Jellyfish eyes

The complexity of eyes is commonly used as an argument by those who do not believe in evolution - "how could something so complex evolve naturally?" etc.  Here's part of the explanation...

Jellyfish may seem like simple blobs but some have surprisingly sophisticated features, including eyes. These are often just light-sensitive pits but species like the root-arm medusa have complex ‘camera’ eyes, with a lens that focuses light onto a retina. Not only are these organs superficially similar to ours, they’re also constructed from the same genetic building blocks.

Hiroshi Suga from the University of Basel has been studying the eyes of the root-arm medusa (Cladonema radiatum). His work strongly suggests that all animal eyes share a common origin, whether they belong to a human or an insect, an octopus or a jellyfish. The details may be different but they’re all under the control of closely related ‘master genes’ that themselves evolved from a common ancestor.

As you might imagine, growing an eye is a complicated business and involves a huge alliance of different genes, switching on and off in a coordinated way. But in humans and other animals, this alliance all comes under the control of a master gene called Pax-6... Only one of these – Pax-A – is actually active in the eyes and Suga clearly showed it’s the jellyfish’s master eye gene. When he transferred it into a fruit fly, he managed to trigger the development of eyes on odd body parts.

There is further explanation at Not Exactly Rocket Science.