The four-chambered heart has a distinct advantage over simpler structures: It allows us to send our "dirty" blood to the cleaners-the lungs-and our "clean" blood to the rest of the body without having to mix the two. That system is very efficient. The blood coming from the left side of the heart is pure, fully oxygenated, and ready to fuel the muscles. A fish heart, on the other hand, has to pump blood, which is only half as pure, to the body because it doesn't have separate chambers that enable it to clean the blood in one cycle and distribute it in the next.
Our sophisticated ticker allows us to process energy more efficiently and, therefore, move further from our energy source. We can do lots of work in between looking for food, while a fish has to live in its energy source, eating all the time.