One way to study evolution is by looking at remains of the past life, or fossils. You could also determine relationship by looking at patterns of development or . Other ways to study evolution is by comparing sequences of their and looking at their structures. If species were once related by a in the past, but became more and more different like the arm bones of mammals, they show evolution. The bones would be structures. If species were not related but somehow have similar structures because of similar selection pressure, they show evolution. Sharks, dolphins, and penguins for example have fins as structures. Sometimes organisms may have a structure, an organ or structure that has no useful purpose anymore and can give evidence of the past ancestor. An example of this is a .