Invited by the University of Miami to address members of the class of 2005, the columnist repaid this courtesy by telling them that even though they surely had showered before donning their caps and gowns, each of them had about a trillion bacteria feeding on the 10 billion flakes of skin each of us sheds in a day. If each 2005 graduate were disassembled into his or her constituent atoms, each graduation gown would contain nothing but atomic dust. But as currently assembled, this star dust--really: we are all residues of the Big Bang--is living stuff, capable of sublime emotions like love, patriotism and delight in defeating Florida State.

The body of every Miami graduate has about 10 thousand trillion cells, each containing a strand of DNA that, uncoiled, would extend about six feet. If that person's DNA were spliced into a single strand, it would extend 20 million kilometers--enough to stretch from Miami to Los Angeles and back 2,270 times.

So says Bill Bryson, author of the delightful "A Short History of Nearly Everything." According to him, everyone now alive contains some Shakespeare. That is, some of the physical stuff he was made of. And Julius Caesar's stuff, and Genghis Khan's and Charlemagne's. And Charlemagne's cook's. There are trillions of trillions of atoms in each of us, so lots--probably billions--of atoms have been recycled in each of us from Beethoven. In that sense we all are, as Bryson says, reincarnations.

Indeed, each member of Miami's class of 2005 is related to every other member and to--facts must be faced--every graduate of Florida State. It took two parents to produce each of us, and four people to produce our parents. If we look back eight generations, to Lincoln's day, Bryson says that more than 250 people contributed to the creation of each of us. Look back to Shakespeare's day, and we are directly descended from 16,384 ancestors. Look back 64 generations, to the era of the Roman Empire, and we have a thousand trillion ancestors.

But wait. A thousand trillion people is thousands of times more than the number of human beings who have ever lived. So everyone is the product of a lot of incest--but incest at what Bryson calls "a genetically discreet remove." This extended single family--humanity--inhabits the little planet Earth, whose continents are wandering.

Bryson says Europe and North America are moving away from each other at about the speed that a fingernail grows--about two yards in a normal human lifetime. The African continent is creeping northward and someday will squeeze the Mediterranean Sea out of existence and will shove up a chain of mountains as high as the Himalayas extending from Paris to Calcutta.

The Earth is restless partly because its molten core retains heat amazingly well: it has lost only about 200 degrees in the 4 billion years since the planet coalesced. Not that we have come close to that core: Bryson says that if the planet were an apple, our underground exploration would not yet have broken the skin.

The sun around which Earth orbits is one of perhaps 400 billion stars in the Milky Way, which is a piddling galaxy next door to nothing much. There are perhaps 140 billion galaxies in the still-unfolding universe. If all the stars in the universe were only the size of the head of a pin, they still would fill Miami's Orange Bowl to overflowing more than 3 billion times.

We should by now be used to strange thoughts. It has been 100 years since June 1905, when Albert Einstein began publishing the scientific papers that taught us that gravity bends light, that space and time are warped, that matter and energy are interchangeable, that the mass of an object increases the faster it moves and that the experience of time is a function of speed.

But there is a not-at-all-strange reason that a Washington columnist would belabor Miami graduates with strange facts. It is this: The more they appreciate the complexity and improbability of everyday things--including themselves--the more they can understand the role that accidents, contingencies and luck have played in bringing the human story to its current chapter. And the more they understand the vast and mysterious indeterminacy of things, the more suited they will be to participate in writing the next chapter.

This is so because the greatest threat to civility--and ultimately to civilization--is an excess of certitude. The world is much menaced just now by people who think that the world and their duties in it are clear and simple. They are certain that they know what--who--created the universe and what this creator wants them to do to make our little speck in the universe perfect, even if extreme measures--even violence--are required.

America is currently awash in an unpleasant surplus of clanging, clashing certitudes. That is why there is a rhetorical bitterness absurdly disproportionate to our real differences. It has been well said that the spirit of liberty is the spirit of not being too sure that you are right. One way to immunize ourselves against misplaced certitude is to contemplate--even to savor--the unfathomable strangeness of everything, including ourselves.