What better way to show the passage of time in Time Travel stories than by having humans lack appendixes and other vestigial organs? Progress!
Never mind that it takes millions of years for significant evolution like that to occur and the fact animals like whales and snakes (specifically snakes like boas, pythons, and slender blind snakes) have had vestigial leg bones for so long that it's unlikely that humans would shrug off their own vestigial organs anytime within a hundred thousand years.
Evolution is a result of natural selection, and while they're unnecessary, there isn't much evolutionary pressure against having an appendix. If something is truly vestigial, there is little advantage in getting rid of it so evolution will be particularly gradual in getting rid of vestigial organs, if it gets rid of them at all. Whenever a feature becomes useless to an animal it does tend to slowly disappear since genetic mutations that lead to its degeneration are not weeded out by natural selection. There is also the issue of the resources, minor as they presumably are, that the animal uses to grow these useless features; there is therefore some benefit to losing these vestiges, but there's not much in it.
Of course, this trope can easily be justified through genetic modification — the appendix didn't naturally evolve away, humans simply altered their own genome to weed out the junk.
A note on the appendix itself: The appendix has historically been thought a useless, vestigial or even dangerously self-destructive organ, but Science Marches On — it's still considered vestigial, but not necessarily useless. In the last few years scientists have begun to suspect that it is actually important to the body, as a reservoir of the "good bacteria" species that populate the colon and are necessary for digestion. When the colon's population is thrown out of whack by infection, diarrhea, or the like, the bacteria "hiding" in the appendix can recolonize it and return the necessary balance. There are also suggestions that this reservoir is used in training the immune system of babies and young children. It's still a lot easier to live without one than it is to live without most of your other organs, but it does seem to serve a useful (if non-vital) purpose. This is what we would expect from evolution; if you have something useless hanging around, it can be repurposed. Moreover, a smaller appendix might more easily become infected, putting up a barrier to its fading away and selecting for a larger appendix.
- In the 2012 Alpha Flight miniseries, those who have been through the Master of the World's evolution/brainwashing machine can be identified because they only have four toes. They also don't have an appendix, but that's a little harder to verify in the street.
- Legion of Super-Heroes would have us believe that in 1,000 years humans will grow an extra heart. Forgot the fact that an extra pump in our chest would cause more problems than it would solve.
- European comic boy genius Olivier Delabranche invents a two-person scooter out of his school bag but then sleeps through class and dreams in class that human laziness will cause evolution to phase out toes... then feet, legs, hands why not? Until humans are big brains on wheels. When he wakes up, he smashes the scooter with a rock, repeating over and over "I don't want to be a brain on wheels!"
- Isaac Asimov, who was actually a biochemist, employed this trope in several works:
- Pebble in the Sky: Imperial officials realize Joseph Schwartz is a time traveler by the fact he has wisdom teeth and an unusually large appendix. Justified by the future Earth being covered in radiation after a nuclear war, which would increase mutation rates vastly. How much time passed between the "present" and the "future" is disputed by the fanbase due to inconsistencies within the series. (One character estimates between fifty thousand to a hundred thousand years since humanity only existed on a single planet, even assuming that humans didn't evolve simultaneously on multiple planets.)
- This is actually a plot point in The End of Eternity, when two of the main characters, both time travelers, who were born three million years apart notice that the only difference between them is the lack of wisdom teeth and the lack of appendix in the one from the later time. They (perhaps hastily) draw the conclusion that Eternity is trying to prevent human evolution.
- While it is not employed in Part 3 of The Gods Themselves, the eugenics-conscious moon separatists want to use genetic engineering to get rid of various organs which they consider unnecessary, like molars (the artificial food of the Moon requires little chewing).
- In Captain French, or the Quest for Paradise, the titular protagonist has 2000 years (subjective time) or 20,000 years (objective time) over his new wife, who is only in her 50s (but looks 20 thanks to the omnipresent cure for aging). The first thing he does after taking her onboard his ship is to have her examined by the ship's Auto Doc. The doc reveals something horrible to French — she still has an appendix. Angry at the carelessness of her home planet's religious fanatics, he has the Auto Doc remove the "offending organ" despite the fact that there's nothing wrong with it. The justification is that it's too risky to have one in space, since it may burst while you're away from the med-bay (e.g., EVA).
- In the Doctor Who New Adventures spin-off novels, the companions Chris Cwej and Roz Forrester have no appendices. They're from the 30th Century.
- An alien version in the Giants Series — the Ganymeans genetically modified themselves to remove their secondary circulatory system, which was primarily a defense mechanism. Then it turned out that they needed it after all.
- The Hive Series by TJ Bass features a race of Nebish — four feet tall, four toes, pink blood due to lack of hemoglobin...
- The Lunar people of I, Weapon have engineered away bellybuttons and bodily hair because neither is considered important in the colony. Pregnancies are handled via test tubes and body hair clogs the air filters.
- The futuristic anatomical doll in "Mimsy Were the Borogoves" lacks an appendix, and a distinguishable large intestine for that matter. It also has at least one entirely new organ system, suggesting that changes were engineered rather than evolved.
- The genetic manipulation side is handled fairly well in Schismatrix. The Shapers are engineered to not only go without appendices, but also to not need any of the bacteria humans currently need to digest food properly, allowing Shaper habitats to be free of any non-engineered nasties. Unfortunately for them, not everybody has this advantage; Shapers going from the aseptic environment to a bacterial one tend to get diarrhea, nausea and horrible rashes. Non-Shapers living in their habitats have to agree to get their intestines pumped and eat regular doses of foul-tasting yoghurt full of digestive enzymes to make up for it.
- This is exhibited by the children in Nicole Luiken's Violet Eyes.
- The War of the Worlds (1898): In designing the Martians, H. G. Wells started from a humanoid body plan, then stripped off everything — limbs, digestive tracts, sexual reproduction — that he thought mechanization and/or medical technology could potentially make obsolete. All that he left them was a giant head, a few tendrils for pushing buttons, and the ability to breathe. Instead of eating, they drain captured aliens and humans of blood and inject it into their bodies.
- Philip José Farmer's The Wind Whales of Ishmael features a race of four-toed future people.
- Last and First Men: The Second Men are noted to lack several of the First Men's "useless excrescences"; for instance, they have no toes, tonsils or appendixes.
- molesworth parodies the idea (which showed up often enough in 1950s SF) in Whizz for Atomms, with typically idiosyncratic molesworth spelling:
Well everyone use their branes so much that in the end they are all going to turn into eggs becos they will have thort a way of getting along without walking. This will not be until 21066 a.d. (approx.) but it makes you think a bit.
- The Man Who Awoke, published in 1933 on Wonder Stories, is a series about Norman Winters, who invents a technology for suspended animation and visits successively more distant futures. By the year 5,000 humans have lost the appendix, followed by teeth in the year 15,000 and tonsils in 25,000.
- In The Big Bang Theory, Sheldon argues that his pinky toes and lateral incisors are smaller than average, indicating that he is a more advanced form of humanity, a "homo novus", and thus is too evolved to learn how to drive. Leonard would beg to differ, but doesn't bother.
- Doctor Who: Lady Cassandra, from five billion years in the future, who appears in "The End of the World" and "New Earth", is the self-proclaimed "Last Human" and is nothing but eyes and a mouth on skin stretched trampoline-like in a frame, attached to her Brain in a Jar. However, in her case it's because she's had extensive plastic surgeries, not evolution, as the latter episode shows humans and human-descendants (and a younger version of Cassandra) who look perfectly normal.
- This trope was facetiously employed in Phil of the Future when Phil ran into a problem: he would have to be seen barefoot to stamp tomatoes in a farm on a class trip, but in his time, no one has pinky toes. To quote Phil's mother, "They won't lose their pinky toes for another 70 years." Ironically, pinky toes are actually used for balance.
- Losing the appendix is mentioned as one of the results of human evolution in Time Trax.
- Parodied in Dr. Katz, Professional Therapist: when Ben is due to have his wisdom teeth removed, Laura mentions she never had them, which her dentist attributed to her being "more evolved". They end up getting into an argument over the veracity of the statement.
- The Simpsons:
- Parodied when a news report says that scientists predict people will evolve an additional finger in the future. Characters on the show have only three fingers and a thumb on each hand.
- In the episode "Lady Bouvier's Lover", Homer is worried that Abe marrying Marge's mother would make him and Marge brother and sister, and as a result turn the kids into "...horrible freaks with pink skin, no overbites, and five fingers on each hand!" Right after Homer says this, the camera points at the kids, who instead of being drawn in the Simpsons style, are drawn "horrifically" normal.
- Some people don't grow wisdom teeth. On the opposite end, some people have them but don't need to get them pulled, either because they're quite fine staying put in the jaw, or are lucky enough to have the space for them to fully emerge without getting stuck or infected.
- If the occurrence of the appendix among humans decreased until only a testimonial number of people had it, it would become what in biology is known as atavism. Horses with lateral toes and whales with leg bones are among the best known examples.
- The tiny arms of Tyrannosaurus rex and other theropod dinosaurs are believed to have been the result of this. Some paleontologists speculate that if the Cretaceous-Paleogene extinction hadn't happened, the theropods would have lost their arms entirely tens of millions of years later.
- Loss of eyes is a frequent phenomenon among cave-adapted and some burrowing animals, as vision is impossible in a lightless environment. In this case, the loss serves a practical purpose, as eyes are a vulnerable point for injury or infection, so become a useless liability if they can no longer collect sensory input.