Just before the Cambrian explosion, approximately 600million years ago, multicellular aquatic animals are thought to have developed new body plans. As a result they split into two groups of organisms which were notably radially symmetrical and bilaterally symmetrical. These early bilaterally symmetrical animals were simple organisms without a body cavity called a coelom; further evolution, led to the development of complex coelomates. Eventually, land-dwellers capable of respiring out of water with the aid of lungs emerged, and subsequently, their adaptations allowed for reproduction in their new habitats.
