The universe is a big place. There are about ten billion stars in our galaxy alone, and some certainly have planets. Even if the odds against an Earth-like world are steep as winning the National Lottery, at least there are plenty of tickets.

The trouble is that all our knowledge of life comes from our Earth - and a sample of one is a precarious basis for universal conclusions. It could be that the origins of life is a trillion-to-one fluke, and we’re the only winners.

Then again, life may be relatively common, but what of intelligent life? Or, if the galaxy if teeming with intelligence, what if technology is a aberration, destined to inflict catastrophe on any species foolish enough to try it?

The Seti programme - the "search for extraterrestrial intelligence" - and its successor, Project Phoenix, have hunted in a desultory, underfunded way for years, using radio telescopes to listen out for alien signals. They’ve found nothing - but absence of evidence is not evidence of absence. Aliens may communicate by means other than radio, or perhaps we have not listened long enough.

It is not unreasonable to assume that, somewhere out there, aliens exist. Our nearest neighbours could be at the far end of the galaxy. But even "nearby" in galactic terms is a very long way off. Interstellar journeys would be extremely long and costly. Why would aliens bother?

Lets assume they had faster-than-light technology and the urge to probe the galaxy. What would such aliens be like?

That brings us to the weakest link in virtually all descriptions or supposed photographs of alien visitors: the creatures are invariably humanoid.

Discounting the dubious coincidence, there is a real objection to the supposed humanoid appearance. Evolution proceeds by random mutation. There is no guiding hand, no inevitable end. In our own progress, there have been perhaps millions of points at which random change "directed" developments in a way that led to mammals, to primates and us. Had the dice fallen differently, we would not exist.

Intelligence may evolve, but not in a four-limbed upright creature with a brain-case bobbing on the end of a spinal column. The odds against the same long chain of random mutations occurring elsewhere are astronomical.