That's a good one, John. One can pretty much compare any mammal's skeleton with any other and find all the same bones. Even the whales and other sea mammals.
Here's another - cellular chemistry. Many simply one-celled organisms (yeast, for example) use a basic anaerobic (non-oxygen) process to metabolize and use energy from food. More complex organisms use an aerobic process (oxygen-using), which is much more efficient - it produces about 15 times as much energy, if I remember my basic biochemistry.
The thing is, those complex cells, with their aerobic metabolic pathways, still have and use the simpler anaerobic pathways, too. They have the cellular mechanisms for the simpler cells rolled in as well. There's not much of an advantage to this - the anaerobic process could never produce enough energy for a complex organism by itself, and the energy it does add is nearly unnoticable when combined with the aerobic process.
So, why would it be there unless those complex cells were the decendants of the simpler anaerobic cells?
When you look at it, animals only differ from each other in superficial forms. At the cellular level, we're very, very much alike.
But like John said, even though you're related to both your parents and your great, great-uncle you look much more like your parents.