Fish People, and the Coelacanth: Bizarre Sci-Fi and Bizarre-er Science

Let’s begin where all good science fiction does: somewhere completely bonkers.

Take "Doctor Who", a show that routinely features humans in rubber lizard suits. One storyline posits that Earth was originally ruled by reptilian humanoids—the Silurians—who evolved millions of years before humans and now live underground in stylish, fluorescent-lit bunkers. Meanwhile, in Japanese science fiction, we have "ningyo" - ancient fish-human hybrids with big eyes, big feelings, and a tragic backstory involving bioluminescence and heartbreak.

These stories sound absurd. Until, of course, you remember that scientists don’t even agree on what a fish is. That’s right - if you ask a group of marine biologists to define a fish, you’re likely to start a fistfight at an academic conference. What it is depends on if you’re a taxonomist, a chef, or a time-traveling lobe-finned fish. The current working definition is something like: "It lives in water, has fins, swims, and has vertebrae... probably." It’s basically the same criteria you'd use to decide if a kid’s pool toy is alive. 

Enter the coelacanth: a five-foot-long fish (with limb-like fins and a primitive lung) that lurks in sea caves and seems to have crawled straight out of a speculative evolution Reddit thread.  

The coelacanth (pronounced SEE-la-kanth - because biologists love a spelling bee challenge) is a true living fossil. It was presumed extinct for 66 million years, until a South African museum curator looked at a fisherman’s catch in 1938 and said something along the lines of, “Well, this shouldn’t exist.”

Spoiler: it did. It still does.

And it doesn’t just look ancient—it is ancient. Fossils show that coelacanths were already old when dinosaurs were still trying out new teeth. Modern specimens look almost identical to their 300-million-year-old ancestors, which is the evolutionary equivalent of wearing the same cargo shorts since the Permian period.

Even more astounding, its genes are evolving in slow motion. Scientists sequenced the coelacanth genome and confirmed that, compared to most species, it’s like reading Shakespeare in molasses. It’s not just a fossil—it's a molecular procrastinator, a vertebrate version of that guy who still uses a flip phone and listens to music on a Zune.

But here's where it gets truly trippy. For decades, people thought coelacanths might be the closest living relatives to the first land vertebrates—our scaly aquatic ancestors who got fed up with gills and decided to invent hiking. But when scientists compared 251 genes across jawed vertebrates, they discovered the coelacanth had been upstaged by the lungfish. Yes, the lungfish (that have both gills and lungs) - possibly the most underrated celebrity in the evolutionary family tree - is actually closer to us than the coelacanth is.

But don’t feel bad for our slow-evolving, cave-dwelling friend. The coelacanth still offers unexpected insights into the move from sea to land and might yet hold the genetic secrets to why humans have legs and dolphins are just whales who believe in cardio.

And if you're thinking, “This is weird but at least it's real,” just remember: someone once pitched a show where the Moon is an alien egg (Doctor Who again), and someone else greenlit an anime where tuna cans are haunted.

So maybe those underground reptile people and Japanese fish-human hybrids aren’t so far off the mark. After all, if the coelacanth can take a 300-million-year nap and still make headlines, who’s to say what’s lurking beneath our feet—or our fins?

Moral of the story: Science fiction is just science that hasn’t gotten tenure yet. And fish? Fish are just a suggestion.

REFERENCE

Amemiya, C., Alföldi, J., Lee, A. et al. The African coelacanth genome provides insights into tetrapod evolution. Nature 496, 311–316 (2013). https://doi.org/10.1038/nature12027

https://www.reddit.com/r/todayilearned/comments/1lzpozm/til_that_lungfish_are_more_closely_related_to/

Zhang Z, Zhang S, Xu Y. The Evolution of Cognitive Abilities in Marine Animals: Insights from Cognition Gene Polymorphism in Coelacanths and Lungfish.