A thesis statement states what your paper or theory is about. A good way to start is to write "the purpose of this paper is". In this way you narrow down what you want to say and write about. It is easy to wonder off the task and end up in places where you don't want to go. You need to write this. Wiki can not do it for you and we can only give you direction.
what is Similar structures that related species have inherited from a common ancestor
They evolved from a common ancestor.
The limbs of Species A have a similar structure to the limbs of Species B, but perform a different function.
the species come from a common ancestor
Scientists use scientific names (binomial nomenclature) to avoid confusion caused by different common names for the same species. For example, the scientific name for the domestic dog is Canis lupus familiaris, which clearly indicates its relationship to the wolf (Canis lupus) and distinguishes it from other species with similar common names.
A homologous structure.
what is Similar structures that related species have inherited from a common ancestor
Back down the line, the species came from a common ancestor.
Groups of similar organisms within a species are called populations. Populations consist of individuals of the same species that can interbreed and share a common geographical location.
A taxon of closely related species that share a recent common ancestor is a genus.
Homologous structures!
Homologous structures.