Why is it that practically every time sci-fi characters discover evidence of extraterrestrial life, they are just as swiftly confronted with creative new ways to die? As “we are not alone” scenarios go, “Life” is no exception, although it’s unusually intelligent for so much of its running time — picture white-knuckle “Alien” hijinks grounded by “Gravity”-strong human drama — that the lame-brained last act comes as a real disappointment (unless you’re determined to read this Sony-released Mars-attacks thriller as an origin story for Spider-Man’s Venom nemesis, which it is not).

Still, overlook its inevitable wah-wah ending (cue sad trombone sound effect), and “Life” is far better than the trailers made this me-too outer-space opus look. Assuming that “Passengers” hasn’t quashed audiences’ appetite for space-station movies, and that sci-fi enthusiasts wouldn’t rather simply wait for Ridley Scott’s fast-approaching “Alien: Covenant,” then director Daniel Espinosa’s mostly-smart, plenty-stylish entry could eke out a nice box-office life.

Working in its favor is an international cast — even more inclusive than “The Martian’s” multi-culti support crew — with the added bonus that everyone, not just white-boy A-listers Jake Gyllenhaal and Ryan Reynolds, has an important role to play. The six-person ensemble make up the Mars Pilgrim 7 Mission, sardined aboard a claustrophobic space station whose Nigel Phelps-designed floorplan proves positively mind-boggling — this despite a stunning establishing tour, during which, via an “unbroken” (but vfx-assisted) nearly-seven-minute single take, the camera makes the rounds of what will soon be a $200 billion coffin. Clearly determined to rival Emmanuel Lubezki’s Oscar-winning work on “Gravity,” DP Seamus McGarvey hovers just over the shoulders of the crew during this opening scene, as they diligently collaborate to recover a Martian-specimen-collecting capsule carrying God knows what.

At first, the alien being — which is soon christened “Calvin” — appears to be an innocuous, inert single-celled life form, visible only beneath a high-powered microscope. But when lead scientist Hugh Derry (British actor Arlyon Bakare, buff-upper-bodied but CG-withered from the waist down as a disabled doc who doesn’t need his wheelchair in zero gravity) feeds the organism glycerin, it swiftly multiplies, exhibiting characteristics that are a credit to screenwriters Rhett Reese and Paul Wernick’s creativity: every cell has muscular, neural, and photoreceptive properties, suggesting the potential for an incredibly strong, fast-adapting entity.

To celebrate the discovery, horns blare on Jon Ekstrand’s constantly shape-shifting score (one moment, he’s waxing optimistic with low-key strings, the next, he’s amplifying the tension via “Inception”-style foghorns). Hugh can hardly contain his enthusiasm, though there are other crew members on board to take precautions, most notably Miranda North (Rebecca Ferguson, the most disciplined character in the motley ensemble), representing the Centers for Disease Control and Prevention — because nobody knows what Calvin is capable of, even after he’s attacked Hugh and face-hugged one of the other crew members.

It’s Miranda’s job to design firewalls the alien can’t breach, while it appears to be everyone else’s (unofficial) task to create opportunities for Calvin to get out. This is where a movie that has taken great pains in its stunning first act (a bit pokey for genre fans, but impressive in its willingness to give characters like Gyllenhaal’s Syria-surviving space medic a backstory before snuffing them one by one) makes a gradual turn for the worse. You see, the aptly named “Life” isn’t just about making a landmark discovery that could provide clues to life’s “nature, its origin, and maybe even its meaning”; it’s also about the biological imperative for survival, and the way in which even the cleverest humans will ignore their training in order to prevent their own deaths, and those of the people they care about.

But “Life” isn’t an especially philosophical movie, and it’s weakest when the screenplay pretends to be making protocol-questioning decisions in the heat of the moment. As Miranda could attest, if Calvin turns hostile — and it doesn’t take long for that to happen — the entire crew should be prepared to sacrifice their lives in order to prevent the “symbiote” (to borrow the term used to describe Venom, even though Calvin never lingers long on a human host) from finding its way back to earth. Instead, the characters — and Gyllenhaal’s David Jordan in particular — are so empathetic, they’re constantly opening hatches that should remain locked shut in order to save goner crewmates, or themselves.

On the plus side, such ill-advised and undisciplined behavior serves to boost the suspense considerably, and even though we can never quite get a handle on what Calvin can do — the invertebrate creature can insta-digest an entire rat, withstand prolonged exposure to fire, go long stretches without oxygen, survive in sub-zero space, and propel itself through narrow apertures — one thing is clear: it ain’t friendly. (It also isn’t true to the screenwriters’ concept for long, eventually taking on a multi-tentacled, menacing-squid form that looks like something out of Patrick Tatopoulos’ playbook.)

Fans of “Deadpool” duo Reese and Wernick may be disappointed to find precious few genre-savvy wisecracks in the finished film (though a nerdy “Re-Animator” reference survives). Frankly, “Life” could have used a few more cathartic laughs, although it’s a relief that the entire movie isn’t as self-aware or sarcastic as the writers’ reputation-making “Zombieland.” While that high-attitude approach may have been the right fit for an undead spoof, “Life” benefits from a certain seriousness of tone — one that Swedish-born director Espinosa (“Easy Money”) sustains even when the characters’ choices start to get silly.

We can understand why senior crewmember Sho Murakami (Japanese actor Hiroyuki Sanada, resuscitated from Danny Boyle’s semi-similar “Sunshine”) might risk his life to be reunited with his newborn son on earth, but Espinosa’s heretofore elegant direction suddenly gets sloppy during the climactic moment, when this Toshiro Mifune-like tough guy might have had an “Aliens”-iconic faceoff with Calvin. Weirdly, the only person who behaves in a responsible way is Russian cosmonaut Ekaterina Golovkina (ethereal “Twilight Portrait” star Olga Dihovichnaya, whom more Hollywood directors should cast pronto), but the movie is better served by bad decisions. “Life’s” a thrill when it’s smart, but it’s even more exciting when the characters are dumb — which is ultimately a paradox the film wears proudly, to the possible extinction of the human race.