Human beings and other vertebrates mammals, birds, reptiles, and so on have eyes whose structure is like a camera, with an iris to control how much light gets into the eye, a lens that forms an image, and a retina that detects the image.
The eyes of insects appear very different, since they are compound eyes, made of many small facets, each of which detects light coming from just one direction.
Both eye structures are efficient, but for a long time the differences between them and differences among other animals' eye structures led biologists to think that different types of eyes might have evolved independently, with no common ancestor.
But some years ago a team of scientists working in Switzerland cast doubt on that notion. They found that nearly identical genes play key roles in the formation of the eyes in mammals and fruit flies. Fruit flies are a favorite subject for experiments in genetics.
In humans and other vertebrates, there's a gene which, if defective, leads to malformation of the eyes as an embryo develops. In other words, that gene is crucial for the proper development of eyes. In fruit flies there's also a crucial eye development gene. What the Swiss scientists found is that those human and fruit fly genes are almost identical. Their molecular structures are almost exactly the same.
That's compelling evidence that those two eye development genes are just variations of a gene carried long ago by a common ancestor of today's vertebrates and invertebrates. Flies' eyes and our eyes may be more closely related than we thought.