Most species on Earth have only two eyes, but there are many species that have more. Evolution has also thrown up lots of different types of eye through the eons (compound eyes, pit eyes, eyes on stalks etc) and some of these are still around today. Had evolution taken a slightly different turn – either as a result of different environmental conditions or simply through pure chance – we might all have eyes in the back of our heads and insect feelers sticking out of our nipples.
You do have to consider the practicality of having three eyes, though. We have two eyes for a very simple reason; depth perception. Would a third eye in the middle of our faces really be practical or would it just get in the way? I like to think that the third eye would be better placed on the back of the head as a defence against predators (so far as I know this has never happened on Earth but I’d love somebody to prove me wrong).
It also stands to reason that aliens living on a world with low gravity might develop an extra set of eyes so as to see predators approaching from all angles. Although life on Earth developed in the simulated low-gravity environment of the sea, this hasn’t happened. However, the most common type of eye remains the apposition eye (a type of compound eye) which typically gives a wide field of vision and is common in all arthropod groups, including some of the oldest species on Earth. Many sea creatures also have eyes on stalks, giving them a 360 degree view, as do snails whose predators (birds) attack from various angles. In a low gravity environment flying would be much more practical and this kind of vision would be an essential defence.
Animals With 3 Eyes in Nature
Some reptiles and fish species do have small light-sensitive spots on their foreheads. These are known as parietal eyes and their purpose is still unknown. The closest thing to a third eye is the parietal eye of New Zealand’s Tuatara – a lizard-like reptile. The eye has a lens and a nerve connection to the brain, suggesting that it has evolved from a fully functional third eye. However it is covered by scales and cannot see.