The word “fish” is a very touchy subject for biologists. It actually has zero scientific meaning today, and is an archaic word that is well past its expiration date only because the general public keep using it and scientists have to use it to make any sense to the general public (“reptile” and “amphibian” both also have that problem, by the way). I could go on a long, passionate rant about how horrible and ucky the word “fish” is, but I will spare humanity this time.
There is actually a scientific meaning for “fish,” but it isn’t what you may expect. “Fish” is now synonymous with “vertebrate,” which means that a fish is basically any animal with a backbone (in biology we are trying to stamp out defining life via physical traits, but I won’t go into the technical stuff). This means that you are a fish. Birds are one of the most common types of fish on Earth. There are herds of fish roaming the African savannah. To argue that this isn’t true—that these animals are not fish—is to argue that it is completely logical for a branch growing on the left side of a tree to suddenly switch to growing from the right side of the tree. It is the exact same argument.
But when I say that I am a “fish” (horrible word), don’t be fooled. I am not saying that I am something like a tuna, and nor are you (unless you are a tuna reading this). It is true that both humans and tuna are “fish,” but I am not descended from anything like a tuna, nor a salmon, nor a sturgeon. Usually when we think of the word “fish,” we think of one of those animals—they are a type of creature known as actinopterygians (we also call them “ray-finned fish”). They are fish (as in, vertebrates) that have fins right up against their torsos. Most of them actually evolved after the dinosaurs.
Humans are not ray-finned fish. We are “lobe-finned fish.” Throw out any idea you may have of what a “fish” is like, and allow me to describe what your typical lobe-finned fish looks like. Most lobe-finned fish have beaks. They have very complicated respiratory systems that can extract a lot of oxygen from the air. They have feathers, and can fly. What’s that, you say? It sounds like I’m describing a bird? I am—by far, the most common lobe-finned fish on our planet are birds. When you picture the average lobe-finned fish, you should be picturing them. The next most common type of lobe-finned fish has hair and feeds its children with nutrient-rich sweat—mammals. Frogs, salamanders, lizards, turtles, snakes, and crocodiles are all lobe-finned fish too. Several lobe-finned fish swim in the water—most of them we call “whales,” “dolphins,” or “seals.”
Lobe-finned fish used to be the dominant group of aquatic animals, well before sharks, rays, and most ray-finned fish evolved (although their ancestors were certainly around back then, trying to swim away from a ferocious lobe-finned fish’s open jaws). They have since lost the marine throne to the ray-finned fish, although lobe-finned fish such as orcas remain at the top of the aquatic food chain. There are only two lobe-finned fish alive today that look anything like the lobe-finned fish back in their aquatic hayday before the dinosaurs. They are the coelacanth and the lungfish. How are they like you? Just like you, the coelacanth and lungfish do not have fins pressed tightly up against their bodies. Although they do not have fingers and toes, coelacanths and lungfish have arms, legs, hands, and feet just like you do (these limbs are the “lobes” that we get the name “lobe-finned fish” from). Just like you, a coelacanth or lungfish has a humerus, a radius and ulna, and carpal and metacarpal bones; just like you, a coelacanth or lungfish has a femur, a tibia and fibula, and tarsal and metatarsal bones. Lungfish, like most lobe-finned fish today, can even breathe air.
Now, certainly, ray-finned fish and lobe-finned fish have a common ancestor. But the ray-finned fish (what you typically think of when you think “fish”) are our cousins, not our siblings or ancestors. We are fish, yes; we are even bony fish (sharks, rays, ratfish, lampreys, and hagfish are not). But we are not anything like a tuna, nor were we ever anything like a tuna. The problem with saying that humans are “fish” is that people hear that and think “ray-finned fish” instead of thinking “lobe-finned fish” and then are incredulous that anybody could say we are one and the same. The word “fish” in general needs to be deleted from our vocabulary, but until the general public catch on that it is meaningless and misleading, scientists will have to continue to use it when speaking to the general public.
^ The above picture is a picture I took from Wikipedia of a lungfish. Notice how the fins of the lungfish are at the tips of arms and legs—these are lobe-finned fish.