Why do you have two lungs but only one heart?

You might imagine that having two of some organs is redundant. We have two lungs, two kidneys, two eyes -- each doing the same job at the same time. But Dr. Tony Neff, a professor of anatomy and cell biology at Indiana University - Bloomington, warns against downplaying the role of duplicate organs. It takes both organs in those sets to carry out their job fully; Although one can function alone, the process it carries out will not be done at full capacity, and the rest of the body suffers. For example, you can see with only one eye, but the eyes' function of providing depth perception will suffer and you'll bump into things much more frequently seeing with one eye than you would with two.

So if you need both lungs to function at full capacity, what would happen if you had an extra heart? Would the performance of the processes it carries out double?

Not at first, says physiologist Bruce Martin, a colleague of Dr. Neff's at Indiana University. Your body is a system, and it's built so that the system is always functioning at its full capacity. When the system is attacked -- for example, through starvation -- all parts of the system suffer at the same rate. Conversely, when one part breaks down, the whole system suffers. If your lungs become irreparably damaged -- say, through emphysema -- the rest of the system will slow down to accommodate the broken part.

So since your system is already functioning at full bore, the addition of an extra heart wouldn't do much. But your system also possesses potential function, as seen in the muscles, when they're called upon to act beyond their normal capacity, like in the case of hysterical strength. We can train our bodies to function at higher levels, like athletes do. Since the heart pumps blood to the muscles, with a second heart your muscles would eventually grow stronger with time. Once the rest of the system is used to having a second heart, a person could grow stronger and have more endurance [source: Martin].

But the same can't be said for your brain. The brain is already getting more than enough blood to it, so it wouldn't function at a higher level, theorizes Dr. Martin.

Interestingly, when we are in the embryonic stage of development, we actually do have two hearts. The heart primordia (which describes the stage of the heart's development) in the embryonic stage is actually two hearts, which eventually fuse together into one heart with four chambers. Embryologists in the 1920s and '30s kept the heart primordia from fusing in embryonic frogs, and the frogs that grew up developed two hearts. The same also goes for our eyes. We begin with one primordia of the eye, which eventually separates to form two. If the primordia is kept from splitting, one central eye develops, like a cyclops, says Dr. Neff.

So it's theoretically possible for us to develop two hearts. And if we could determine how to use both fully, we could also advance ourselves into a species of super-strong, intellectually average beings. But wouldn't tampering with our own evolution as a species be dangerous?

"We've already taken ourselves out of evolution," says Rutgers' Susan Cachel. "[Humans are] all effectively tropical animals, and through our use of technology, like winter clothes, we've shielded ourselves from the effects of cold weather."

So we've beaten natural selection by the elements. We'll see what we can achieve with two hearts.