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 nostrils called choannae
– Greek for ‘funnels’. These connect to the throat and are what allow us to breathe through our noses.
To do this they somehow had to work their way back through the teeth. This sounds unlikely but
scientists in China and Sweden have recently found a fish called Kenichthys campbelli – a 395-
million-year-old fossil – that shows this process at its half-way stage. The fish has two nostril-like
holes between its front teeth.
Kenichthys campbelli is a direct ancestor of land animals, able to breathe in both air and water.
One set of nostrils allowed it to lie in the shallows and eat while the other poked out of the water a
bit like a crocodile’s.
Similar gaps between the teeth can also be seen at an early stage of the human embryo. 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, breathing different amounts of air into each to create a kind of nasal stereo.