It’s easy to forget when reading about James Cameron’s latest ventures, from pro-environmental short films to the 10 million Avatar sequels we seem to have been waiting eons for, that this was once the youthful master of a kind of grounded, blue-collar futurism, a purveyor of science fiction able to conjure up stories in which ordinary people were often just one or two wrong turns away from encountering vicious, multi-jawed, acid-blooded beasties, or grim, unflinching red-eyed robots from the world to come.

In many ways 1984’s The Terminator is now remembered best as one of the twin pillars of Arnold Schwarzenegger’s early Hollywood career. The hulking Austrian had previously been considered virtually uncastable thanks to his preposterous frame and thick Styrian accent, but along with John Milius, whose film Conan the Barbarian hit cinemas two years earlier, Cameron worked out that it was only necessary to find freakily eccentric roles for the oversized actor to introduce the world to perhaps the biggest star of the 1980s.

And yet The Terminator’s true quality, especially when taken alongside its sequel, 1991’s T2: Judgment Day, is the manner in which Cameron brought sci-fi to the people. The genre often ignores ordinary lives in favour of those lived by the elite, in the form of space explorers, scientists and engineers. Hollywood seems to think that those who have reached society’s top rung will be the most likely to discover that we are not alone in the universe, whether by encounter with aliens, AIs or evil cyborgs from the machine-ruled future. Yet Terminator’s real hero is Linda Hamilton’s Sarah Connor, a perfectly ordinary woman who time-travelling resistance fighter Kyle Reese (Michael Biehn) first meets in a nightclub.

Even in the film’s blockbuster follow-up, T2, Cameron is at pains to flag up the simple family life of the movie’s Oppenheimer figure, the Cyberdyne Systems engineer Miles Dyson. Whenever the movie’s themes become too fantastical, Cameron ruthlessly pulls them back down to earth, building brutal, humanist foundations for his futuristic castles in the air. So Dyson is first imagined as a figure of evil, the destroyer of worlds, until we visit his home and see his kids cowering at the thought of their daddy being hurt by Connor and her sinister and unwelcome compadres.

Cameron must have jumped at the chance to take on a sequel to Alien, Ridley Scott’s pioneering slasher flick in space, because the British film-maker’s 1979 sci-fi horror appears to have been built with exactly the same toolbox. Influenced by the vision of grinding cosmic boredom presented in John Carpenter’s early 1974 venture Dark Star, Scott gave us a crew of blue-collar workers who are clearly only in space for the money, the diametric opposite of the wide-eyed, cosmic Boy’s Own optimism of Luke Skywalker in the Star Wars films.

Aliens, Cameron’s 1986 sequel, presents another vision of ordinary people dealing with horrific circumstances, from Sigourney Weaver’s Ripley (forced back to the alien-infested planetoid in exchange for the return of her unfairly revoked flight licence) to the heavily armed but ultimately helpless grunts sent to take down the xenomorphs. This almost unspeakably extreme vision of humanity’s future in space is constantly grounded in the cold reality of life beyond the borders of our solar system, and is never allowed to slip into the realms of starry-eyed Spielbergian space fantasy.

Cameron did his best to inject Avatar, his 2009 blockbuster megalith, with the same icy reality-shot to the heart. It’s made clear in the opening scenes, as we meet Sam Worthington’s disabled soldier Jake Sully, that future Earth remains a place of poverty and shrinking resources, where the working-class are forced to waste their lives in military servitude. The tiny details that push Cameron’s players beyond the realms of caricature are in place for Weaver’s chain-smoking exobiologist Grace Augustine and Stephen Lang’s ruthless alien-hating bigot Colonel Quaritch. But it’s hard to conjure up a sense of spiky realism when your movie’s key characters are 14ft-tall space elves with ethernet-capable tails, dragons for pets and symbiotic relationships with sentient, god-like megaflora.

The Terminator films and Aliens should be required viewing for new sci-fi film-makers, especially those hoping to break into the big budget arena. Without Cameron’s eye for detail and obsession with the lives of ordinary people, later movies in both futuristic sagas have languished in a sea of vapidity, unable to interest their audiences in protagonists with less apparent humanity than the merciless extraterrestrials and metal killing machines whose eternal task it is to hunt them down.

Compare Emilia Clarke’s version of Sarah Connor in last year’s Terminator Genisys to the original, Hamilton-essayed iteration, or Christian Bale’s shouty take on John Connor with Edward Furlong’s vivacious turn as mankind’s teenaged sci-fi messiah. In each case, the recast versions are thinly drawn facsimiles, plastic people about whom we know next to nothing and who make us care even less.

Cameron has now been working almost exclusively on Avatar and its sequels for at least a decade, and one suspects it could easily be another 10 years before all four (yes, four) new films find their way into cinemas. In his absence, no one has quite taken up the mantle – though Neill Blomkamp has at times essayed a similar street-level vision of things to come.

The Titanic director is only 61, but you wonder if his passion for the environment and fondness for the untapped possibilities of motion-capture tech will leave time for a return to the stark sci-fi of those early films. Like Furlong’s ever-optimistic Connor in T2, we can only hold out hope that good things eventually come to those who keep the faith.

