the brain and the body have reached some kind of strange collusion about fear. Forget logic, the body says; respond to danger. The brain remembers any fearsome situation, but can't tell the difference between physical and cultural fear. We patrol the senses for either. It is as if the brain had butted into the equation and created fancy tricks to fool the body about what to fear. An example--my body prickles with fear when I am thinking about muggers or trying to parallel park.

I can't help wondering what this fear induced by everyday challenges means about truly life-threatening dangers. Will our bodies be so used to the excitement of our mundanely fear-filled lives that our worst nightmares come true will produce no more than a sigh from our overworked amygdala?

And the little fears that we face daily, perhaps never conquering them, but always battling, getting to know them, improving our reactions to their insidious ways--will they always loom over us like walls of rock, impenetrable, slick and craggy, tempting and threatening us at the same time?

Perhaps this precise balance explains the attraction of rock climbing. Finally we have an experience whose magnitude can match our daily physiological reactions to those puzzling and embarrassing mundane threats. Finally we can offer an acceptable explanation to our friends about why we were "scared to death." High up there on the rock, who wouldn't be?