The evolutionary change in the pattern of heart among the vertebrates are given below:
The vertebrate heart has evolved from a two-chambered heart of a fish to a four-chambered heart of mammals and birds.
The heart of the fish is two-chambered. The heart pumps deoxygenated blood to the gills where it is oxygenated and sent to the body. The deoxygenated blood is carried to the heart.
In amphibians, there are three chambers—
the left atrium
the right atrium, and
The left atrium receives the oxygenated blood from the gills, the lungs or the skin. The right atrium receives deoxygenated blood from the body organs. However, eventually, both types of blood are mixed in the ventricle and the body is supplied with the mixed blood.
In the case of reptiles, there is half septum which divides the ventricle incompletely. Here, oxygenated and deoxygenated blood do not mix.
In crocodiles, birds, and mammals, the heart is completely divided into halves which keep the oxygenated and deoxygenated blood separate.
There is a structural modification in the heart from fish to mammals, which ensures the supply of oxygenated blood to the body. At the same time, the four-chambered structure makes sure that the flow of blood is regulated.