The concept of identifying a single “first fish” is a complex scientific endeavor, largely because evolution is a gradual process without clear-cut starting points for new groups of organisms. Instead, scientists piece together evidence from the fossil record and comparative biology to trace the incremental changes that eventually led to what we recognize as fish.
Defining Early Fish
Scientists classify early fish based on specific anatomical characteristics that set them apart from simpler invertebrates. A key feature is the presence of a notochord, a flexible rod-like structure that provides support along the back of the organism. In more advanced forms, this notochord develops into a vertebral column. Early fish also possessed gill slits for respiration, allowing them to extract oxygen from water. Additionally, rudimentary fins or fin-folds were present, indicating a move towards more controlled movement in water.
From Simple Chordates to Early Vertebrates
The evolutionary path to fish began with simpler organisms called chordates, possessing a notochord at some life stage. A notable early chordate fossil is Pikaia, discovered in the Burgess Shale, dating back approximately 505 million years. This small, worm-like creature had a distinct notochord and muscle segments, suggesting it could swim by undulating its body. While not a fish, Pikaia represents an important evolutionary precursor, demonstrating the foundational chordate body plan.
Further along this evolutionary trajectory, early vertebrates emerged, characterized by the development of a rudimentary skull and, in some cases, early vertebral elements. Fossils such as Haikouichthys and Myllokunmingia, found in China and dating to about 518 million years ago, are considered some of the earliest known vertebrates. These small, jawless creatures possessed a notochord, a distinct head with eyes, and gill pouches, indicating they were active swimmers. They represent a transitional stage, showing the initial development of features that would define true fish.
The Reign of Jawless Fish
The earliest undisputed fish belong to the group known as jawless fish, or agnathans, which appeared around 530 million years ago during the Cambrian period. These ancient fish lacked jaws and paired fins, but many developed protective bony plates or scales on their bodies, leading to the name “ostracoderms” (shell-skinned). They primarily fed by filter-feeding or scavenging, using their circular, suction-like mouths. Ostracoderms were diverse and abundant in the Paleozoic seas, with some developing primitive fins.
Modern relatives of these ancient jawless fish include lampreys and hagfish, part of a group called cyclostomes. Lampreys, for instance, have a unique funnel-like mouth lined with teeth, which they use to attach to other fish. Both lampreys and hagfish possess a cartilaginous skeleton and retain a notochord throughout their lives, reflecting characteristics of their ancient ancestors.
The Emergence of Jawed Fish
A revolutionary development in fish evolution was the appearance of jaws, which allowed for a dramatic expansion in feeding strategies and ecological roles. Jawed fish, known as gnathostomes, likely evolved from jawless ancestors during the late Ordovician or early Silurian periods. The first jawed fish were the placoderms, an extinct group characterized by heavy bony armor covering their head and front body, often with a movable joint between the head and trunk shields. This armor provided protection, while their newly evolved jaws, likely derived from gill arches, enabled them to bite and consume a wider range of prey.
Placoderms diversified extensively during the Devonian Period, often called the “Age of Fishes,” occupying various marine and freshwater habitats. Some placoderms, like Dunkleosteus, grew to impressive sizes, becoming apex predators of their time. Following the placoderms, two major lineages of jawed fish emerged and continue to dominate aquatic environments: cartilaginous fish (Chondrichthyes), which include sharks and rays, and bony fish (Osteichthyes), the most diverse group today. The evolution of jaws fundamentally reshaped aquatic ecosystems and set the stage for the diversification of all jawed vertebrates.
The Challenge of Pinpointing the “First”
This means that any attempt to draw a precise line marking the “first” fish is an oversimplification of a complex, incremental transition from pre-fish chordates to early fish forms. The fossil record, our primary source of evidence, is also inherently incomplete. New fossil discoveries can continuously refine or even alter our understanding of early evolutionary pathways. What might be considered the “earliest” today could be superseded by a new find tomorrow. Therefore, the “first fish” is more accurately viewed as a conceptual milestone, representing a stage in the long evolutionary journey where organisms acquired a combination of features that scientists collectively define as fish-like.