This is a true story. It’s a late winter day in Plano, Texas. A high school geology class is walking along a drainage ditch near school. As the teacher points to the white limestone rock and lectures, the students are shivering and muttering amongst themselves. “This is a hands-on lesson,” the teacher explains. “I want you to look around and see what you can find.” Then he picks up a thin sheet of chalk-white limestone and points to the design ­inscribed in the rock: a coiled, ribbed shell from a ­being that roamed the earth millions of years ago.

The students split up; some kick the rocks over, ­uninterested, while others look more carefully. One or two move methodically, examining the cold limestone. Here and there they find a clam shell frozen and lithographed into the stone. Snail shells are ­everywhere.

One student walks a little farther from the class, eyes down, bored. He’s new, having moved recently from New Orleans. He’s looking halfheartedly at a bed of fossilized oysters when his eyes fall on ­something odd. His interest peaks, and he calls the teacher over.

It’s a fist-sized vertebra, and it is not alone.

This was three years ago. Four months before, a storm of near-biblical proportions rolled over the Gulf Coast, smashing levies and flooding New Orleans, leaving nearly 2,000 dead and 700 missing. The ­student in this story was one of thousands of ­displaced people who fled from the storm, many escaping with just the clothes on their backs.

For many, Hurricane Katrina was a disaster on par with the September 11 attacks four years before. Just like 9/11, it forced us as a nation and as a species to contemplate our mortality. What will we leave behind when we disappear from this world?

Everyone considers this question at some point: when we are swallowed by oblivion, when we check out of this life, how will we have shaped our surroundings and what void will be left by our passing? Will it be fame or notoriety? Material things or a new idea? And, most important, how long will it last? A lifetime is often considered a mere 80 years; empires rise and fall in 500; civilizations might last a thousand.

This student, so recently arrived, stands at the threshold of an unimaginable 60 million years of ­history, in a place that was once buried under a ­shallow sea. And as he’s standing there, just for an ­instant, the sea comes back.

The vertebra is one of eight, quickly identified by the teacher as belonging to Xiphactinus audax, a 15-foot monster of a fish resembling a fanged tarpon. The following weekend, more than 20 people arrive to help excavate the remains. Among them are students, teachers, curious neighbors, and me. That weekend, we uncover more than two dozen vertebral spines, a rib, and many unidentifiable fragments of bone and teeth. Nearby emerge the foot-long skeleton of a smaller fish, skull fragments of another, and shark teeth. All around are countless oyster shells and clams, remnants of the inland sea. It is an exciting ­experience for everyone, but it leaves a deep mark on me. I am a teenager who is crazy about fossils, and I’m having my first experience with deep time.

Humans’ concept of time is necessarily limited. Our comprehension begins to dwindle around 500 years, and becomes fuzzy and vague as we approach the thousands. A hundred thousand years seems an unimaginably long time; in fact, it would encompass all of recorded human history and a good bit of recent prehistory too. Even today, there are some who draw the line, claiming the world is a youthful 10,000 years. “Isn’t that long enough?” they ask.

No, it’s not nearly long enough. Once you are ­contemplating spans of time that immense, you are beyond the realm of easy comprehension. You are swimming in deep time. This is the time it takes a continent to move, an ocean to advance, a mountain range to rise, a valley to be cut from rock. In such a concept, all human history and human achievement is lost, with no more effect or importance than individual molecules have on the flow of a stream. In the words of John Playfair, a mathematician of the Scottish Enlightenment, “The mind seemed to grow giddy by looking so far into the abyss of time.”

The concept of deep time was introduced by James Hutton, a friend and colleague of Mr. Playfair. Hutton envisioned a world built by uncounted eons of cyclical geology, shaped by winds and tides, deposition and uplift and erosion. Most significantly, he realized that a world like this could not have been formed out of a recent catastrophe but instead the long processes of geologic time. In Hutton’s words, “We find no ­vestige of a beginning, no prospect of an end.”

It’s a simple statement, but the implications are staggering.

The ultimate fate of the Xiphactinus was to be ­displayed in a glass case in our high school’s library, for the interest and edification of students. Once dug up, it is supposed to remain and no one imagines it might be lost once again. But of course the school could burn down, close, or may be ravaged by a tornado. The bones could be sold, misplaced, vandalized. In a mere 20 years, they could be erased from our knowledge. That this Xiphactinus has an impact on the world today is also by mere chance – a fleeting coincidence of the right conditions, the right time, the right people. If that particular hurricane-displaced student hadn’t been there, the creature might never have been discovered.

What, then, of humanity? When all is said and done, when we have bowed out of the great game of life, what will our species leave behind? Artifacts of one kind or another. Perhaps fossils as well, although that is by no means a certainty. What is more likely is that all knowledge of our existence will simply be erased. Hurricanes will come; fossils will appear from erosion of the hillsides, unremarked; time will march on.

Many of us know this, in our heart of hearts, but we refuse to acknowledge it or in many cases even consider it. If it’s true, we say, then what purpose does our existence serve? Must we be rendered meaningless before deep time?

It is a sentiment we seem to both fear and find ­oddly comforting. Percy Bysshe Shelley gloomily wrote: “‘My name is Ozymandias, king of kings:/ Look on my works, ye mighty, and despair!’/Nothing beside remains. Round the decay/Of that colossal wreck, boundless and bare,/The lone and level sands stretch far away.”

T.S. Eliot went still further in a famous passage from “Choruses from the Rock,” composed over half a century ago and reeking with a self-pitying gloom: “And the wind shall say: ‘Here were decent godless people:/Their only monument the asphalt road/And a thousand lost golf balls.’”

Is that indeed our fate? Perhaps so. In billions of years, the Sun will die, and the Earth will die with it. But by then there will have been billions more years of marching life; it is just as foolhardy to assume we will have no impact as it is to assume we are the end result. Along with every other living thing, our actions help determine the shape of the far-off future, in ways both subtle and immediate. To walk on fossils is like staring into the night sky; if nothing else, it forces a kind of perspective.