From Ancient Oceans to the Age of Fishes: The Evolution of Vertebrates

The roots of vertebrates trace back to the oceans, where the earliest ancestors emerged around 550 million years ago. The Devonian Period, known as the Age of Fishes (417 to 359 million years ago), marked a remarkable phase in their evolution. Among vertebrates, fish now stand as the most diverse group, accounting for 32,000 of the 52,000 known vertebrate species. They are collectively characterized as gill-bearing vertebrates, devoid of digits on their limbs.

Agnathans, the first fish to emerge during the Cambrian Explosion around 530 million years ago, were jawless with plated armored heads. Their round mouths served for sucking or filter feeding, and today's lampreys and hagfish are their sole surviving descendants. The advent of jaws, evident in contemporary cartilaginous and bony fish, expanded their dietary range and made them active hunters. Cartilaginous fish (Chondrichthyes), without true bone but equipped with lightweight, flexible cartilaginous skeletons, include agile predators like rays, sharks, and skates.

Among the 19,000 true bony fish species (Osteichthyes), diversity reigns, from eels to horse fish, trout, and tuna. Most possess swim bladders, gas-filled sacs ensuring buoyancy control at specific depths. However, sharks and rays, devoid of swim bladders and heavier than water like all fish, must decide between resting on the sea floor or expending considerable energy to stay afloat. Water holds only a fraction of the oxygen found in the air, yet multiple pairs of efficient gills continuously filter oxygen from water while removing metabolic carbon dioxide.

Two major groups of bony fish emerge: the ray-finned fish, abundant and named for the bony rays supporting their fins, and the lobe-finned fish, exemplified by the "living fossil" coelacanth. These lobe-finned fish feature rod-shaped bones enveloped by muscles in their pectoral and pelvic fins. Intriguingly, these lobed fins laid the foundation for the limbs and feet of tetrapods—four-legged land animals, including humans.

The earliest fish, exemplified by the sea lamprey, lacked jaws and relied on their circular mouths for filter-feeding. In the North American Great Lakes, lampreys are regarded as invasive species, thriving without natural predators and preying on economically significant fish like lake trout.