2. Life Develops in the Sea

At the beginning of the Palaeozoic 545 million years ago, a great explosion in the diversity of life occurred. The seas became populated with a wide variety of animals in a very short period of time. By the end of the first period of the Palaeozoic, the Cambrian, all of the major groups (phyla) of animals alive today, had evolved. In later periods, fish evolved and diversified. For the first several hundred million years of the Palaeozoic, animals were found only in the sea.

The first animals with hard body parts—Cambrian explosion
Animals with hard body parts, such as shells or other skeletal elements, appeared at the beginning of the Cambrian Period, around 545 million years ago. Organisms may have evolved hard body parts as a protection from predators. The evolution of hard body parts may also have been helped by an increase in the amount of phosphate or carbonate in the oceans, as these are the minerals used by organisms for making hard parts. The ability to build hard parts was a major evolutionary development that led to a spectacular diversification of life in the seas. During this period almost all the major groups (phyla) of animals alive today appeared, including arthropods, brachiopods, molluscs, echinoderms and sponges. The fossilised hard parts from the Cambrian reveal a great diversity of marine life at a very early stage in the history of multicellular life.

Evolution of jaws in fish

Jawless Fish. Artist: Kate Nolan. Source: Museum Victoria.

The earliest fish, from the Late Cambrian, Ordovician and Silurian, had no jaws, just a mouth opening which sometimes contained teeth. Fish with jaws first appeared during the Devonian Period about 400 million years ago. Their jaws evolved from modified gill arches, which are the bones that support the breathing structures

Jawed Fish. Artist: Kate Nolan. Source: Museum Victoria.

(gills) of fish. Jawless fish are thought to have fed by sucking food from the ocean floor, although modern jawless fish (lampreys and hagfish) suck the blood of other fish. Having jaws made many different ways of life available to fish that were not available to jawless species. There was a great explosion in the diversity of jawed fish species during the Devonian, while most of the jawless fish species disappeared.