The tendency for several sci-fi alien species to be one facial feature away from humanity.
Sometimes they're not even that far away. They look totally human and sound human. In some cases, this may well be a disguise or A Form You Are Comfortable With, but in others this appears to be their natural appearance. See Human Aliens.
You'd think that alien species would be radically different — insectoids, three-legged wombats, giant cats, etc. — but animal-cruelty laws tend to discourage fitting animals with prosthetics, and the effects budget only allows for latex and makeup, so we get humans with brow ridges, humans with extra nostrils, humans with Pointy Ears, humans with bony protrusions, and so on. One odd consequence of this, however, is that in the Federation Council scenes in the Star Trek movies, you often see very strange, non-humanoid (or only partly humanoid) aliens, because the movies have the necessary additional budget for them. These additional races are, of course, never seen in the TV series at all.
Gene Roddenberry gave more reasons for this in an interview once. Budget constraints aside, if you try to make aliens look completely alien, you'll firstly make them look ridiculous (cf. Doctor Who), and secondly make it doubly hard for the actor playing the alien to do anything mildly resembling acting. This has actually been isolated to extremely specific requirements: if an audience can't see an actor's eyes or mouth, their ability to empathize with or emotionally invest in that character is significantly impaired. This is one reason why mooks, especially SF mooks like the Cylons or the Imperial Stormtroopers, are so often uniformed in face-obscuring helmets. Additionally, Gene Roddenberry had always insisted that Star Trek was about human issues and that the aliens are intended as vehicles for social commentary. This required aliens that may have been scientifically implausible (humanoid appearance, ability to communicate in English and emote like humans, etc) but easy for the human characters to interact with and the audience to relate to in the narrative.
The anime equivalent is the alien with Pointy Ears, colorful facial markings, or cutesy animal-like traits.
Rubber foreheads also tend to be paired up with Humans Are White for some reason, likely the fact that back in the 60s/70s it was easier to get a black, Latino, or Asian actor on TV by gluing something to their heads and claiming that they were raceless otherworldly beings instead. You will virtually never see a Caucasian Klingon, which means that the makeup department actually has to do a lot more work when a white actor plays one.
The next step past Rubber-Forehead Aliens (catlike or buglike or lizardlike aliens that can still sit in chairs and hold weapons) is Humanoid Aliens, possibly overlapping with Intelligent Gerbils. Contrast with Starfish Aliens.
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In UK advertising, the Tefal Eggheads. Later parodied by Ant McPartlain's actual forehead.
Anime and Manga
Crest of the Stars: The Abh were distinguished by their blue hair though some of them also had pointy ears.
This one does get justified, though, in that the Abh are in fact genetically altered humans, who even call their stellar nation the "Humankind Empire Abh" (or a variant, depending on how you translate it); the Abh see themselves as humans with a few different traits, while their (non-modded) enemies tend to see them as vile aliens, wholly different from humanity. One of the narrative thrusts of the work is examining just how human they really are - or aren't.
UFO Robo Grendizer: The Vegans and other alien races have strangely-shaped heads or unusual skin or hair colours at the very least. Gandal has blue skin and his head looks cubic due to his very angular features, his wide forehead, and that the top of his head is nearly flat. Blackie has dark-purple skin, pointy ears and a very pointy, hairless head. Even the most human-looking aliens -like Rubina- have bluish or anormally pale skin, black irises or green or aquamarine hair (keep in mind no human character in this anime is blue-haired).
The Saiyans in Dragon Ball Z are just humans with tails in appearance; that turn into giant monkeys during a full moon.
Dragon Ball Z is very bad about this. There's even a whole race of "humans but skin is different color"; the Brench-seijin, which Jeice and Salza belong to.
Practically all aliens in Leijiverse. Mazone, Illumidas, Tokarga... they're all humans with slightly different skin colour, even the Mazone who are plants. Miime's race is unrevealed, but she's also fully humanoid apart from not having visible mouth in most? story versions.
Asobi ni Iku yo! features Little Bit Beastly aliens like the Catians (cats) and Dogisians (dogs). Hilariously, the Catians originally called their home planet Earth and referred to themselves as Earthlings — they changed the name of their planet and species to Catia out of courtesy. A race of bunny girls is also briefly mentioned.
The Zolans in Macross 7 appear mostly similar to humans (due to the Protoculture messing around with their evolution), except for very Pointy Ears, two-toned brightly-colored hair, and prominent fur on the forearms of the men. They are also implied to be marsupials (like most of the lifeforms on their homeworld).
Elves in ElfQuest are humans with pointy ears and four fingers. This is because their shapeshifting alien ancestors deliberately took on a human-resembling form before landing (they even reshaped their spaceship to look like a palace). But even before that (flashbacks), said ancestors already looked fairly human in shape, and would have qualified as Humanoid Aliens at least.
Though technically Human Aliens, Viltrumites from Invincible are also Hairy Upper Lip Aliens. All Viltrumites have black hair, and all male Viltrumites have mustaches, which makes it pretty easy to tell them apart from humans. In fact, one character simply removed his fake beard, showing his Viltrumite mustache, in a pretty hilarious reveal.
Lampshaded in some Space Agent Valerian book. There are quite a few non-human aliens but also a multitude of practically humans, to the point where it's mentioned that "one head, two hands, two feet, two eyes, could be anyone".
The Kree, from Marvel Comics, are extraterrestrials with two main races: the blue-skinned (such as Ronan the Accuser), and others who seem completely human (such as Captain Mar-Vell and Marvel Boy). There's a little problem: the blue Krees are dominant in the Kree social hierarchy, and treat the others as worthless slaves. With such a background, it's nobody's surprise Ronan's opinions of us humans.
Films — Live-Action
The movie This Island Earth's aliens were similar to humans except for huge foreheads and white hair.
In Galaxy Quest, the character of Dr. Lazarus from the Show Within The Show is played by Alexander Dane (who is in turn played by Alan Rickman) wearing a rubber forehead. The Thermians think he is a real alien, even though his rubber forehead begins to show damage and develop holes over the course of the adventure.
The Thermians themselves are a subversion of this: on first appearance, they look like short-ish humans who have Vulcans for hair stylists. However, it turns out it's just an illusion. They're really Starfish Aliens.
The movie Trail of the Screaming Forehead takes this to the logical extreme. The aliens are foreheads that attach themselves to humans. The movie is pure, high quality B grade.
Mangalore warriors from the same movie appear to be this but were actually the products of CGI and animatronic costumes.
The Battlefield Earth film featured the Psychlos, whose main distinguishing features were that they were big, had eyebrows that joined their hair, high foreheads, and dreadlocks.
Prince of Space: the men of Krankor are rubber nose aliens. This might have been slightly more badass had the noses not had a silhouette much like a chicken beak.
Unlike Star Trek, Star Wars generally tries to avoid hewing to this trope by striking a balance between Rubber Forehead Aliens and Starfish Aliens, making most alien species look like bizarrely tweaked, vaguely humanoid versions of non-primate animals. However, there are quite a few species that look almost human; the most famous example is probably the head-tentacled Twi'leks, who are often considered sexually appealing to Humans and can interbreed with them.
Averted in Animorphs. Not only do all the other alien races look nothing like humans, but it turns out that humans are the only civilization in the universe that walks upright on two legs.
Well, Hork-Bajir have two legs, but they have their tails for balance, and generally look like dinosaurs.
Word of God is that Applegate originally intended to use Rubber-Forehead Aliens just in case the series got a Live-Action Adaptation. Then Scholastic rejected the idea and told Applegate to be more creative. So she did and made the aliens damn-near IMPOSSIBLE to adapt to live-action (at the time). Then the TV series was made and we all know how that turned out.
This trope is lampshaded and mocked countless times in the books. Due to the book's tendancy to describe what the same aliens look like each book, the number of times the Animorphs have said something to the effect of "REAL aliens aren't just humans with rubber foreheads and putty on their faces" could fill an entire page on it's own.
Played with in Alastair Reynolds novel House of Suns; All of the civilizations in the Milky Way originally came from Earth, but over millions years (the novel is in 6.3 million AD) they have diverged somewhat. One of the characters seen in the story has a elephant-like trunk, and other humans are mentioned as having scales or full body hair.
Star Wars has a lot of humanoid aliens, most of which find the nitrogen/oxygen atmosphere found on human-inhabited planets tolerable, if not comfortable (there are exceptions to this rule, such as the Kel Dor, who must wear goggles and breather masks at all times on human-habitable worlds). There are also, however, several non-humanoids, including a handful of insectoids, a lobster-like species, more than a few quadrapeds or hexapeds... and one that looks like nothing so much as a floating brain. The most human-like aliens are called near-human, and are considered to have descended from humans (the blue-skinned, red-eyed but otherwise human Chiss are a typical example).
The Yuuzhan Vong look like big, muscle-y humans with a few deliberate errors- their skin tones are varying shades of grey rather than brown, they have talons instead of fingernails, their foreheads are prominent and sloped, and their hair is almost always black when they're not bald (which is more common among them than it is among humans). Artists also commonly depict them with pointed ears, though this is never described i the novels. Since they treat ritualized Body Horror as a mark of high status, the higher-ranked a Vong is, the less humanoid they usually look.
The Psychlos in the book version of Battlefield Earth are vaguely-described, but come across as big, hairy humans, save for inexplicable "eyebones" and "mouthbones" instead of eyelids or lips.
The Classic Space OperaThe Lensman Series had human, humanoid and utterly alien species. It also had a guiding sentient race that was controlling evolution on many different planets.
Justified in the early science fiction novels, Last and First Men. The varieties of human aliens are a result of original humanity escaping from a dying Earth. For two billion years, humanity evolves through nine different stages and splits off into a smaller set of subgroups.
Wicked Lovely tends to go this route with a lot of The Fair Folk. Niall, for example, is described as having "too-sheer skin, like parchment by a flame, and too many joints."
The aliens in Deathscent by Robin Jarvis are somewhere between this and Humanoid Aliens. While both races look quite like humans - enough that when one first arrives the human characters don't realise what he is until they see him in the light - their biology is very different. One of them perceives the world primarily through sense of smell, has four nostrils (one of which is in the forehead) and speaks a musical Starfish Language.
A large number of the Culture's alien species are near human. It was explained away as a convergent evolution thing.
In fact, most of the books take place before humanity as we know it achieves spaceflight. Most of the Culture's citizens are Human Aliens and no distinction is made between them and us.
C. J. Cherryh's series Foreigner deals with the deceptively humanoid alien race known as the Atevi. While they look similar to us, they think entirely different then Humans.
Many of the land-dwelling races in the Books of the Raksura are human-looking, but with a trait such as fur, horns, oddly colored skin, or the like.
Subverted in David Brin's Uplift novel Brightness Reef. The Rothen, a human-like alien race trying to con human beings, wear artificial foreheads and other facial prosthetics to make themselves appear more human-like. The Tymbrimi are genuine Rubber Forehead Aliens, mostly-human looking though with different proportions and facial features, granted they're minor shapeshifters who can gradually change their features over time. All the other aliens in the series are Intelligent Gerbils or Starfish Aliens.
In Robert A. Heinlein's The Star Beast, the Rargyllian Dr. Ftaeml is described as a Medusiod Alien, being a humanoid with tentacles on his head resembling snakes. There is also passing mention of exchange students from Procyon VII who are "cephalopods".
Older Than Feudalism: The very first "science fiction" novel, A True Story by the Ancient Roman author Lucian, has rubber forehead aliens living on the Moon... and the Sun and several stars. A Moon-person looks basically human but has one toe on each foot, a marsupial pouch in his belly, a leaf growing out of his butt, and leaves for ears. They do have a lot of Bizarre Alien Biology (removeable eyes and genitals, hermaphroditic/all-male reproduction, the ability to grow people on trees, etc.)
The Catteni from Anne McCaffrey'sFreedom quartet would be Human Aliens except for their grey skin. This is made fairly explicit when several human characters successfully disguise themselves as Catteni with face paint.
In Star Trek, Klingons, Vulcans, Bajorans, Ferengi, and Cardassians, just to name a few. In fact, the majority of all races encountered in every Star Trek series has two arms, two legs, a head, and a general chest area. The exceptions are usually Monster of the Week types. Most common is a small latex prosthetic on the bridge of the nose or eyebrow. It's no wonder that Stargate SG-1 (Voyager's contemporary) gave up on all pretense and filled the universe with plain humans.
Unfortunate Implications began to arise by the time Star Trek: Voyager rolled around. Recurring enemy races each have an elaborate, distinctive look. Due to budget constraints, however, everyone else is stuck with tiny, inexpensive prosthetic. This results in all of the "bad" aliens wearing dark makeup, while all the hapless "good" aliens are Caucasians with Silly Putty on their noses.
Laas: And humanoids are not very tolerant of difference.
Odo: Some of them are. There are dozens of species on this station. They tolerate each other's differences very well.
Laas: He has bumps on his forehead. She has a wrinkled nose. But they're basically alike. They're bipeds that eat, sleep, breathe. You and I are nothing like them.
For example, the Bajorans. Small ridges on the nose are the only visible differences between fully-clothed Bajorans and fully clothed Humans. There is a reason for this: the Bajoran makeup was designed the way it is specifically to make sure that the (numerous) Bajoran females who would appear in the franchise would all still be good-looking. The Bajorans were also planned from the outset to be refugees. The minimal makeup was convenient for costuming large crowds and child actors.
The producers tended to do a good job of giving the Bajorans distinctive (albeit uniform) features in other respects, though. Bajorans almost always have short hair, regardless of sex, and they are normally always short and of thin build. Clothing is pretty much always either bergundy red or light beige, and both sexes almost always wear elaborate earrings. This had to do with the religion of the Prophets; a Bajoran psychic could gain information about a person's "pagh," or soul, by holding onto their left ear.
Klingons only gained their rubber foreheads when the movies' increased budget permitted it. Prior to the retcon, they were Entire Bottle of Bronzer and Upswept Eyebrows Aliens. In fact, until attention was called to it in Deep Space Nine, the Literary Agent Hypothesis was the official explanation: in-universe, they weren't considered to look exactly like humans. There just wasn't the budget to portray them as they actually looked. (There's actually an onscreen reference, sort of: a Klingon posing as a human was said to have been surgically altered to appear human. If we take what's onscreen at face value, it wouldn't take surgery, just a haircut.)
The 2009 movie seems to be going out of its way to give us a new variation with the large eyed aliens.
It also makes the Romulans worse than the Klingons in the "where'd the foreheads come from?" department. They went from having ridges to to not having them to having them again to having lost them again. What? Also, while Klingons' gaining ridges after TOS has been referenced, and explained much later, there has never been any onscreen acknowledgment of the changes in Romulans.
The Klingons' appearance in a deleted scene of the movie is actually something of a Lampshade Hanging of this issue, as the Klingons we see have ridged... masks (as in what the characters are wearing). We don't actually see their faces.
Even more curiously, photos of Victor Garber with his Klingon helmet off show that he had been given a ridged nose... even though it would have not been visible onscreen.
The Star Trek franchise has racked up several nominations (and wins) for awards in makeup because of how often they have had to pull some crazy stunts with the rubber. One nomination, for instance, was for a Ferengi who put bigger earlobes over her own to look male. That's right, prosthetics on top of other prosthetics.
Betazoids are notable in that their One Facial Feature is so subtle that they have an entry on the Human Aliens page: they have all-black irises.
All of this was parodied by the production staff on Deep Space Nine: when they had a test printout of their episode script formatting and needed a fake episode title for it, they went with "ATTACK OF THE ALIENS WITH BUMPS ON THEIR FOREHEADS!"
The early 1970s Roddenberry production Genesis II had post-humans with two navels as their "distinguishing characteristic". That was mostly a "screw you" towards the censors. For some reason, up until then navels were considered taboo.
Babylon 5 is almost a case study on alien race-types on television. The Shadows and Vorlons were Starfish Aliens. Then, the League of Non-Aligned Worlds, the minor powers of the region, were a mix of Humanoid Aliens and Rubber-Forehead Aliens. The Minbari were Rubber-Back-of-the-Head Aliens, being bald humans with a boney crest covering the back of their skulls, the Narn were fairly elaborate Rubber Forehead Reptilians, and, finally, the Centauri were Human Aliens distingushed only by crazy haircuts and, sometimes, eyebrows and fangs.
The Twilight Zone episode titled "Will the Real Martian Please Stand Up". The setting of this episode is a rural restaurant. During the 25 minute episode, we wonder which one among a group of people is the alien. It turns out, we were seeing the alien all along, and that there were two of them. One alien has an extra arm (this one is from Mars). The other one has a third eye (he's from Venus).
There are some hilarious rubber headed aliens in Mr. Dingle, the Strong which feature one race as two guys stuck together with metal antennas on their heads and another race as children with painted mustaches partially inflated balloons on their head and two bug-like antennas. Thankfully that episode was meant to be a comedy.
The infamous To Serve Man featured tall aliens with larger brains.
Most "aliens" in the Stargate Verse are just humans, transported from Earth in antiquity. But of those that don't, some — particularly other species used as hosts by the Goa'uld — still fit this Trope.
The Unas are humans with scales, chin spikes, and only 4 fingers, which puts them more in the Humanoid Alien category.
Buffy the Vampire Slayer and Angel have a lot of demonic races that are essentially humans in various different colors of facepaint. However, this is partially justified because almost all demons that appear are possessing or have interbred or been contaminated by humans. The few Pure Demons that have appeared didn't look remotely human:
Thals are essentially no different from humans other than bleach-blonde hair, while Kaleds are all black-haired humanoids, until they mutated into Daleks.
The Expanded Universe has offered an explanation; Rassilon, the founder of Time Lord society, was a bit of a racist and deliberately tinkered with evolution so that races resembling his own would be more likely to evolve. While it's uncertain if this is considered canon, the show has addressed that humans and Time Lords look almost identical.
Amy: You look human. The Doctor: No, you look Time Lord. We were here first.
Multiple Votan species in Defiance look a lot like humans.
Irathients have bright red hair, red and white skin patterns, and a slightly more prominent forehead.
Castithans have very pale skin and white hair.
Indogenes have no hair and white (actual white) skin covered with hexagon patterns, justified as they are cyborgs.
Bio-men on the other hand are hulking, blue-skinned, and bald, but are actually human Super Soldiers.
A recurring sketch on Human Giant is an actor who plays one of these on a show called Battle Sector 17. Fed up with having to spend the time to apply the make-up he gets surgery to make it permanent... just before the show is canceled. He then spends several epsodes trying to get work but no will take him because of the make-up, not even for his old Battle Sector 17 character. When he finally gets a job he has to spend hours to get make-up to make him look human, it doesn't quite work.
The in-universe setting book Xenology references this; while the Eldar and Tau, and to a lesser extent the Orks, look outwardly like slightly modified humans inside the races are nothing alike. The Tech-Priest doing the dissections is extremely confused by this, especially since his other subjects aren't remotely humanoid.
Space Munchkin the RPG parodies this trope with the "Bumpy Headed Alien" racial choice. You choose, among other things, your facial bumps, the concept your entire species is devoted to, and the one aspect of human culture your species doesn't understand ("we do not have a word for this thing you call 'hygiene'")
The Split race in the X-Universe games are tall human-like aliens with very rough looking narrow faces, and odd colored skin.
The dominant race in the Jak and Daxter series are humans with excessively long pointy ears.
Subverted in Phantasy Star with Newmans/Numans, and later Beasts in Phantasy Star Universe. Sure, they look human enough, save for their ears and (in the case of Beasts) their harelips and eyes ... but they aren't actually aliens at all. They're actually genetically engineered humans.
Mass Effect: Asari fit the trope perfectly, being blue-skinned alien space babes. Although their tentacles are actually on the back of their heads, where a human would have hairnote In the third game, Liara describes them as "semi-flexible cartilage-based scalp crests that grow into shape". While they wear the same armor as humans in-game, every race sees them as their equivalent of blue-skinned alien space babes except possibly the Krogan, who still see them as attractive.
In Mass Effect 3, if a male Shepard romanced Tali and saved the quarian fleet, Tali will leave a photo of herself without mask or helmet on his nightstand. It turns out that were it not for the skin markings, the three-fingered hands, and the avian legs, she could pass for human. It helps that her "photo" is literally a photoshopped stock image of a model◊
On the technical side, the developers of Mass Effect admit that when it comes to fighting their alien designs were limited to bipeds with human proportions because of the Unreal engine's combat system. That's the reason you never see any of the more alien races such as the Elcor or Hanar in-combat; each race would require whole new skeleton rigs of their own.
In the Star Ocean games, Nedians and Expellians are identical to humans, while Roakians all have tails (and there are "Lesser Fellpool" who are more similar to cats, including cat ears. Interestingly, Roddick makes sure the Earthlings know that they're related to cats rather than monkeys, seeming to indicate that they descended from them. The third game introduced a bunch more alien species, some of which are humanoid dolphins, dwarves and such, others of which look practically identical to humans.
Miriam in Shining Force Feather might be a living Lampshade Hanging. She meets our protagonists and is immediately amazed, as she hasn't ever seen a human before. Never mind that Miriam is an elf, and that the only difference between her and Jin are her pointy ears, slanted eyes, and skinnier build. One scene later, she meets Alfin and is equally wowed, as she's never seen a Core Unit before, despite that Core Units are... Ridiculously Human Robots. Meanwhile, she meets all the varieties of Petting Zoo People with no more than chipper enthusiasm.
Subverted in Freefall: Sam Starfall looks humanoid, but it's really a suit to let him operate in an Earth-like environment. We don't get to see his true appearance, but it involves tentacles, and humans apparently find it disgusting.
The trolls in Homestuck make reference to a lot of Bizarre Alien Biology endemic to their species, like "chitinous windholes", "auricular sponge clots", "porous cranial plates", and various colors of blood, but outwardly just look like grey-skinned humans with yellow-orange eyes, horns, and fangs.
Complicating this is the fact that some of the Bizarre Alien Biology seems actually more likely to be bizarre alien terminology, instead, or a very idiosyncratic translation convention if you presume they're not speaking English in the first place.
All the known nonhuman species in Terra fall here. The Azatoth would be Human Aliens but for a heavier brow ridge and a couple nodules on the sides of their noses; they also tend to be a foot taller than the average human. The Vareliens look like asari with longer, more flexible head-tentacles. The Shintari have swept-back conical heads and orange skin.
The Pelkons from the Global Guardians PBEM Universe are humanoid, and have the right number of fingers and toes and such, but their eyes are pure milk-white, their hair is thicker and more brush-like, and they have spots like a leopard. on their forehead that run down the back of their necks and across their shoulders.
In the Homestar Runner episode "buried", Strong Bad unearths what he believes to be an alien artifact. He states that the earth was colonized by extraterrestrials, and that it explains "why all beings look the same except for slight differences of our foreheads!"
Due to non-professional special effects and costumes, Noah Antwiler (from The Spoony Experiment) portraying Terl from Battlefield Earth in some episodes of Channel Awesome's review shows and the fourth year anniversary To Boldly Flee looks much more human than the version portrayed by John Travolta. It also means most "rubberhead" effects could not be recreated. However, it's still clearly a Psychlo.
Warhok and Warmonga, the Proud Warrior Race aliens who appear in season four of Kim Possible are an animated version of this. They're nine feet tall and have green skin.
In another animated example, the cast of Futurama is virtually all two-arms two-legs one-head humanoid (for most of the time). This is probably more to make it easier to animate jokes for them than anything else, as the show has otherwise shown a fair amount of ingenuity in depicting odd aliens (sentient nebulae, swarms of flies, etc). Subverted with Leela, who is revealed in an early season to be a human mutant who was raised to think she was a Last of Her Kind alien.