The evolutionary journey of fish spans hundreds of millions of years, marking a pivotal moment in the history of life on Earth. The appearance of the first fish-like creatures set the stage for the evolution of all vertebrates, including humans. Fish represent a diverse group of aquatic animals.
The First Vertebrates Emerge
The earliest forms resembling fish, specifically jawless vertebrates, first appeared during the Cambrian Period, approximately 530 million years ago, with significant diversification following in the Ordovician Period. Fossils like Myllokunmingia, found in the early Cambrian Maotianshan shales of China, represent some of the oldest known chordates with primitive vertebrate features. Myllokunmingia was a small, eel-like creature, about 28 mm long, possessing a distinct head and trunk, a notochord, and rudimentary vertebral elements.
During the Ordovician Period, which began around 485 million years ago, jawless armored fish, known as ostracoderms, became more common and left a richer fossil record. These early vertebrates had large bony shields covering their heads and small, plate-like scales on their tails. Fragments of dermal armor from jawless fish, dating back about 450 million years, have been discovered in Upper Ordovician strata in North America.
Characteristics of Early Fish
These earliest fish were primarily jawless (agnathans), a group that includes ancient ostracoderms and cyclostomes, which are related to modern lampreys and hagfish. Ostracoderms were typically small, less than 30 cm long, with a flattened, fish-like body. Lacking jaws, they were likely filter feeders or detritus feeders, consuming small organisms or particles from the seabed.
Their bodies were covered in heavy dermal bone plates, particularly on the head and thorax, which likely served as protection against predators like giant scorpion-like arthropods called eurypterids. While they possessed a cartilaginous skeleton, which rarely fossilizes well, their bony plates and scales are well-preserved in the fossil record. These early forms also lacked paired fins, having only median and caudal fins, which limited their maneuverability compared to modern fish.
The Evolutionary Leap
The appearance of early fish represented a significant evolutionary leap due to the development of a notochord and, subsequently, a vertebral column. The notochord, a flexible rod-like structure, provided mechanical support and served as a signaling center during embryonic development, influencing the formation of surrounding tissues. In early vertebrates, this notochord provided a basis for muscle attachment and more controlled movement, particularly for swimming.
This fundamental body plan, with an internal skeletal axis, laid the groundwork for all subsequent vertebrates, including amphibians, reptiles, birds, and mammals. The evolution of gills in these early fish allowed for efficient aquatic respiration, enabling them to thrive in diverse aquatic environments. The development of early fin structures provided the foundation for the more complex paired fins seen in later fish, which greatly enhanced locomotion and paved the way for the eventual colonization of land by their descendants.