What is some conlusive evidence that organisms of two different species share a common ancestor?

Well, according to the theory of evolution, the reason that organisms of two different species would share a common ancestor is that way back before evolution really took place, there were species that were all grouped together. After time, these species needed to evolve in order to adapt to the way the environments were changing. For example, dogs and wolves are not the same species, but are related and share a common ancestor, causing them to look alike.