The word “fish” is deeply ingrained in our language, typically describing any aquatic, cold-blooded vertebrate with gills and fins. This common definition, however, is at odds with how modern biology classifies life. Scientists use a system that prioritizes evolutionary relationships, and under this framework, the traditional group called “fish” does not exist as a valid biological category. This counter-intuitive idea stems from the fact that a true biological group must include an ancestor and all of its descendants, a rule that the common understanding of “fish” fails to follow.
Defining Biological Groups
The modern system for classifying organisms, known as cladistics, is based on tracing the history of life through shared ancestry. This method requires that all accepted groups, called clades, must be monophyletic. A monophyletic group includes an ancestor and every single one of its descendants, forming a complete branch on the tree of life.
An example of a monophyletic group is Mammalia, which includes all mammals from their common ancestor, including humans, whales, and bats. The opposite is a paraphyletic group, which consists of an ancestor and some but not all of its descendants. Traditional classifications often created paraphyletic groups based on superficial similarities, like a shared environment or lifestyle.
Paraphyletic groups are invalid because they do not accurately represent an entire evolutionary lineage. For instance, “Reptilia” is paraphyletic because it includes lizards and snakes but excludes birds, despite birds evolving directly from a reptile ancestor. Classification must reflect the actual branching pattern of evolution, which is why these groupings are rejected.
The Problem with “Fish”
The traditional grouping of all aquatic, gill-bearing vertebrates—the “fish”—is a classic example of a paraphyletic group. This category includes jawless fishes, cartilaginous fishes like sharks, and bony fishes like salmon, but deliberately excludes all land vertebrates, known as tetrapods. The common ancestor of all these so-called “fish,” however, is also the ancestor of all amphibians, reptiles, birds, and mammals.
If a biologist were to trace the lineage of a shark, a tuna, and a lamprey, the common ancestor they share must also include the entire lineage that eventually gave rise to tetrapods like frogs and humans. By excluding tetrapods, the classification fails the monophyletic rule, meaning the term “fish” does not represent a natural, complete evolutionary unit.
The classification is artificial because it relies on excluding organisms that evolved a different body plan, specifically four limbs instead of fins. This arbitrary exclusion breaks the rule of including all descendants of the common ancestor. The scientific community has largely abandoned the term “fish” as a formal taxonomic rank, recognizing it only as a convenience term for aquatic vertebrates without limbs.
Where Tetrapods Fit In
The evolutionary evidence shows that the first tetrapods arose from within a specific group of bony fishes called the Sarcopterygii, or lobe-finned fish. This lineage includes species like the lungfish and the coelacanth, which are the closest living non-tetrapod relatives to humans. The fins of these ancient lobe-finned fish contained a central skeletal element connected to the shoulder or hip, a structure homologous to the limbs of land animals.
This arrangement means a lungfish is more closely related to a cow or a human than it is to a ray-finned fish like a tuna or a trout. The split separating the lungfish line from the tetrapod line occurred after the split separating the lungfish line from the ray-finned fish line. Grouping a lungfish with a tuna, while excluding a frog, is akin to grouping a bird with a lizard while excluding a mammal.
Tetrapods are merely lobe-finned fish that successfully colonized land. Features like digits and lungs are modifications of structures already present in their sarcopterygian ancestors. This direct line of descent requires a classification that places tetrapods within the lineage traditionally called “fish,” rendering the old definition scientifically obsolete.
Modern Scientific Groupings
Rather than using the non-existent category of “fish,” scientists classify aquatic vertebrates into several distinct monophyletic groups. These groupings accurately reflect the true branching pattern of vertebrate evolution. The major accepted clades include the Chondrichthyes, the Agnatha, and the vast Actinopterygii.
Chondrichthyes is the monophyletic group containing cartilaginous fishes, such as sharks, rays, and chimaeras, which share a skeleton made entirely of cartilage. The Agnatha, or jawless fishes, represent a distinct and ancient lineage that includes lampreys and hagfish. These two groups separated from the bony fish line very early in vertebrate history.
The most diverse and abundant group is the Actinopterygii, or ray-finned fishes, which includes over 30,000 species and accounts for nearly half of all vertebrates. This group contains most common aquatic species, from salmon to seahorses, and its members share fins supported by bony rays. The remaining aquatic vertebrates, the lobe-finned fish, fall into the Sarcopterygii clade, which is only monophyletic if it includes all of its descendants, including the entire lineage of tetrapods.