What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor?

Homologous organs are the organs inherited from the common ancestors, having similar basic structures, but performing different functions.

For example, the forelimbs of humans, lizards, frogs, birds, etc. have a similar basic design but perform different functions.

In homologous organs, the similarity in origin and basic structures is observed, because they share a common ancestor.