Are we alone in the Universe? It is often alleged that aliens in UFOs have visited us. But the claimed manifestations - crop circles and the like - are as banal and unconvincing as the messages from the “other side” routinely reported in the heyday of spiritualism 100 years ago.

I certainly don't believe that the declassification yesterday of Ministry of Defence files reporting encounters with unidentified flying objects in British airspace shows that we have found aliens. But I am open-minded about the possibility and would dearly like to know if they exist.

The great 18th-century astronomer William Herschel thought that the Sun was inhabited. In the late 19th century, the science fiction of Jules Verne and H.G. Wells further popularised the idea of alien life. Percival Lowell, a wealthy American, believed that Mars was criss-crossed by “canals”, dug by an advanced civilisation to channel water from the frozen polar caps to “deserts” near the equator. We are less optimistic about Martians than our forebears.

However, space probes have revealed dried-up river beds - evidence for water, suggesting that Mars may once have been hospitable to life, even if it isn't today. Life could also exist in the ice-covered oceans of Jupiter's moon, Europa.

Even if there is life elsewhere within our solar system, nobody expects it to be other than rather primitive. But within our galaxy there are billions of stars. What are the prospects of more advanced life on planets orbiting some of these?

In 1584 Giordano Bruno, a Dominican monk, published a book On the Infinite Universe and Worlds. In 1600, he was burnt at the stake in the Campo de Fiori, Rome, where he is now commemorated by a fine bronze statue. Among Bruno's conjectures was that: “There are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours”.

In the last years of the 20th century, his prescient belief was vindicated: there are, assuredly, planetary systems around many other stars. Bruno had a further conviction: “No reasonable mind can assume that heavenly bodies which may be far more magnificent than ours would not bear upon them creatures similar or even superior to those upon our human Earth.” Our conception of the physical Universe has been utterly transformed since Bruno's time, but we still cannot gauge the likelihood of extraterrestrial intelligence.

The more difficult issues lie in the province of biology, not astronomy. There are two great questions, which it is important to distinguish from each other. First, how did life begin? I think there is a real chance of progress here, so that we will know if life is a “fluke”, or whether it is near-inevitable in the kind of initial “soup” expected on a young planet.

But there is a second question: Even if simple life exists, what are the odds against its evolving into something that we would recognise as intelligent? This is likely to prove far more intractable. Even if primitive life were common, the emergence of advanced life may not be.

Simple life on Earth emerged quite quickly. But it took nearly three billion years for multicellular organisms to come on the scene. Most of the standard body types for animals date from the “Cambrian Explosion” half a billion years ago. The immense variety of creatures on land emerged since that time - punctuated by major extinctions, such as the event 65 million years ago that wiped out the dinosaurs.

This disparity of timescales suggests that intelligence could be exceedingly rare even if simple life were widespread. Certainly our own emergence was the outcome of time and chance. If the Earth's history were rerun, the fauna may be quite different. If the dinosaurs had not been wiped out, the chain of mammalian evolution that led to humans may have been foreclosed. We cannot predict if any other species would have taken our role.

Even if we have not been visited, we should not conclude that aliens do not exist. It would be far easier to send a radio or laser signal than to traverse the mind-boggling distances of interstellar space. (Aliens equipped with large radio antennas could, in any case, pick up signals from anti- ballistic missile raiders and the output of our TV transmitters - if they could decode them, it is not hard to think what they might conclude about “intelligent” life on Earth.) It makes sense for us first to “listen” rather than transmit - if a signal were detected, there would be time to send a measured response. But there is no scope for snappy repartee - a two- way exchange would take decades.

Searches for extraterrestrial intelligence (SETI) are a worthwhile gamble, even if one suspects that there are heavy odds against success, because of the huge philosophical import of any detection. A manifestly artificial signal - even if it were as boring as a set of prime numbers or the digits of pi in binary notation - would convey the momentous message that intelligence (although not necessarily consciousness) was not unique to the Earth and had evolved elsewhere, and that concepts of logic and physics were not peculiar to the kind of hardware that we carry around in our heads.

The SETI@home programme, based in California, is spearheading these searches, supported by hefty private benefactions. Any interested amateur with a home computer can download and analyse a short stretch of the data stream from a radio telescope. Several millions have participated - each, no doubt, inspired by the hope of being first to find ET. But we should not hold our breath for success. Maybe we are alone. Even if intelligence were widespread, we may never become aware of more than a small and atypical fraction of what is out there.

Some brains may package reality in a fashion that we cannot conceive. Others could be living contemplative lives, doing nothing to reveal their presence. Absence of evidence would not be evidence of absence. The only type of intelligence we could detect would be one that led to a technology we could recognise.

It would, in some ways, be disappointing if searches for alien intelligence were doomed to fail. On the other hand, it would boost our cosmic self-esteem: if our tiny Earth were a unique abode of intelligence, humanity would have greater cosmic significance than it would merit if the galaxy teemed with complex life.

Charles Darwin wrote that: “Judging from the past, we may safely infer than not one living species will transmit its unaltered likeness to a distant futurity.” And artificially genetic modifications can now induce far faster changes than natural selection. Our Sun formed 4.5 billion years ago, but it has six billion more before the fuel runs out. And the expanding Universe could continue for ever. As Woody Allen said: “Eternity is very long, especially towards the end.”

There is abundant time for posthuman intelligence (organic or silicon-based) to spread through the entire galaxy. Even if life were now unique to Earth, we should not conclude that it was a trivial “afterthought” in the Universe. The cosmos is still nearer its beginning than its end.

Lord Rees of Ludlow is Astronomer Royal and President of the Royal Society

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