The search for the first prehistoric fish requires tracing an evolutionary journey through hundreds of millions of years, beginning long before the appearance of anything recognizable as a fish. This narrative starts in the Cambrian Period, a time of rapid diversification, and extends into the Ordovician, a geological epoch marked by the first true vertebrates. The question of what the first fish looked like is answered by examining the organisms that developed the single defining characteristic of all vertebrates: an internal skeletal structure. The transition from soft-bodied invertebrates to creatures with a primitive spine set the stage for the evolution of all fish, amphibians, reptiles, birds, and mammals.
From Worms to Spines
The immediate ancestors of fish were small marine animals that appeared during the Middle Cambrian, approximately 505 million years ago. These creatures, including fossils like Pikaia, were not fish but represented the earliest known members of the chordate family. Pikaia was a flattened, eel-like animal, averaging about five centimeters in length, with a soft body that lacked bone or hard scales.
Its importance lies in the presence of a notochord, a flexible, rod-like structure that ran along the length of its body. This notochord served as the precursor to the vertebral column and provided attachment points for segmented muscle blocks, called myomeres. This internal support system enabled the animal to swim with efficient S-shaped, side-to-side movements through the water column.
The combination of a dorsal nerve cord, a notochord, and myomeres confirms Pikaia as a primitive chordate. While it lacked the definitive characteristics of true fish, such as fins or a complex skeletal system, it possessed the fundamental anatomical blueprint for vertebrates. This tiny, worm-like organism defined the entire chordate phylum.
Armored and Jawless
The first organisms definitively called fish evolved later, during the Ordovician Period, around 480 to 443 million years ago. These pioneering vertebrates, known as Ostracoderms, or “shell-skinned” fish, were jawless and heavily armored. They were the first creatures to use bone for protection, appearing far different from modern fish, dominated instead by large, protective plates.
Early examples, such as Arandaspis, reached lengths of only 12 to 14 centimeters. The head and front portion of the body were encased in rigid armor composed of hard plates or bony scutes. This heavy dermal skeleton served as a defense against large invertebrate predators, such as the giant sea scorpions (eurypterids), common at the time.
The body plan featured a massive head shield with small openings for the eyes, nostrils, and a series of gill slits. Lacking paired fins, they were poor swimmers with limited maneuverability, often dwelling near the seafloor. Their mouth was a simple, non-biting opening, positioned ventrally, suggesting they were primarily bottom-dwelling filter feeders or scavengers.
Another contemporary, Astraspis, or “star shield,” reached up to 20 centimeters. Its head shield was constructed from thousands of small, polygonal, star-shaped bony tesserae. These jawless fish, collectively known as Agnatha, relied on drawing water through the mouth using a muscular pharynx to filter food particles. The rigid armor and filter-feeding lifestyle defined this early stage of fish evolution, where survival depended more on defense than on speed.
The Development of Jaws
The next major evolutionary leap occurred with the appearance of the Gnathostomes, or “jawed mouths,” during the Silurian Period, around 430 million years ago. This development transformed the trajectory of vertebrate life. The earliest jawed fish were the Placoderms, a group whose name translates to “plate-skinned,” reflecting their continued reliance on external armor.
The jaw evolved from the skeletal supports of the anterior gill arches of their jawless ancestors. These modified arches became a hinged, movable structure, allowing the mouth to open and close powerfully. This new ability meant fish could actively bite, grasp, and shear food, transitioning them from passive filter feeders to active predators.
The acquisition of a jaw led to a rapid diversification of feeding habits and body forms. Placoderms retained heavy armor on the head and thorax, but they also developed the first paired pelvic fins alongside their pectoral fins. These paired fins provided greater stability and control, making them much more agile swimmers. The combination of a functional jaw and paired fins unlocked the potential for aquatic vertebrates to become apex predators, driving the explosion of fish diversity in the subsequent Devonian period.