We are all familiar with the idea that there are strikingly
different kinds of eyes in animals: insects have compound eyes
with multiple facets, while we vertebrates have simple lens
eyes. It seems like a simple evolutionary distinction, with
arthropods exhibiting one pattern and vertebrates another, but
the story isn’t as clean and simple as all that. Protostomes
exhibit a variety of different kinds of eyes, leading to the
suggestion that eyes have evolved independently many times; in
addition, eyes differ in more than just their apparent
organization, and there are some significant differences at the
molecular level between our photoreceptors and arthropod
photoreceptors. It’s all very confusing. There has been some
recent press (see also this press release from the EMBL) about
research on a particular animal model, the polychaete marine
worm, Platynereis dumerilii, that is resolving the confusion.
The short answer is that there are fundamentally two different
kinds of eyes based on the biology of the cell