Predicting eyes

David Campbell (
Tue, 18 Nov 1997 15:01:57 -0400

Bilateral symmetry is predictable from evolution-it makes sense for a
mobile organism to specialize so as to have a front and a back end. Sense
organs are most useful up front, to see/smell/taste/feel/hear/etc. where
you are going, so having eyes symmetrically distributed on the front end
makes sense. Two eyes rather than one medial eye allows better vision to
each side, and if the eye is good enough, can allow binocular vision.
Since we're usually going forwards, extra eyes in back have not been a
particular advantage, and the pineal "eye" is reduced or lost in many
vertebrate lineages. Additionally, there's the "other mammals have two
eyes so we are likely to maintain this pattern" line of reasoning, which is
less a priori. Directionality is probably important in the presence of
more than one eye-we also have two ears, but one nose and one mouth-light
and sound have more direction to them than smell or taste.