We all evolved from the oceans some 3.7 billion years ago during the Archean period (Ohtomo et al. 2013), when the earth had no visible continents at the surface for it was a water world according to our best estimates (Dong et al. 2021). That all vertebrates evolved from fishes should not be a surprise. If we need to find clues about our early evolutionary history, it makes good sense to defer to the common fish. Brains of fishes have all the basic components characteristic of the human brain: a telencephalon (including a hippocampal homologue with which it mediates thinking), a thalamus and the optic tectum (which mediates the transfer of sensory information), and brain stem and spinal cord upon which sits the cerebellum (which converts thinking into executable movement, Tehovnik, Hasabegović, Chen 2024).

In my living room, I have a fish tank that is populated by two species of fish, the tetra fish (the Gymno-corymbus ternetzi, better known as the tetra glofish, which originated from southeast Asia before being genetically modified) and the Corydoras catfish (Corydoras habrosus) which are originally from South America, the Amazon area (see Figure 1). The tetras and the Corydoras catfish are both schooling fish, so they thrive best living in groups of six or more. But the social nature of each is radically different. The tetras are in constant disputes such that they frequently attack each other to establish a hierarchy. After months of such activity, they have yet to finalize a hierarchy, since often the smallest and most junior member of the group will attack (without making contact) the largest member of the group when it ‘feels’ violated by the larger member. This tit-for-tat behavior has gone on daily since the establishment of the aquarium two months ago (see Footnote 1).

The catfish, on the other hand, do not display any overt rivalry and instead work together to collect food from the bottom of the tank. This activity goes on throughout the day with no interruption. When a tetra attempts to attack a catfish, the catfish continues to swim without incident. Since catfish are faster and more agile it is almost impossible for a tetra to interfere with the activities of catfish. Also, each occupies a different sector of the tank: the tetras are mid-level feeders, whereas the catfish are bottom feeders.

So, what does this have to do with humans? Well, if I were looking to come back to life as a fish, I would avoid the high stress confrontational life of the tetra in favor of the busy, but minimally-confrontational life of a catfish. We humans can make choices. If you want to join an overly ambitious Wall Street firm that may require the occasional snort of cocaine to remain competitive then the life of the tetra is for you, but if you prefer a highly-productive life with minimal stress then maybe carpentry is a better choice. I am not sure if fishes have the endowment to make career choices (but maybe humans don’t either).

Footnote 1: The tit-for-tat behavior of tetra fishes reminds me of reports about the Logothetis laboratory in Tübingen, in which commonly housed rhesus monkeys were always testing their place in the social hierarchy, such that at one time when a particular monkey of high status lost his tongue due to an attack, he was thereafter demoted to the lowest status—and we all thought that monkeys could not talk.

Figure 1: View of the fishes being discussed. The brightly colored fishes are genetically engineered tetra fishes (Gymno-corymbus ternetzi), and a pair of Corydalis catfish (Corydoras habrosus) can be seen at the lower right of the figure.