So the aliens we choose are indicators of our cultural condition. The life on Mars imagined - that is, for the moment, the right word - by Nasa scientists is tiny, bacterial, suitably scaled for an age obsessed with the power of the virus, the prion and the molecule.
We could not have found these traces had we not first been concentrating on the microscopic. Another age might have found planet or galaxy-sized life forms. But we are not looking for that. Indeed, when James Lovelock proposed his Gaia hypothesis - suggesting that the Earth itself was, in some sense, a giant living system - he was at once ridiculed by the molecularly inclined mainstream. So there was some real wit in the joke of the novelist Douglas Adams about a mighty alien invasion fleet that launched an assault on Earth only, because of a fatal miscalculation of scale, to be swallowed by a small dog.
But, for aesthetic convenience, we usually imagine aliens to be about the same size as us. That way they can be nice or nasty in recognisably human ways. And the good news is that aliens are nasty again. Independence Day, the hit sci-fi movie of the moment, is all about extremely nasty aliens. These creatures, with satanic, goat-like legs, are pure predators. They roam space seeking out suitable planets where they can wipe out all life and exploit the resources that remain. They are, like Saddam Hussein or Gaddafi, beyond negotiation. When the American president asks one captured alien what we can do for them, the only response is the unhelpful "Die!" This is good news for American audiences as they like their killing to be justified by the knowledge that the bad guy is beyond redemption and the even better news is that, one on one, these aliens can be killed with old-fashioned handguns. If one turns up on his doorstep, Joe Six-Pack will know exactly what to do.
So they are just plain bad and their badness inspires a worldwide, concerted human effort to defeat them. This goes against the dominant tendency of the past 30 years of movie sci-fi. Close Encounters of the Third Kind and ET were both about aliens of supreme, lovable goodness. Even in the shoot-outs of the Star Wars trilogy, the baddies are human while the vast menagerie of aliens are, for the most part, weird but nice. Indeed, the tautly uniformed, human fascists of the evil empire are only finally defeated with the aid of a woodland tribe of teddy bear-like woodland creatures.
There is an early form of political correctness at work here - a benign alien plurality evidently evokes a benign racial, religious and sexual plurality. And it is noticeable that Independence Day tries to avoid the potential incorrectness involved in hating aliens by ensuring that the American side consists of a carefully tossed racial salad.
As a result of those earlier global hits, the cuddly alien has become an icon of our time - represented in comics, toys and on TV. Of course, there were alternative movie visions, notably in the Alien trilogy. But the conflict with that monster was a significantly private drama, essentially a sophisticated acting out of sexual traumas. The benign, often foetal- like alien, from 2001 onwards, was intended to provide a warm, glowing message of hope to the world as a whole.
What, then, do the evil aliens of Independence Day mean? Well, apparently, different things on different sides of the Atlantic. I saw the film in America, where the audience applauded wildly when the good guys finally cracked the alien defence system. Over here, I gather, audiences have been falling about in embarrassed laughter at the crude patriotism of it all, notably at the president's pallid, vernacular version of Henry V's speech before Agincourt. And the climactic conceit that, from now on, 4 July will be not just America's but the whole world's Independence Day is just too vulgar a display of American exceptionalism.
But I think the laughter indicates that our own native form of vulgarity - a crude, lazy sophistication - is lagging behind the latest twist in American idealism. For what this film is really about is a nostalgic yearning for a real, unarguable enemy. The Cold War, of course, provided such an enemy. In its early days this was simply the Soviets - Invasion of the Bodysnatchers, in 1956, used silent, hidden alien invaders as a metaphor for silent, hidden communist subversives. Latterly the enemy became a more generalised sense of a human failure to overcome our conflicts - in Close Encounters and ET the ideal, harmonious alien life is contrasted with our own life of petty division and brutality. Indeed, the hero of Close Encounters in effect boards the space ship as a way out of a bad marriage. No wonder the aliens were seen as foetal innocents.
But, since the Cold War, the world has remained divided without either clear enemies or easy, ideological rationales for our divisions. Our guns are pointed at nobody, the American hero has become a cowboy searching for extinct Indians. It's no good trying to imagine an ideal alien order, because, now, this should be ideal, but it is not. Conquering big conflicts doesn't work because little ones start up all over the place.
The alien saga must therefore reinvent the big, systemic conflict. But this is not against an economic theory, it is against an uncompromising, predatory civilisation. A book by Harvard Professor Sam Huntington, to be published later this year, will speak of a "clash of civilisations" which will supersede the old wars between ideologies and nation states. Well, one step ahead of Harvard, here it is on the big screen.
I think this is good news because, though Independence Day is a poor film when set against those earlier alien operas, it resurrects the idea of evil. A warm, beneficent glow is all very well in its way, but it's a pretty useless basis for social order. Evil exists and it's not going to be wished away. If the infinite sentimentality, ephemerality and ingenuity of Hollywood decides that the fashion now is in favour of uniting us against evil, as opposed to drifting dreamily away from it, then, for this fleeting moment, Hollywood has got it right.