Why do we have two kidneys, but just one heart? Wouldn't it make more sense to have extras of everything in case something goes wrong? Why can't we be like the lowly earthworm, with five different hearts? Or those girls with two bladders and two uteruses? Or that other girl with four kidneys? Some extras make sense -- for example, having two eyeballs gives us better, binocular, vision. The heart and brain have two parts each, of course, but each side has a different function. One chamber of the heart pumps out oxygenated blood, the other pumps in deoxygenated blood; and each lobe of the brain is responsible for different functions and operations. The liver regenerates, so need for two of those, and you can live without the spleen and gallbladder. But as for the double kidneys, we're not sure why one wouldn't suffice to clean the bloodstream, and why do we need two lungs? Rutgers University anthropologist Susan Cachel told Discovery Health the one heart/two lung system started about 300 million years ago, when we emerged from the muck onto land. For whatever reason, that's what we needed to survive on land, and it's remained the same ever since. Not a very satisfying answer, if you ask me, but there you go.