I've been curious about the ratio of original-to-adapted projects in filmed science fiction for some time now, in big part because a cursory survey of significant SF movies from past decades shows that most of them were in fact reworked from short stories or novels. The originals mostly seemed to be the tons of awful cheapies that flooded the market and vanished without a trace, only to end up on midnight TV or Something Weird Video.
Among the early exceptions I can think of, apart from Metropolis, was Forbidden Planet. It was adapted — shilling for lifted wholesale — from Shakespeare's Tempest, but it wasn't reworked directly from any pre-existing SF novel. In fact, an adaptation of it was written from the script (something Frederik Pohl never stopped kicking himself for turning down, since he ended up loving the movie). 2001 was a hybrid: it was written both as a literary and as a cinematic project, more or less at the same time.
It feels like only recently, in the last few decades, there's been a ramping-up of quality SF work written directly for the screen. Star Wars (and before that, Lucas's original if also derivative THX 1138) helped kick things off, although before that there were a few bright spots, like Westworld. Close Encounters of the Third Kind further cemented the deal started by Star Wars. Tron and E.T. were original creations, although Blade Runner was not (even if it strayed a good deal from its source material), and later on Spielberg's idea factory also gave us Back to the Future, which has held up a lot better than I thought it would.
Now we're getting more and better original SF for the screen than ever, it seems: Inception, Looper, Primer, Cube, Gattaca, the better installments of the Terminator and Alien franchise (and for the latter, Prometheus, for all of its flaws and divisiveness), Twelve Monkeys, The Abyss, RoboCop, Dark City, The Matrix, In Time, eXistenZ, and tons more I'm probably forgetting. (Chime in below.)
Two major reasons for this come to mind. First is how many of the people responsible for getting films made these days were raised during that first boom of populist SF in the 70s and 80s. The other is a gentle erosion of the presumed barriers between SF and mainstream fiction. Instead of more people becoming "SF fans", they're simply becoming that much more receptive to the presence of "SF" in their entertainment, however it manifests. They may not seek it out on their own, but they don't mind as much when it comes their way. It's just another way for a story to be brought their way. My long-standing parallel example of this is the recent Batman films: they've been massively successful, even if they haven't converted moviegoers into Batman fans per se.
On the whole, I think this is a good thing. It not only means more SF written directly for the screen, but better odds of any particular SF favorite being adapted, too. Audiences become more familiar with SF as just another kind of film, and so they react less to where something comes from and more what it actually is. I sensed something of the same thing going on when Stieg Larsson and Jo Nesbø hit the best-seller lists. People didn't care where the stories were from, only whether or not they were interesting on their own terms. The fact that people weren't seeking these things out because they were exotic imports actually worked in their favor this time.
That said, some part of me wonders if we are better off with original SF work written for the screen, versus adaptations. The best of SF doesn't adapt well to the screen because it is not really designed to be there; it's meant to be read and not filmed. And then there are many stories that could be filmed, but to what end? There's nothing wholly prohibitive on a technical level (not anymore, anyway) about filming, say, Stranger in a Strange Land, but the larger question of whether it would be worth it goes unasked. That particular book has dated badly, and I suspect the continued championing in SF-fandom circles of a film version is a case of rooting for an underdog, as various attempts to film the book (including one with a script by David Gerrold, which he swears he got fired from for doing it right) all died a-borning. Maybe for the best.
I feel the same way about the unpleasant prospect of having Asimov's Foundation stories adapted by Roland Emmerich. It is bad enough given who's at the helm, but the other problem is that the Foundation books just plain don't lend themselves to cinema. They're mostly about people standing around and arguing their particular position, with almost all the major action happening somewhere offscreen. That doesn't make them any less interesting; it just means they're a poor choice for a medium where action and imagery come first, dialogue second, and ideas dead last.
Further down the ladder are things like the proposed live-action film versions of AKIRA and Evangelion (god, why?), with the fan enthusiasm for either one again being a case of underdog syndrome. Anime fans want validation by having their story picked for the next Big Movie, since the Tolkien fans have already been that lucky. But there aren't enough AKIRA or Evangelion fans to make a proper live-action film of either project an economically feasible proposition — and there are plenty of anime fans themselves who don't number either of those projects among their favorites, and for good reason. If we must have anime-to-live-action projects, let's start with titles that won't run into pettifogging relocalization issues and have some mainstream appeal to boot: Claymore, Vampire Hunter D, Soul Eater [Tim Burton, call your office], the long-heralded Battle Angel Alita film [Mister Cameron, ahem!], the Hong Kong and Hollywood pastiche of Black Lagoon, the epic-scale steampunk of Fullmetal Alchemist — you get the idea. I think I am letting my prejudices show, so I will close there with an appeal for some more thought in this vein.