Emerging in the Cambrian seas around 530 million years ago, the first vertebrates were small, jawless creatures that would hardly be recognized as fish today. Species like Haikouichthys and Myllokunmingia, discovered in China's Chengjiang fossil beds, measured just a few centimeters long. Yet they carried a revolutionary innovation: a notochord stiffened by cartilaginous elements -- the precursor to the vertebral column that defines all vertebrates.
These early fish lacked jaws, instead possessing simple, round mouths suited for filter-feeding or scavenging. Their bodies were covered in bony armor plates for protection against the formidable invertebrate predators of the time. Over the following 100 million years, fish diversified enormously. The evolution of jaws, around 430 million years ago, was a transformative moment -- repurposing gill arch bones into grasping structures that opened up entirely new feeding strategies.
By the Silurian and Devonian periods, fish had become the dominant vertebrates in the oceans. Armored placoderms like the massive Dunkleosteus -- with its guillotine-like bony plates instead of teeth -- grew to over 6 meters long. Meanwhile, two lineages of bony fish diverged: ray-finned fish (which would eventually account for nearly half of all vertebrate species) and lobe-finned fish, whose muscular, limb-like fins would prove crucial for the next great chapter in evolution.
The internal skeleton of these early fish was more than just structural support. It provided attachment points for muscles, enabling more powerful and controlled movement. It protected the brain and spinal cord. And it could grow with the animal, unlike the rigid exoskeletons of arthropods. This flexible, scalable body plan would become the foundation for all amphibians, reptiles, birds, and mammals to come.