Here's why:

* Homologous structures share a common ancestor and have similar underlying bone structures, even if their functions differ.

* The human forelimb, along with the forelimbs of other mammals, birds, reptiles, and amphibians, all have a similar arrangement of bones: humerus, radius, ulna, carpals, metacarpals, and phalanges.

* While the exact form and function of these bones may be adapted to specific needs (e.g., a human hand for grasping, a bat wing for flight), their underlying structural similarity points to a common evolutionary origin.

This is strong evidence for the theory of evolution, as it suggests that all these animals descended from a common ancestor that possessed this basic forelimb structure.