Anil Aggarwal's "The Freak" aka "Time and Again" (flash fiction, time travel, free): A human anatomical adaptation necessary for time travel!
This is a very unusual time travel story; at least I've not seen anything like this before.
I'm unqualified to comment on parts of the story that deal with human anatomy - something central to the story. So I had to take author's assertions about that on faith.
Story summary.An early human starship is captured by aliens, who treated its occupants as lab animals - doing extensive genetic modifications in the process. Centuries later, harassed descendants of those men return to earth - equipped with time travel capability.
So humanity now comprises of two species - "three dimensionals" & "four dimensionals". They cannot interbreed, but are otherwise compatible enough to live in the same society.
This is the story of the discovery, by the unnamed doctor narrator, of how "four dimensionals" differ from "three dimensionals" in an anatomical structure that help us balance when, e.g., walking. He's treating Rajiv, a "four dimensional", who suffers from an imbalance problem when traveling along time.
Rajiv will die, & his autopsy & later post-postmortems of healthy "four dimensionals", will reveal their difference. Inside the human "inner ear" (which one? both?), there is a structure comprising "three semicircular canals [that] are at right angles to each other, each representing a single dimension of space. Whenever someone tips in one or the other dimension, the otoliths inside the appropriate semicircular canal would start moving and activate the vestibular nerve which would send the information to the brain immediately. The brain would then send orders to proper muscles and joints to correct the body position." In healthy "four dimensionals", autopsy reveals this structure to be a "tesseract" - a projection of the 4D structure in 3D. Rajiv had this balancing structure similar to "three dimensionals"; so had problem balancing during time travel.
- Lewis Shiner's "Nine Hard Questions about the Nature of the Universe"; download from Manybooks (Manybooks seems down at the time I'm posting; this link found via Google; should work when Manybooks comes back); F&SF, December 1983: Another story where aliens kidnap humans & use them as lab animals. This also has a - not time travel but time displacement - where a child gets older than his parents! But I found it a very ordinary & generally a pointless story, compared to Aggarwal's.
- Of course, there are other kidnapped humans as alien lab/zoo animals stories, but none I've read is close in spirit or situation to this story.
Fact sheet.First published: Science Today, July 1988.
Download full text from Anil's site.