Scientists have found single celled creatures with structures similar to animal eyes. Perhaps this tells us something about how structures like the eye evolved. https://cosmosmagazine.com/life-sciences/single-cell-eyeball-creature-startles-scientists
How fascinating! These single celled things are just eyeballs then.......lol I always wondered what was guarding all of those 20, 000 leagues under the sea alien civilizations in that they watch for intruders...... Just kidding....but it does boggle the mind......
They have no brains to interpret the information, so why the eye? If this is a clue as to how the eye evolved, does this mean that eyes exist before brains exist, that the senses are primary, and the brain is strictly just our visual and sensory interpretation of the way that the senses filter information? What does it mean to have the features of an eye? It has been said that nothing but an eye can truly have the features of an eye. Either it is an eye or it isn't, but there's no way it just has the features of an eye and isn't also an eyeball. Or does it just have some of the features of an eye? In which case, what is the purpose of having just a handful of features that also happen to be in an eye? It must have some function if it's an organ at all, I presume. But if it has duplicates of features found in an eye, but with no apparent function, this is particularly fascinating... What an amazing discovery this is. These are heavy times. Heavy times indeed. Most engaging.
Brains are not the only way a cell communicates. A light sensitive region in a cell could convey information without a brain.
Many of these plankton use photosynthesis for some or all of their energy. The lens could have simply evolved as a means to distribute light energy throughout the cell. It could also be a simple light/dark reflex trigger to snap at prey above the lens. That is about as complex a function as it gets, I suspect.