At the beginning of the Cambrian period hard-shelled animals appeared for the first time in large numbers. As normally only a creature's shell becomes fossilized, the history of life is only well known from this time onwards. By the Cambrian all major groups (phyla) of animals, both radially and bilaterally symmetrical, had evolved. The animals with radial symmetry consisted of the coelentrates (jellyfish and corals) and the echinoderms (starfish and sea urchins). Those with bilateral symmetry fell into four main groups; the brachiopods - an almost extinct group of shellfish; the molluscs - bivalved shellfish, sea snails and nautilus-like cephalopods; the arthropods - represented primarily by the trilobites; and several classes of worms and worm-like creatures.
From one group of these worm-like animals, the chordates, came the first backboned animals in the Silurian - a class of primitive jawless fish and the ancestors of all vertebrates. At this time, too, the plants first came on to land. From shallow coastal waters emerged a group of plants that could survive without being totally immersed in water. They evolved stiff stems, to give them more support, and an internal plumbing system to transport water and dissolved minerals up from the ground and carry manufactured food down from the leaves.
As a side effect of photosynthesis free oxygen was liberated into the atmosphere; the proportion of oxygen increased while that of carbon dioxide decreased, making the composition of air more congenial to animal life. The arthropods were the first animals to take advantage of the improved atmospheric conditions and both scorpions and millipedes existed among the early plants.
The succeeding Devonian period is known as the Age of Fishes. First to evolve from the primitive jawless types were placoderms such as Dinichthys � the armoured fishes, which had jaws evolved from the bones of the gill arches. Before the end of the Devonian they were largely replaced by cartilaginous fish such as Cladoselache, the forerunners of the sharks and rays. Bony fish, more versatile and widely distributed, existed alongside these cartilaginous species. They formed two main groups - the ray-finned fish, which were to prove most successful, and lobe-finned fish such as Eusthenopteron. The last named is the most significant of the two from an evolutionary point of view. Living in shallow freshwater pools which periodically dried out gave them the evolutionary stimulus to survive out of water. When the pools disappeared Eusthenopteron dragged itself overland to the next area of water by means of a pair of muscular fins evolved from stabilizing organs. At these times it was able to breathe air through primitive lungs developed from outgrowths of the pharynx. Vertebrate life on land had begun, even though it was only as a temporary measure to allow the continuation of an aquatic existence. By the end of the Devonian the amphibians, able to spend most of their adult lives on land, had appeared. One of the earliest, Ichthyostega, showed the typical arrangement of five-toed limbs supported on strong girdles of bones found in land animals. It nevertheless retained fish-like features in the shape of the tail and skull.
The Carboniferous period that followed was the time of the great coal forests. It was also the Age of Amphibians; the lush swamps that characterized the lowlands of the period were ideal for their development and consequently a large number of new forms appeared. Some were small and eel-like, such as Dolichosoma, others such as Eogyrinus assumed an alligator-like form and existence. Still others, such as Diplocaulus, became broad and flattened and lived entirely in mud. The skulls of these creatures were more advanced than the fish-like structure of Ichthyostega. The nasal passages were well defined, indicating that they belonged to sophisticated air-breathing animals. These animals gave rise to both the later more highly advanced amphibians and to the reptiles.