IN 1969, after graduating from Harvard but before starting further study in astrophysics at Princeton University, I took a summer holiday in Europe and visited the Berlin Wall. It was the height of the Cold War, and the wall was then eight years old. Standing in its ominous shadow, I began to wonder how long it would last. Having no special knowledge of East-West relations, I hadn't much to go on. But I hit on a curious way to estimate the wall's likely lifetime knowing only its age.

I reasoned, first of all, that there was nothing special about my visit. That is, I didn't come to see the wall being erected or demolished - I just happened to have a holiday, and came to stand there at some random moment during the wall's existence. So, I thought, there was 50 per cent chance that I was seeing the wall during the middle two quarters of its lifetime . If I was at the beginning of this interval, then one-quarter of the wall's life had passed and three-quarters remained. On the other hand, if I was at the end of this interval, then three-quarters had passed and only one -quarter lay in the future. In this way I reckoned that there was a 50 per cent chance the wall would last from 1/3 to 3 times as long as it had already.Magazine 

Before leaving the wall, I predicted to a friend that it would, with 50 per cent likelihood, last more than two and two-third years but less than 24. I then returned from holiday and went on to other things. But my prediction, and the peculiar line of reasoning that lay behind it, stayed with me. Twenty years later, in November 1989, the Berlin Wall came down - unexpectedly, but in line with my prediction. [...]

This is all good fun. You can predict approximately how long something will last without knowing anything other than its current age. But in the past few months, in the light of the spectacular success of the NASA's Mars Pathfinder mission, I've been reminded of a far more serious implication of this way of thinking. Applying it to the human race forces me to conclude that our extinction as a species is a very real possibility, and that we had better take steps to improve our survival prospects before it's too late. Let me explain why I have such a sense of urgency, and why we had better begin colonising space - and very soon.

In the 16th century, Nicolaus Copernicus pointed out that the Earth revolved about the Sun, rather than vice versa, and in one swift move, displaced humanity from its privileged place at the very centre of the Universe. We now see the Earth as circling an unexceptional star among thousands of millions of others in our unexceptional Galaxy. This perspective is summed up more generally in the "Copernican principle", which is the supposition that one's location is unlikely to be special.

Early this century, when astronomer Edwin Hubble observed approximately the same number of galaxies receding from Earth in all directions, it looked as if our Galaxy was at the exact centre of a great explosion. But reasoning with the Copernican principle, scientists concluded instead that the Universe must look that way to observers in every galaxy - it would be presumptuous to think that our galaxy is special. As a working hypothesis, the Copernican principle has been enormously successful because, out of all the places intelligent observers could be, there are only a few special places and many nonspecial places. A person is simply more likely to be in one of the many nonspecial places. But the Copernican principle doesn't apply only to placement of galaxies in space - it works for the placement of moments in time as well.

What does it imply for "Homo sapiens ?"We have been around for about 200 000 years. If there is nothing special about the present moment, then it is 95 per cent certain that the future duration of our species is between 1/39 and 39 times 200 000 years. That is, we should last for at least another 5100 years but less than 7.8 million years.

Since we have no actuarial data on other intelligent species, this Copernican estimate may be the best we can find. It gives our species a likely total longevity of between 0.205 million and 8 million years, which is quite in line with those for other hominids and mammals. The Earth is littered with the bones of extinct species and it doesn't take much to see that we could meet the same fate. Our ancestor "H. erectus" lasted 1.6 million years, while "H. neanderthalensis" lasted 0.3 million years. The mean duration of mammal species is 2 million years, and even the great "Tyrannosaurus rex" lasted only 2.5 million years.

For us, the end might come from a drastic climate change, nuclear war, a wandering asteroid or comet, or some other catastrophe that catches us by surprise, such as a bad epidemic. If we remain a one-planet species, we are exposed to the same risks as other species, and are likely to perish on a similar timescale.

Some people might think that the discoveries of our age - space travel, genetic engineering and electronic computers - place us in a special position. These breakthroughs, they might say, could lead us to spawn new intelligent species, including intelligent machine species, enhancing our chances of survival. But such thinking may raise false hopes. For, according to the Copernican principle, you are likely to be living in a century when the population is high because most people will be born during such periods. And since it is people who make discoveries, it is not surprising that you will live in a century when many interesting discoveries are being made. But your chance of being born 200 000 years after the beginning of your intelligent lineage, in the very century when a discovery is made that guarantees it a billion-year future, is very small, because a billion years of intelligent observers would be born after such a discovery, and you would be more likely to be one of them. If you believe that any current discovery will dramatically increase our longevity, you must ask yourself: why am I not already one of its products ? Why am I not an intelligent machine or genetically engineered ?