Homologous tetrapod limbs (4 of 6)
This evolutionary tree shows the relationships between different tetrapod lineages, all of which evolved from a single common ancestor. This 350 million year old animal, the first tetrapod, had limbs with one long bone (the humerus) attached to two other long bones (the radius and ulna). Its descendants, including whales, lizards, humans, and birds, as well as many others, inherited the tetrapod limb from this ancestor.
Homologous tetrapod limbs (5 of 6)
Not all similarity is homology. Since the octopus, sea star and grasshopper limbs don't have bones, they are not homologous to tetrapod limbs.