How many nostrils do you have? Four. Two you can see, two you can’t. This discovery came from observing how fish breathe. Fish get their oxygen from water. Most of them have two pairs of nostrils, a forward-facing set for letting water in and a pair of «exhaust pipes» for letting it out again. The question is, if humans evolved from fishes, where did the other pair of nostrils go. The answer is that they migrated back inside the head to become internal. To do this they somehow had to work their way back through the teeth.

Similar gaps between the teeth can also be seen at an early stage of the human birth. When they fail to join up, the result is a cleft palate. So one ancient fish explains two ancient human mysteries. The most recent research on noses, incidentally, shows that we use each of our two external nostrils to detect different smells.