The Big Picture: Evolution (PG)

Director, Ivan Reitman, Starring David Duchovny, Julianne Moore, Orlando Jones, 120mins

Difficult to pinpoint exactly when it started – perhaps the rise of the Farrelly brothers and their cloacal comedy – but there is currently a fixation in American movies upon all things anal. Road Trip, Nutty Professor II, Scary Movie, they've all been there. The derrière is de rigueur, the bottom is back on top.

Last week's cinema bore witness to the trend twice over. First we were presented with the regrettable sight of a young man charging through town with his hand up a cow's arse in the Farrelly-produced Say It Isn't So. Yet the obsession is by no means confined to the lower reaches of the New Moronism, for the outhouse has reached the arthouse, too: in Julian Schnabel's Before Night Falls, Johnny Depp plays a resourceful transvestite who, for the price of a thousand cigarettes, smuggles out of prison the hero's manuscript scrolled up in that place where the sun don't shine.

That the fundamental things very much apply is evidenced several times again this week in Evolution, Ivan Reitman's alien-terror comedy, and I hope it won't offend the more delicate sensibilities to report that the climactic moment in the film involves a "jumbo enema". There are, for sure, less vulgar notes of comedy in the script by David Weissman and David Diamond, but what may alarm is the way the "bum" jokes are meant to be the crowd-pleasers. As Alan Bennett wrote: "When a society turns to the lavatory for its humour, you know the writing is on the wall."

The plot gets under way when a meteor strikes Earth and triggers a superdynamic strain of DNA within a remote desert cavern in Arizona. On hand to investigate are a lowly college lecturer, Ira Kane (played by David Duchovny) and his even lowlier colleague Harry Block (Orlando Jones), who senses that they may be on the verge of, well, fame and riches at the very least: "Is the Nobel Prize paid in instalments or a lump sum?"

Casting David Duchovny in a movie about alien life-forms looks like horses for courses, but he's a good sport about sending up X-fellow Mulder's deadpan paranoia. When Harry suggests they report their findings to the authorities, Ira shakes his head: "No government. I know these people."

His wariness proves justified: when they return to the site the next day it's crawling with boiler-suited military and a cordon sanitaire has been thrown around the cavern. There's no hope of collaboration on the project, either: Ira has been persona non grata with the army since an experimental serum he tested on soldiers caused a catastrophe of sicknesses including skin rashes, temporary blindness and, to return to a theme, chronic diarrhoea. So notorious were its effects that it earned the name "the Kane Madness".

Meanwhile, the alien DNA is evolving at an unprecedented rate and threatening to outmode humanity as the ruling life-form on Earth. In other words, we can expect updates every 15 minutes or so on the progress of those two old standbys of the sci-fi genre, mumbo and jumbo. And just as Bill Murray and co made the world safe from spooks in Reitman's 1984 hit Ghostbusters, so it falls to Ira and Harry to rescue the planet from the kind of gonzoid special effects I thought we'd seen the last of in Men in Black.

Duchovny can't do this without the help of a cool red-headed rationalist, of course, and into Gillian Anderson's shoes steps Julianne Moore as Dr Allison Reed, a top-drawer scientist who has a prickly romantic flirtation going with Ira. The scriptwriters evidently thought that this might not be enough character detail, so they have cast the naturally elegant Moore as a klutz – she can't walk across a room without slipping on her butt and flashing her garter belt. Perhaps this qualifies as sophistication, though, given that Duchovny gets to moon out of a car window like a rugby player and Orlando Jones is given an anal probe after a foreign body incubates in his nether regions.

Could Evolution enjoy a more ironic title? Even if it is a summer movie aimed at kids, Reitman and his team seem intent on reversing the progress of comedy, if not of cinema. Everything seems to have been borrowed from somewhere else, and one needs to have seen only Alien, Men in Black or an episode of The X Files to realise just how old this particular hat is.

Either out of shamelessness or a shortage of ideas, the writers turn this familiarity into a virtue. About to step over a threshold into the unknown, Harry quails: "I've seen this movie before. The black dude dies first." The suspicion that different fan bases are being covered tells in the casting: the presence of the leering Seann William Scott (American Pie, Road Trip), playing a trainee fireman who stumbled on the original discovery, should catch the gross-out comedy crowd, while Dan Aykroyd does a turn as a bellicose state governor for those old enough to remember Ghostbusters.

In case you were wondering, by the way, it transpires that the alien menace is vulnerable to certain chemical constituents in Head & Shoulders shampoo, which entails one of the most bizarre product placements in movies – though still not as bizarre as the anatomical placement of the product. Let's just say it's inserted well below the head and shoulders.