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 (6 of 6)
This tree shows how the octopus is related to tetrapods, and the points in their evolutionary histories when their limbs evolved. Tetrapod and octopus limbs evolved independently after their point of common ancestry, so they were not inherited from a common ancestor. Therefore, they are not homologous. The same is true of the grasshopper leg and the sea star arm.