->"The too-perfect security of the Upper-worlders had led them to a slow movement of degeneration, a general dwindling in size, strength, and intelligence."
-->-- '''Creator/HGWells''', ''Literature/TheTimeMachine''

Evolution isn't [[GoalOrientedEvolution goal-directed]]. Sometimes the directions it takes, in RealLife and in fiction, can be surprising. Certain species of New World monkeys, for example, re-evolved secondary claws from their fingernails, having lost their true claws earlier in their evolution from more Prosimian-like ancestors. Similarly, in fiction, a species (often but not limited to humanity) will sometimes evolve into a more feral, less civilized, sometimes even non-sapient variety, regaining "primitive" characteristics. These "primitive" characteristics can include behaviors and/or physical traits. Whether or not the result of this evolution is still recognizably related to the parent species varies from work to work and species to species.

This trope tends to be expressed in a matter of degrees. Most basic is the scenario common to Post-Apocalyptic settings AfterTheEnd, where humanity (or another species) is still physiologically more or less the same, but society has collapsed and technological and cultural regression have set in. The people are living a more primitive existence than their more technologically and culturally advanced ancestors. They may even intentionally evoke a "tribal" aesthetic, and may raise a BarbarianHero or two. The scruffy survivor of a ScavengerWorld is usually not an example of this trope, but the tribe of abandoned feral children he's likely to encounter on his trek across the wastelands almost always is.

In more extreme scenarios, the population may have evolved into a new subspecies or another species altogether with more "primitive" or "feral" behavioral and physical characteristics than their more "civilized" ancestors. The change may be subtle [[FrazettaMan (like increased muscle mass, more body hair and a neanderthal-like face)]] or [[BizarreHumanBiology much, much more radical]]. Whether these changes in the population constitute a new species or are simply a variation or subspecies of the original depends on the work, but typically most ScavengerWorld-type future settings are not far enough removed from the PresentDay for natural selection to favor such drastic changes.

May overlap with WasOnceAMan. Sometimes, it's the motivation of an EvilutionaryBiologist to try and take control of evolution in order to avert this fate.

Common to AfterTheEnd settings. Likely to exist in [[HumanitysWake Humanity's Wake]].

Contrast EvolutionaryLevels, UltimateLifeform and TheSingularity (all of which tend to assume evolution's [[GoalOrientedEvolution a linear, goal driven process]]). Has nothing to do with the effects of a DevolutionDevice. The change must be a result of cultural and/or biological evolution; it cannot be the result of a DevolutionDevice or other similar transformation on an individual or a group.

Please note that for an example to fit this trope, it must feature an entire population subjected to evolution (whether that evolution is cultural or also physiological). Isolated persons raised by animals or left to fend for themselves in the wild would not count. Neither would any creatures created by a MadScientist by combining human and animal traits (such as in ''Literature/TheIslandOfDoctorMoreau'' and its various adaptations and homages in science fiction works). A virus with the same effect on individuals as a DevolutionDevice would not be an example of this trope. This trope applies to populations, not individuals or groups with too small a gene pool for viable reproduction. However, evolution does not have to be portrayed realistically for this trope to apply. "Evolution" can work however the writer wants it to work in his/her [[TheVerse 'verse]], so long as it's stated to be evolution, and not the result of some other process or event, like a MadScientist's experiments, a DevolutionDevice, or a transformative virus. However, if a gradual change began with experimentation, or a virus, etc., which altered the terms of natural selection, resulting in a dramatically altered evolutionary trajectory, it '''would''' count as an example.

Also note that in RealLife, as already stated, evolution is not goal directed; the only "value" of a trait in evolutionary terms is how much that trait enhances the fitness of an organism or its offspring. Like this trope, real evolution can involve the loss of traits as well. For example, parasitic leeches often lack many traits in comparison to their free-living cousins, which include other annelid worms like Earthworms and beachworms. This is because these traits, unnecessary for the parasitic lifestyle, had no fitness advantage; it's possible they even detracted from a parasite's fitness. This does not mean that parasitic leeches are any more or less "evolved" than non-parasitic annelid worms. In evolutionary science, "primitive" simply means "retaining ancestral traits." And events like Anthropoid monkeys evolving secondary claws from their fingernails are the rare exception, not the rule.
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!!Examples

[[AC:ComicBooks]]:
* ''ComicBook/{{Kamandi}}'', a comic book adaptation of ''Franchise/PlanetOfTheApes'' with the SerialNumbersFiledOff: instead of just apes, all sorts of animals have become more advanced, while humans act like savage animals.

[[AC:{{Film}}]]
* In the original ''Franchise/PlanetOfTheApes'', humans in the far-distant future have regressed to animalistic mutes hunted and enslaved by the Apes.
** The female love interest starts to speak in the second movie, implying that there was no physiological change (IE muscular or neurologically-related loss of speech faculties).
** In the original book version of ''The Planet of the Apes'' Ulysse (the equivalent character of Taylor) teaches Nova how to talk and their son can talk from the get-go. Also TheReveal that [[spoiler:many many years ago apes were servants to humans]] is discovered through GeneticMemory in humans; when a certain portion of the brain is stimulated via electricity they can talk, recounting the stories of their ancestors which lead up to the [[spoiler:ape takeover]]. Also in the original book the humans run around nude rather than clad in skins (which no doubt was done to get the film past the censors).
* In ''YorTheHunterFromTheFuture'' a plot twist halfway through reveals that it is not set in the distant past, but a post-apocalyptic future. At least it would be a twist if the title of the movie didn't explicitly state it was set in the future.
* In ''Film/{{Idiocracy}}'', after 500 years of mind-numbing media and natural selection favoring large numbers of poor-quality offspring over smaller numbers of higher quality offspring, humanity has become rather stupid, and struggles on with only two things in mind: loads and loads of sex and cheap entertainment.
* Both the 1958 and 2002 versions of ''Film/TeenageCaveman'' feature a post-apocalyptic world where the descendants of human survivors have regressed to primitive tribalism.
* In ''Film/{{Pandorum}}'', the man-eating Hunters are presumed to have evolved from people who caught [[SpaceMadness space madness]] and turned into cannibals. Their evolution into feral predators was "accelerated" by artificial means.

[[AC:{{Literature}}]]
* The morlocks (ape-like violent savages) and eloi (docile and meek) from ''Literature/TheTimeMachine'' are the TropeCodifier, if not the UrExample as well. The Time Traveler theorizes that morlocks are the descendants of the working class, and the eloi are the descendants of the upper class. A brief visit even farther into the future (omitted in some editions of the text and restored in others) sees humans evolved into gray rabbit-like animals, preyed on by giant insects.
** Of course, there is a bit of a subversion here, since [[spoiler:the eloi are, contrary to first impression, the less "civilized" of the two species, having been reduced to cattle by the morlocks.]]
* ''Literature/{{Galapagos}}'' describes descendants of the human survivors of a global epidemic, stranded on the titular islands, evolving into a non-sapient, semi-aquatic, furred, seal-like species, due entirely to natural selection.
* Happens several times in Olaf Stapledon's ''Literature/LastAndFirstMen'':
** The seventh men, who were designed with wings and did not care about technology, however the "wingless mutants" who evolved into the eighth men did.
** The tenth through fourteenth men, sentience briefly re-emerging from the whole ecosystem of animals that evolved from the ninth men. The fifteenth men manage to restore civilization to humanity though.
* In one of Creator/DavidBrin's ''{{Literature/Uplift}}'' novels, intelligent beings are trying to revert to unintelligent ones.
* Invoked in ''[[Literature/ManAfterManAnAnthropologyOfTheFuture Man After Man]]'', in which far-future biologists re-stock Earth's denuded ecosystems with engineered humanoids of animal-like intelligence. Most of their creations give rise to descendants that are even more different from their human ancestors.
* The original novel ''Monkey Planet'', known more commonly in English as ''Franchise/PlanetOfTheApes'', featured primitive, mute humans on a planet dominated by a civilization of intelligent apes. See the above example in {{Film}}.
* The Thrintun ("Slavers") from Creator/LarryNiven's ''Literature/KnownSpace'' stories ''seem'' to be this trope to the extreme, as their only living descendants are the Grogs: sessile creatures whose limbs degenerate into useless stumps when they reach adulthood, and which weren't even recognized as sentient when first discovered. Then we find out [[spoiler:the Thrint "[[MindControl Power]]" became so developed that [[WeWillNotHaveAppendixesInTheFuture the rest of their bodies atrophied]]. They're sessile because you don't need to move when you can glance at a prey-animal and ''tell'' it "get in my mouth"]].
* In ''Ape and Essence'', New Zealand and Equatorial Africa remain untouched by WorldWarIII because they were simply "too remote to be worth anyone's while to obliterate." Therefore, while a remnant of Western civilization has survived in New Zealand, Europe has been repopulated by the tribes of DarkestAfrica. This is zig-zagged with the main setting of the post-apocalyptic story, the Los Angeles basin, where the inhabitants retain barely enough trappings of civilization to call themselves a "democracy," yet radiation-induced deformities have become increasingly common to the point that the rulers have adopted the rites of HollywoodSatanism in a desperate attempt to ward off the extinction that will surely happen in a hundred years.

[[AC:LiveActionTelevision]]
* ''Series/DoctorWho''
** The "Futurekind" in "[[Recap/DoctorWhoS29E11Utopia Utopia]]", a savage, cannibalistic race from the end of the Universe. It is hinted that they are the early stages of humanity turning into the [[BitPartBadGuys Weevils]] from ''Series/{{Torchwood}}''.
** In "[[Recap/DoctorWhoS29E3Gridlock Gridlock]]", the Macra have "devolved" (in the Doctor's words) since their first appearance in season 4 from powerful, mind-controlling creatures to beasts who are living only to feed.
* In the ''Series/StarTrekTheOriginalSeries'' episode "[[Recap/StarTrekS3E1SpocksBrain Spock's Brain]]", the ''Enterprise'' visits a planet where an ancient catastrophe split society in two: the Female Eyemorg, a technologically advanced, automated subterranean civilization; and the male Morg, who live a Stone Age existence on the frozen surface.
* Tom Paris's "accelerated evolution" into a non-sapient salamander-like creature in the [[CanonDiscontinuity infamous]] "[[Recap/StarTrekVoyagerS2E15Threshold Threshold]]" episode of ''Series/StarTrekVoyager'' where they built a working Transwarp Drive on a shuttlecraft. The writer of this episode has stated that his idea was that in the distant future, humanity would evolve beyond the need for sapience due to technology providing for all our material needs.
* ''Series/BlakesSeven'': Servalan claims that the savage, ape-like Links, products of "accelerated evolution" on the artificial planet Terminal, are humanity's future. Presumably their name derives from the phrase "missing link."
** Many other episodes of ''Blake's 7'' featured primitive descendants of old Earth colonial expeditions; a few of them were genuinely HumanAliens, though, and would not count as this trope, and there were a few other ambiguous cases.
* The Sleestaks in ''Series/LandOfTheLost'' are the savage descendants of Enik's people, the Altrusians. At first, Enik refused to believe this, and thought he had traveled into his people's past. When he saw the ruined Lost City, he realized he had been wrong, and that he was actually in the future relative to his own timeline. Sleestaks and Altrusians look similar but with a few key differences such as height (Altrusians are shorter), skin color (Altrusians are brown, Sleestaks are green), and the presence (in Altrusians) or lack (in Sleestaks) of a third opposing digit. Altrusians are also more intelligent and able to function in daylight, whereas Sleestak are nocturnal and require low-light conditions.
* The setting of ''Series/TheTribe'': child and teenage survivors of an [[OnlyFatalToAdults adult-killing]] [[DepopulationBomb virus]] wear facepaint and form social units called "tribes," intentionally invoking this trope (despite the series taking place only TwentyMinutesIntoTheFuture).
* On ''Series/{{The 100}}'', the Ark and Mount Weather have maintained much of the technology and civilization from before the nuclear apocalypse, but the Grounders have reverted back to a tribal, hunter/gatherer society.

[[AC:VideoGames]]
* The Falmer in ''Franchise/TheElderScrolls'' series have degenerated from graceful snow elves into a race of blind, subterranean [[Literature/TheTimeMachine Morlock]]-like beasts with a primitive, xenophobic tribal culture. Possibly it has something to do with the dwarves, who forced them to blind themselves with poison in exchange for saving their race from genocide, used them as servants and slaves, and may have performed experiments on them that caused them to mutate into their present form.
* In VideoGame/{{Fallout 2}}, you start out in a tribe founded by the first game's protagonist. They have all the tropes associated with primitive tribes (spears, loincloths, face-paint), but they live in the post-apocalyptic future.
* The krogan in ''Franchise/MassEffect'' are portrayed as once being a more civilized race. After turning their homeworld into a nuclear wasteland, only the toughest and most psychotic krogan survived. In fact, krogan able to go into the now infamous "blood rage" were once only a small part of the population- and they were medicated for it!
* Ogres in ''[[VideoGame/ArcanumOfSteamworksAndMagickObscura Arcanum]]'' are speculated to be the descendants of a race of giants who lived in ancient times, but have since become extinct. Ogres are a good deal smaller than their ancestors (the average ogre is about 2m tall, the length of a giant's thighbone), and while giants were civilised and intelligent enough to use magic, ogres are savage and feral.

[[AC: WebOriginal]]
* This happens several times in the diverse setting of ''WebOriginal/OrionsArm'':
** The Epimethian Movement was a group who voluntarily removed their higher cognitive abilities to revert back to simian intelligence.
** Abdication happens when a post-singularity entity descends to a lower singularity level for various reasons.

[[AC: WesternAnimation]]
* In the ''WesternAnimation/{{Futurama}}'' episode "The Late Phillip J. Fry," ''Literature/TheTimeMachine'' is parodied when it is shown that in the year 5,000,000 AD human evolution has diverged into two species. A unnamed race of small pink and purple creatures "advanced in intellect and morality", and the "Dumbloks", "stupid vicious brutes who live underground". The montage song [[https://www.youtube.com/watch?v=GHDGKvh4vgk "In the Year 252525"]] displays some less extreme examples such as medieval-looking knights riding ostriches.
* {{Exaggerated}} in the ''WesternAnimation/TomAndJerry'' short "Guided Mouse-Ille". Tom, living in a futuristic facility, prepares an incredibly potent explosive to deal with Jerry. The explosive is apparently so strong that when it goes off and the smoke clears, Tom and Jerry are both cavemen living in a prehistoric jungle.
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