OK this is totally crazy. All animals have a set of genes (sequences of atoms in their DNA) that regulate how they develop from a miniscule single cell to their final enormous multicellular form. But, the super amazing thing is, those genes are all the same. For every animal. Of every species. They're the same exact genes in whales and people and tax accountants and leeches. These genes are called homeobox genes. ("Homeo" from the Greek word for "same", and "box" from the Greek word for "cardboard box".)
Some of these genes encode for high level structures, like "make an eye here". The actual details of how to make the specific kind of eye are encoded elsewhere in the DNA. However, you can take the high level "make an eye" gene out of a mouse, and splice it into the middle of a fly's genes that control how its legs grow, and the fly will wind up with a semifunctional fly's eye on its knees. Scientists have actually performed this experiment. The flies were not amused.
All of this implies that flies and mice share a common ancestor. Scientists have recently determined that this ancestor is named Steve and lives in rural New Hampshire with his parents and it's about time he moved out and got a job.
Curiously, plants and animals do not share the same set of homeobox genes. This suggests that the homeobox genes evolved separately in plants and animals. I say we patent the concept of homeobox genes and then sue the plants for all they've got.