This being my hundredth column, I asked a number of people what they would most like to read about. Of their suggestions, the subject that I have chosen, perhaps somewhat morbidly, is the end of the world.

Poets, philosophers, and priests have all speculated on this, usually with little agreement even within a single group. For example, among the poets Robert Frost suggested options: "Some say the world will end in fire, some say in ice." T.S. Eliot had a different idea: "This is the way the world ends, not with a bang but a whimper."

None of these is particularly specific, and anyway I am more scientist than poet, philosopher, or priest. I tend to begin by asking questions. What do we mean by the "end of the world?" Are we referring to the end of the human species, or to the final fate of planet Earth, or perhaps to the end of the universe itself?

Curiously enough, we have better answers to the second and third questions than to the first. The end of the Earth is the easy one. According to today's best physical theories, in about eight billion years the sun will swell in size. Inflated from within by radiation pressure, it will grow like a great balloon to become a type of star known as a red giant. It will shine two thousand times as brightly as it does today, and its disk will fill half of Earth's sky. The oceans will evaporate and the land surfaces melt. All life will be long gone.

So much for the fate of the Earth, and anything on it. This does not mean that humans are necessarily doomed. Eight billion years is a very long time, and well before the seas boil, our distant descendants may have moved to a different home. Where might they go? Well, very small stars last a lot longer than big ones. Humans might find breathing space orbiting a slow-burning dwarf star, on a new home good for up to a hundred billion years.

After that, survival becomes more difficult. According to today's cosmology, the universe is not just expanding, it is expanding more and more rapidly. Distant parts therefore recede faster and faster, and the places we can hope to reach - the universe accessible to us, even in principle - become less over time. The very long-term prospects are bleak indeed. Around a million billion years from now, everything in that accessible universe will be tied up in stellar leftovers - neutron stars, black holes, and cold dwarf stars. To give us perspective on a period like a million billion years, let's note that the universe is now perhaps 15 billion years old.

Other things happen after that, such as temperatures dropping slowly toward absolute zero, the eventual decay of black holes, and the change of everything into an expanding ocean of chilly radiation, with maybe a sprinkling of widely separated electron-positron pairs.

I am not going to bother much with these far-future events, because the chance that anything remotely like humans will be around to worry about them is negligible. Our species is homo sapiens sapiens (translating roughly as wise, wise man; a glance at human history suggests that a species was never less aptly named). We have been around for, at most, a few million years. Our ability on the one hand to destroy our environment and ourselves and, on the other hand, to change our basic form and structure, has increased profoundly during the past century. I am an optimist, and I do not believe that humanity will be wiped out by nuclear war, or by genetically modified and more lethal strains of disease, or by pollution and climatic change extreme enough to make the whole surface of the planet uninhabitable.

You will notice that each of those possibilities results from the works of humans, and not from actions of Nature. Nature could also do its part, contributing a vast solar flare, world-wide volcanic activity, or perhaps delivering a meteorite so much bigger than the one that wiped our the dinosaurs that the extinction of all life on Earth would be inevitable. These, however, are low-probability events and I am not too worried that they will occur within, say, the next ten million years.

On the other hand, I feel absolutely convinced that we will find the temptation irresistible to dabble with our own genetic makeup. We will do this not after the ten million years before a killer meteorite, nor after the eight billion years it will take the sun to swell to a red giant, nor in the million billion years before the accessible universe is reduced to cold leftovers. We will do it, almost certainly, during the coming century. The tools for genetic change are being developed today in hundreds of laboratories all around the world. I see little or no possibility that any number of speeches, or the passage of any number of laws, will do more than slow the rate of that experimentation. The human species, at the most fundamental level, is going to change. We are going to recreate ourselves.

There is one other way in which the world ends for each of us, and that is at the moment when we draw our final breath. Whether this is truly the end of everything is outside my realm of knowledge or competence, and I hand that question back to the philosophers and priests. If, however, I am permitted a preference, I will go along with Ben Franklin, scientist, inventor, philosopher, diplomat, statesman, author, and maker of books. In his epitaph he begins, "The body of Benjamin Franklin, Printer (like the cover of an old book, its contents torn out and stripped of its lettering and gilding) lies here, food for worms..."

But if the coming genetic revolution works for good rather than evil, the final words of Franklin's epitaph may apply not so much to an individual as to the human race itself: "...but the work shall not be lost, for it will (as he believed) appear once more in a new and more elegant edition, revised and corrected by the Author."

"Borderlands of Science" is syndicated by: