> For one thing, it starts midway through the story, taking for granted the existence of light-sensitive cells, lenses and irises, without explaining where they came from in the first place. Nor does it adequately explain how such delicate and easily disrupted components meshed together to form a single organ.

Err yes it does. It's called random mutation. One day a critter was born with something lens like in front of it's light sensitive cell. That allowed it to utilize light slightly better than other critters, and it reproduced far more, sending it's DNA down the evolutionary chain. The result is superior models of critters reproduced with other different superior critters, produced super-critter kids, who then mutated and reproduced themselves.

Repeat millions and millions of times.