What could be the possible explanation for the evolution of four-chambered heart in mammals as opposed to two-chambered heart in fishes?
Fishes do not require energy to maintain body’s temperature.
Fishes do not have complex body structure like mammals.
In fishes and reptiles, the heart is two-chambered and there is mixing of oxygenated and deoxygenated blood. They are cold blooded (the body temperature varies according to the surrounding temperature), so energy is required only for locomotion and hence less amount of oxygen is sufficient in them. Birds and mammals are warm blooded, so more energy is required to keep up the body temperature. To maintain high energy requirements of the body, more oxygen supply is required. Therefore, the heart has four chambers and oxygenated blood and deoxygenated blood is kept strictly separated. Also, complexities in the body structure and different mechanisms have increased with evolution. Therefore the mechanism of circulation has also evolved.