The humble paramecium shows us how to avoid being annihilated.

IF YOU WANT TO GET KICKED OUT of a party sometime, try saying this very loudly: "We're not immortal, you know. We're all going to die."

The clinking of glasses will dwindle. Someone might cough nervously, diverting the thoughts of the other guests to tuberculosis. If the hostess is skilled at ditching bozos, she will take your arm and lead you to the door with a sweet "So sorry you have to leave, darling."

Don't expect a kiss on your way out. Evoking the conversational specter of our mortality is a faux pas of the highest order, equal to head lice or intestinal gas. The very function of parties is to forget momentarily that we are stamped with expiration dates. When we do talk about death, we are trained to hold euphemisms like shields. Far more people pass on, push up daisies, kick buckets, visit Davy Jones locker, or journey to the great beyond, than simply die. In fact, linguistically speaking, we are close to overcoming death.

Not all deaths are equally repugnant. The technicolor killings perfected by Hollywood, for instance, are almost consoling because the lessons they impart are so simple. All we have to do is avoid Vietnam and androids from the future, duck bullets and dinner with flesh-eating psychopaths and we'll be O.K. Screen writers don't want to remind us that regardless of what body armor we buy, or how much wheat germ and vitamins we chomp, we are only delaying the inevitable.

The true enemy is the cold hand of mortality, and it approaches, quiet and unstoppable. This taste of mortality on our lips makes dispatches from medicine's front lines taste very sweet. "WE JUST BOUGHT YOU ANOTHER FEW YEARS OF LIFE BUDDY STOP PLEASE SEND MORE GRANT MONEY STOP." But medicine is only the latest -- and most expensive -- group activity whose purpose is to conquer death. More tried and true methods involve denying death's finality. Stories of the journey after life, and ceremonies to bid the deceased bon voyage, are among the marks that distinguish human civilization. Next life, Shirley MacLaine will emerge in another glamorous body. I'd like to believe that insurance agents and telemarketers will visit an other-worldly, and much hotter, place.

But why should people believe there is any way to escape death? All around us life and death are served as a single course. Neither one is available a la carte. I sit here at a desk of dead wood, having eaten some dearly departed corn and chicken only a few hours ago. Someday, I'll be an entree for the worm and microbe. One day we are vital, the next we are food. That is our destiny, our legacy as living beings.

Or is it? It turns out there is a world in which death is merely an option: the world of bacteria. Of course, bacterial lives ocassionally end, but only when annihilated with the drama of a made-for-TV movie. We poison the nastier germs with antibiotics. If they can't find food, they wither and perish. But bacteria don't die of "natural causes."

Aside from lethal physical or chemical insult, bacteria live until they divide. Then, as two cells, they beget generation after generation with Biblical flourish, repeating the same cycle endlessly. Each bacterial parent vanishes into its offspring, and gets to live again. Even Ms. MacLaine must be jealous at their inexhaustable potential for life. Death, as inescapable future, as we know it, is alien to them.

Now consider this: bacteria were the sole rulers of Earth for billions of years. That means mortality, from an evolutionary point of view, was a fairly late invention. Somewhere between the production of germs and of Germans, evolution began including mortality absolutely free with purchases of life.

The first organism on the block with mortality factory-installed may have been the protozoan Paramecium. This single-celled animal moves with the graceful beating of tiny hairs around its potato-shaped body. It dines on bacteria, catching them in its mouth like peanuts at a bar.

Paramecium cells replicate by dividing in half, just like bacteria. But the protozoa aren't nearly as fecund. Each succeeding generation of isolated Paramecium cells grows weaker and weaker, their zest for life constantly diminishing. Finally, they stop life's dance, and the whole population expires at about the same time.

But for Paramecium, death is a matter of lifestyle. If they remain celibate, they will die. But sex saves them. Yes, Paramecium cells are sexual. And the courtship between the protozoan pairs is no chaste, arm-length affair. The lovers meet mouth-to-mouth and fuse in a kiss that lasts up to 24 hours. They exchange and mix DNA and, for some unknown reason, both partners emerge from the erotic clasp rejuvenated. A cell that is one division away from its finale will divide hundreds of times following a conjugal visit.

Paramecium cells cannot live virgin lives if their species are to survive. Death, then, can be seen as one of evolution's socializing forces. The grim reaper removes the celibacy-minded from the gene pool. And sex is more than its own reward, it is life's elixir.

Perhaps the fatal consequences of social isolation in organisms like Paramecium helped to draw cells together, not only for mating games, but into the large bustling communities we call animals. Indeed, our own cells seem to operate within a defined social contract.

While bacteria grow until all food is gone, our cells will halt well before they are full. You cut yourself and the skin cells divide and move in to close the wound. Then they stop. Even removed from the body's tight confines, skin cells will grow only until their edges touch, and then they desist, respectful of their neighbors.

Self-interest is at the heart of this respect. A cell's genes will be passed on only if the body is healthy enough to mate, because each cell has little hope of making it on its own. For example, even when given plenty of space and good things to eat, skin cells can only eke out a few dozen divisions before they poop out.

So, humans are mortal at two levels: the organism and the cell. But the connection between these two types of mortality is unclear. We are not our cells. Cancer cells, for example, are vigorous and healthy -- and immortal. Tumors have given up social niceties and will invade any tissue they like and grow indefinitely. Descendants of cells removed from the cervical cancer of a Chicago resident, pseudonymously known as Helen Lane, have been growing and dividing in research labs since 1952. These cells have become so widely used for biological studies that there is undoubtedly more of Lane around now than when she was alive.

Few of us would choose this type of immortality. Still, it is tempting to draw an analogy between cells and ourselves. Paramecium cells can flip between mortal and immortal modes, as easily as we can change jewelry. Perhaps one day humans will work the same process more rationally, dispense with our fatal accessories and keep the rest.

Until then, we will continue to gather at cocktail parties to forget death, at funerals to mark it with ceremony, and in medical research institutes to try to vanquish it. The function of mortality in cells and in society may not be that different after all. Death has brought us together.