More complex animals are composed of different organs, which have evolved over time. For example, the liver and heart evolved in the chordates about 550-500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, molluscs, and worms about 700–650 million years ago.
More complex animals are composed of different organs which have evolved over time For example the liver and heart evolved in the chordates about 550-500 million years ago while the gut and brain are even more ancient arising in the ancestor of vertebrates insects molluscs and worms about 700–650 million years ago