What is common descent? why is it important to the theory of evolution?

The theory of common descent states that all living organisms are descendants of a single ancestor. Hence, common descent helps scientists better understand why species living in different geographical regions exhibit different traits.  Some of these traits are highly conserved among broad animal classifications (e.g., vertebrates or tetrapods), and seemingly different species (e.g., birds and reptiles) share inherited physical and genetic traits, and successfully adapted organisms typically produce more offspring.

Explanation:

In evolutionary biology, it is theorized that a group of organisms share common descent if they have a common ancestor.