The search for the first fish is complex because evolution is a continuous process, not a sudden event. Tracing this lineage requires examining a sparse fossil record that points to a gradual transformation from simple swimming organisms to true vertebrates. Scientists define a “true fish” as having a backbone or a cartilaginous precursor, distinguishing them from earlier, simpler ancestors. This evolutionary journey began over 500 million years ago, establishing the fundamental blueprint for the vertebrate body plan shared by all fish, amphibians, reptiles, birds, and mammals.
The Earliest Chordate Ancestors
The Cambrian Explosion (541 to 485 million years ago) saw the emergence of the first organisms belonging to the phylum Chordata. These early forms possessed the defining characteristics of our lineage, though they were not yet true vertebrates.
A creature like Pikaia, discovered in Canada’s Burgess Shale, represents one of these primitive chordates. This small, eel-like animal lived approximately 505 million years ago and possessed a notochord, a flexible, rod-like structure running along its back. This notochord provided internal structural support and was the evolutionary precursor to the vertebral column found in later vertebrates.
Pikaia also featured a dorsal nerve cord and segmented muscle blocks (myomeres) for side-to-side swimming. Fossils from China, such as Myllokunmingia and Haikouichthys, are slightly older and represent some of the earliest known craniates. These chordates possessed a distinct head region and primitive skull elements made of cartilage, establishing the foundational body architecture that led to the first true fish.
The Emergence of Jawless Fish
The first animals widely considered true fish and vertebrates belong to the Ostracoderms, a group of jawless, armored fish meaning “shell-skinned.” These creatures emerged during the late Cambrian and diversified between 500 and 419 million years ago. Ostracoderms represented a major evolutionary leap because they were the first to develop hard, bony tissues, marking the advent of the skeletal system.
Their most distinctive feature was heavy dermal armor—a shield of bony plates and scales covering the head and the front of the body. This armor, which provides the majority of their fossil record, likely protected them against large predatory arthropods, such as sea scorpions (eurypterids). The presence of this mineralized tissue, known as dermal bone, is considered a definitive feature of the first vertebrates.
Ostracoderms, such as Pteraspis and Cephalaspis, were small, typically less than 30 centimeters in length, and their bodies were often flattened. They were jawless (Agnathans) and lacked the paired pectoral and pelvic fins that characterize most modern fish. Their mouths were small, fixed openings, and they were likely bottom-dwellers that fed by filtering small particles or sucking up soft prey from the seafloor. The lack of jaws and paired fins meant they were relatively slow swimmers with limited maneuverability, relying on their bony armor for defense.
The Evolutionary Innovation of Jaws
The transition from jawless Ostracoderms to jawed fish, or Gnathostomes, represents one of the most significant evolutionary innovations in vertebrate history. This shift occurred around the late Silurian and early Devonian periods, roughly 440 to 419 million years ago. The development of a hinged jaw fundamentally changed the ecological landscape, transforming vertebrates from passive filter feeders into active predators.
The jaw is thought to have evolved from the skeletal supports of the anterior gill arches in jawless ancestors. These cartilaginous arches, which originally supported the gills, were gradually modified and enlarged to surround the mouth opening. The initial advantage of this modification may have been improved respiratory efficiency, helping to pump water over the gills, before the structure was adapted for biting and grasping prey.
The rise of jawed fish also coincided with the evolution of paired fins, specifically pectoral and pelvic fins, which dramatically improved swimming control and maneuverability. These two innovations—jaws for capturing and manipulating food, and paired fins for steering—allowed early jawed groups, such as the armored Placoderms and the spiny Acanthodians, to rapidly diversify and dominate aquatic environments during the Devonian Period, often called the “Age of Fishes.”
The Legacy of Early Fish
The evolutionary steps taken by these early aquatic vertebrates established the framework for nearly all subsequent life with a backbone. The notochord provided the flexible axial support that preceded the bony vertebral column, and Ostracoderms introduced the concept of a mineralized skeleton using dermal bone.
The later development of jaws and paired fins in the Gnathostomes set the stage for the massive diversification of fish that followed, including the cartilaginous fish (sharks and rays) and the bony fish (Osteichthyes). The bony fish lineage gave rise to the ray-finned fish, the most numerous group of vertebrates today, and the lobe-finned fish. It was from the sturdy, muscular fins of the lobe-finned fish that the limbs of the first four-limbed land vertebrates, or tetrapods, eventually evolved.