What factor of phylogenies enable us to determine that all terrestrial mammals are quadrupedal (walk on all four legs) except humans, the one bipedal twig on the many branches of the mammalian tree?

Common ancestor is the main factor that determines that all terrestrial animals are quadrupedal.

Explanation:

Phylogeny means the evolution of the organisms or genetically related organism and how they are related. The study of phylogeny is called as phylogenetics. Terrestrial means those who live on land. Mammals are one who gives birth to their offspring directly. They are generally warm-blooded, breath air. Studying each state of the ancestor and also tracing the organism characteristics during evolution.