The evolutionary history of fish spans over half a billion years, representing the deepest branch on the vertebrate family tree. Informally, the term “fish” refers to all vertebrates that are not four-limbed land animals (tetrapods). This vast and diverse group includes ancient, jawless forms and the more than 30,000 species of modern aquatic vertebrates existing today. Tracing this lineage requires a journey through the Paleozoic Era, a time of profound biological innovation.
The Earliest Ancestors
The deepest roots of the fish lineage extend into the Cambrian period (541 to 485 million years ago), marked by the appearance of the first creatures bearing a notochord. One example is Pikaia, a small, slender animal known from fossils in the Burgess Shale. Pikaia possessed a stiff rod along its back and segmented muscle blocks (myomeres), defining characteristics of the phylum Chordata, which includes all vertebrates. It lacked a true backbone or skull.
The first true vertebrates, animals with a skull and a rudimentary backbone, appeared later in the Ordovician period (485 to 443 million years ago). These early fish were the Agnatha, or jawless fish, including the heavily armored Ostracoderms. Ostracoderms were small, bottom-dwelling creatures characterized by bony external armor covering their head and the front part of their bodies. This armor likely protected them against dominant predators like giant scorpion-like eurypterids. Ostracoderms lacked paired fins and used muscular gill pouches for filter-feeding, distinguishing them from later jawed fish.
The Evolutionary Leap: The Origin of Jaws
A fundamental transformation in vertebrate history occurred during the Silurian period (443 to 419 million years ago) with the emergence of the Gnathostomes, or jawed vertebrates. The development of a hinged jaw, which allows an animal to grasp and tear food, arose from the modification of the first pair of gill arches. The shift from stationary filter-feeding to active predation dramatically changed the trajectory of life in the oceans.
The acquisition of jaws, combined with two sets of paired fins—pectoral fins near the head and pelvic fins near the tail—gave these early gnathostomes superior mobility and control. This combination allowed for more precise maneuvering and the ability to pursue, capture, and process larger prey items. Early jawed fish, such as the now-extinct placoderms and acanthodians, first appeared in the late Silurian. This initial increase in functional variety set the stage for jawed vertebrates to rapidly become the dominant life forms in the aquatic environment.
The Devonian Explosion and Armored Giants
The subsequent Devonian period (419 to 359 million years ago) is often called the “Age of Fishes” due to the explosive diversification of fish groups. During this time, the Gnathostomes rapidly radiated, filling almost every available aquatic habitat. Placoderms, the armored fish, reigned as the dominant group in both marine and freshwater environments.
These plate-skinned fish were characterized by a bony shield covering the head and thorax. Some species, such as Dunkleosteus, grew to immense sizes. Dunkleosteus was a massive apex predator of the Late Devonian seas, utilizing heavy armor and powerful, shear-like bony plates that functioned as jaws.
Crucially, the Devonian period also witnessed the fundamental evolutionary split between the two lineages that comprise almost all modern fish: the Chondrichthyes (cartilaginous fish) and the Osteichthyes (bony fish). All placoderms, including Dunkleosteus, were wiped out during the Late Devonian mass extinction events, clearing the way for the diversification of these newer groups.
The Rise of Modern Bony and Cartilaginous Fish
The two surviving lineages from the Devonian extinction, Chondrichthyes and Osteichthyes, continued their evolutionary trajectories.
Chondrichthyes (Cartilaginous Fish)
This group includes sharks, rays, and chimaeras, characterized by a skeleton made entirely of cartilage rather than bone. While early forms appeared in the Devonian, this group underwent significant diversification during the Mesozoic era, leading to the forms recognizable today. Modern sharks and rays have maintained their success through powerful jaws, placoid scales, and a reliance on dynamic lift for buoyancy, a different strategy from bony fish.
Osteichthyes (Bony Fish)
The Osteichthyes, or bony fish, became the most numerous and diverse group of vertebrates. This group is divided into the Sarcopterygii (lobe-finned fish) and the Actinopterygii (ray-finned fish). The ray-finned fish are the most successful, accounting for over 95% of all living fish species, with over 30,000 species today. Their success is attributed to refinements in their anatomy, including lighter, more flexible scales, the development of a swim bladder for neutral buoyancy, and increasingly mobile fins and jaws. This led to the highly specialized Teleost fishes that dominate contemporary aquatic environments.