To answer your question, you first have to understand the selective forces that have shape hearts and circulatory systems during evolution.

All vertebrates (fish, frogs, lizards, birds, mammals) use oxygen from the air (or dissolved in water) to efficiently extract energy from food and release carbon dioxide as a waste product.

In some of the smallest aquatic organisms, this exchange of gases (O₂ in, CO₂ out) occurs directly through the skin.

As organisms because larger however, specialized structures were required to deliver O₂ and remove CO₂ throughout the body.

This specialized system is known as the circulatory system: it consists of blood, which contains cells that carry oxygen and CO₂, blood vessels (the tubes through which the blood flow), the heart (which pumps the blood through the blood vessels) and gas exchange organs, such as gills and lungs.

Although it is common to think of fish has having only gills, many also have lungs or an air bladder (both of which arise in a similar manner developmentally from the upper intestine).

In many fish, the circulatory system is a relatively simple loop. The heart consists of two contractile chambers, the atrium and ventricle.

In this system, blood from the body enters the heart and is pumped through the gills where it picks up O₂ and releases CO₂.

As the blood moves through the body, the oxygen is used up and the blood returns to the heart O₂ poor.

One problem with this system is that heart itself requires O₂ to function. The simple loop design supplies it with O₂ poor blood.

If the organism is a very active swimmer, the heart may be starved for O₂ a condition known generically as anoxia.

Colleen Farmer, an evolutionary biologist at the University of Utah, proposed a hypothesis (Farmer. 1999. Ann. Rev. Physiol. 61:573) that the origin of the lung was driven by the need to supply the heart with O₂.

The gill-lung heart loop found in some fish brings highly oxygenated blood to the heart, where it mixes with O₂ poor blood coming from the body.

This gill-lung-heart loop enables the heart to beat rapidly and over a prolonged time without damaging itself.

However, with a simple fish-like heart, the system is relatively inefficient. O₂ rich blood from the lung mixes with O₂ poor blood from the body to produce blood of intermediate O₂ content.

The process of vertebrate evolution through amphibians and reptiles has been one of making the sysetm more efficient.

The approach has been to divide the circulation into two interconnected paths. One circuit takes O₂ poor blood from the body and sends it to the lungs under low pressure. The second loop takes O₂ rich blood from lungs and sends it to the body at high pressure.

In adult amphibians (above) these two circuits still partially mix in the ventricles of the three-chambered heart.

In reptiles (left) the extent of the mixing is reduced by barrier that partially seperates the ventricle into two chambers.

In mammals (below), the seperation is complete and there is no mixing of O₂ poor and O₂ rich blood in the heart.

This four chambered heart has its own functional challenges, however. In two- and three- chambered hearts the mixing of O₂ poor and O₂ rich blood in the heart can provide the heart muscle cells with O₂.

Since there is no such mixing in mammals, the side of the heart that pumps O₂ poor blood must receive its O₂ via a special coronary circulation system.

How efficient a heart needs to be depends on the organism's life style.