Kirk and crew are surveying the water planet Argo, when they are attacked by an alien sea creature. The venom from this extraterrestrial fish monster transforms them both into aquatic beings. Although the captain and science officer are now able to breathe underwater, they are unable to survive outside of it. Since it would be hard to captain the Enterprise from a fish tank, Kirk and crew must interrogate the elusive people of Argo to find a cure.
While this was a simple story about two Starfleet officers magically turning into fish people, and then conveniently turning back, could humans actually be retrofitted into creatures capable of breathing underwater? The first thing to note is why humans couldn’t simply develop gills without changing everything else. Fish gills are not underwater lungs. They are elaborate exchange surfaces designed to extract dissolved oxygen from water, which contains less available oxygen than air. A human-sized animal would need an enormous, highly vascularized gill surface, and a way of pumping huge quantities of water across it. Our lungs, circulatory system, and metabolism are all optimized for breathing air. Real functionality is more than just gill slits in the neck.
Genetic engineering could perhaps create gill tissue in a developing embryo, ensuring it was connected to the bloodstream and the blood was able to absorb it. The body would also have to prevent those delicate structures from drying out whenever the person returned to land. However, Kirk and Spock were transformed via a venom injected into them by the sea creature. Venoms are biological cocktails, mostly comprised of toxic proteins, which are actively delivered by a bite or sting. Their danger is in how they alter the victim’s physiological processes. Some interfere with nerves, others affect blood clotting, cell membranes, or metabolism. Perhaps a fictional venom injected at the right time could trigger dormant developmental pathways in alien victims as an unintended effect on their physiology. Retroactive gene editing is something we are already doing with new technologies such as CRISPR, just not with venom.
On Earth, our ancestors evolved in water, and the genetic and developmental machinery underlying structures such as gill arches remains deeply embedded in vertebrate biology. Modern humans obviously cannot spontaneously grow functional fish gills, but embryonic development still reflects some of that ancient evolutionary architecture. Trek lore is a little unclear as to the origins of Vulcans, but given that Spock is humanoid, with ears and facial features in the same place as Kirk, he may well have a similar inner physiology. At least, this episode suggests he does, because he was transformed in the same way Kirk was.
The magical venom could be a biochemical mechanism capable of temporarily rewriting adult human development. Far from creating an aquatic organism from nothing, it would be exploiting biological machinery that has been with vertebrates for hundreds of millions of years. It’s not about teaching a human to breathe underwater, but teaching the human body to remember that it once could.