Water makes up more than 76 percent of the Earth’s surface. Barring ecological degradation, wherever you find water you will find fish. Therefore, no one should be surprised to learn that life first appeared in the oceans and seas and that the first vertebrae were jawless fishlike animals called ostracoderms because external body plates made of a material similar to bone covered their bodies. (Ostraco comes from the Greek word osteon, which mean bone. Derm also comes from a Greek word, derma, which means skin.) These early ancestors flourish about 540 million years ago during a period scientists call the late Cambrian Period. About 100 million years later, during the Ordovician and Silurian periods, these fish underwent major physiological changes. Some of these new fish were jawless fish, but with less body armor. Most of these fish ultimately became extinct during the Devonian Period about 350 million years ago. A few descendants remain of these ancient fish: the lampreys and hagfish. Another group of fish to emerge during the Ordovician and Silurian periods developed jaws, a feeding adaptation that revolutionized the evolution of vertebrates.
Jaws allowed these new fish to expand their capabilities. They were no longer limited to small particles of food filtered from the water. Jaws allowed the fish to nibble on plants, eat other animals, and to defend themselves by biting. Most modern fish are descendants of these new “jawed” fish. The early jawed fish had another evolutionary innovation, a pair of pectoral (anterior) and pelvic (posterior) fins that were attached to girdles of cartilage or bone. These fins gave fish greater control over their movement in the water. These fins also provided the bone structure for even later evolutionary innovations. The pectoral fins eventually evolved into the forelimbs and shoulders of later vertebrates; the pelvic fins, the hind limbs and hip bones. These early jawed fished vanished, but two major classes continued to evolve. These were the cartilaginous fish (Chrondrichthyes), which today includes sharks and rays, and boned fish (Osteichthyes). Today, boned fish comprise more than 97 percent of all living fish species. Salmon and trout belong to this later class.