In the fall, many animals put on an extra layer of fat. Some collect berries and nuts and store them for the cold winter months. They seem to plan ahead. However, "plan" is probably a poor choice of word. So far as we know, humans are the only creatures on the planet actually able to conceive of the concepts of "tomorrow" and "the future," and to plan consciously for times to come.

We are not very good at it, either. A politician's time horizon is said to be two years for someone in the House, six for a Senator. In our own lives, most of us worry about what will happen to us, and we do our best to provide for our children. That's usually our limit. Have you made plans for your grandchildren? I certainly haven't. Very few programs, personal or public, concern themselves with what may happen a century from now.

I think that makes a lot of sense. Three thousand years ago, most changes in society came slowly and a man or woman might have been transplanted in time by a century or two and perhaps notice little difference. But my great-grandfather, jumped forward Rip Van Winkle-style from 1900 to 2000, would surely be bewildered by almost everything he saw, in the home or out of it. Think telephones, televisions, microwave ovens, hearing aids, computers, automobiles, heart transplants, superhighways, jet planes, humans in space - you can add to the list, because there are plenty more to choose from. Jump me a century ahead and I would expect to be at least as confused as great-grandfather.

The world, a century from now, seems unknown and unknowable. Make that a thousand years rather than a hundred, and conditions on Earth disappear into a mist of uncertainty. Will democracy survive, or go the way of communism? Will the United States exist? Will English itself be a dead language? It's easy to say, of course not, its use becomes more widespread every day. But a Roman, two thousand years ago, could have said exactly the same thing about Latin - and it was still the international language of Europe less than four centuries ago.

All this suggests that we can't say much about any future beyond the next few years. Yet if we are willing to consider longer time scales, science provides some quite definite statements about future times.

Let's start by looking backward. The sun is about five billion years old, and this planet close to the same age, formed maybe 4.5 billion years ago. Life appeared on Earth rather quickly, almost four billion years ago (Did it arise here, or was it transported from elsewhere in the universe? That is a topic for another column).

All life in early times was confined to the seas and oceans of the world. Much later, maybe 430 million years ago, plants came ashore. The first land animals followed a few tens of millions of years later. The earliest mammals, with which we can feel a special kinship, appeared maybe 225 million years ago. The flowering plants, everything from asters to zinnias and all their ancestors, were relatively recent arrivals and have brightened the world for only about a hundred million years. Recognizable humans with human intelligence, able to appreciate those flowers, are real newcomers, on Earth for no more than three or four million years.

All this, however, is rather encouraging. Since life has existed here for close to four billion years, the planet must be very hospitable to life. It is reasonable to ask, could the Earth remain equally hospitable for another four billion years?

According to everything that we know, it could. The first requirement for our own long future is a stable sun. We have one. The sun has shone with only a modest increase in brightness for the past four billion years. Detailed calculations of the physical processes taking place inside the sun indicate that it will continue to shine, looking much the same as it does now, for at least another five billion. It will provide the energy needed by all plant life, which in turn allows for our own air, food, and continued existence. The distance of the Earth from the sun is calculated to change little over the same period. This planet will remain a good place for life for as long or longer than life has already existed here.

Five billion more years sounds good to me, but the pessimists may say, "What happens after that?"

Well, things don't look so good. According to the best physical models - which now appear to be on very firm ground - some time around eight billion years from now, the sun will begin to increase in size. It will swell like a giant balloon, inflated from within by radiation pressure, until it becomes a type of star known as a red giant. It will shine two thousand times as bright, and with a diameter of a hundred million miles its disk will fill half of the sky. The oceans will long since have evaporated, and the land surface will be hot enough to melt lead. Earth will be utterly uninhabitable.

The pessimist will say, "Told you so. It takes a long time, but we're doomed."

I don't believe that for a moment. Five billion years provide plenty of breathing space. Given that much time to plan and act, the humans of the future (who will surely look and think nothing at all like us) should have moved far beyond the bounds of the solar system. Our descendants will, if they choose, go and sit around a planet of a different star, good as a new home for another five or ten billion years. Beyond that, they will begin to make really long-term plans.

The human species, the first life form on Earth capable of thinking about its future, has been around for only a few million years; but it could have a very long future.

Now, if we can just get through the next few centuries without doing something really stupid...

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