
The reason we have two eyes is so that we can judge distance by stereo vision. The reason we have two ears is that the difference between the sound they pick up tells us what direction that sound is coming from. You can argue that these aren't really reasons, but advantages that ensure that we kept those dual systems. So why do we have two nostrils? Is it a side effect of a symmetrically-split face? Not really- since we have one nose at the center of our faces, it could easily have evolved to have one big hole.
You might be surprised to learn that two nostrils do give us a slight stereo effect in smelling things, so that we can detect which direction a smell comes from. But the real reason two nostrils are an advantage to us is that they don't work together. One nostril carries a bigger load in breathing while the other rests- and then they switch. Read how this happens and why its an advantage for animals that have two nostrils at Popular Science. -via kottke
(Image credit: Karabinkrok)










