Question: why do we have two eyes and not three?

Anastasia Wass answered on 17 Mar 2014:

The first ‘eyes’ that evolved were just patches of cells that were sensitive to light. These patches evolved right at the start of life and so all animals had them. Since then more complicated eyes (the sort we recognise) have evolved multiple times separately in lots of different animals. This shows how important eyes are for life as every animal needs them.

Animals evolve by adapting to their environment. To make organs costs energy, not just to make the actual organ but also to maintain it for your entire life. Whilst eyes are obviously important for life to help survival you want to use as little energy as possible making your organs. If you can get away with having two eyes then that’s what happens as the benefit of having a third eye is outweighed by the cost of having to make and maintain it!