Common ancestors only exist within a single species.

It is not true to say that common ancestors occurs only within a single species rather two species with similar traits might have a common ancestor.

Explanation:

If the ancestor is same for both the species then it is said that both the species are closely related and share common ancestor with similar evolutionary relationship.

Phylogenetic tree are drawn and studies are done to understand the ancestor of a species. The evolution has occurred from the ancestor to the new species formed.