The evidence for evolution - What do homologous similarities tell us?
Homologies suggest common ancestory
The evolutionary explanation of the pentadactyl limb is simply that all the tetrapods have descended from a common ancestor that had a pentadactyl limb and, during evolution, it has turned out to be easier to evolve variations on the five-digit theme, than to recompose the limb structure.
If species have descended from common ancestors, homologies make sense; but if all species originated separately, it is difficult to understand why they should share homologous similarities. Without evolution, there is nothing forcing the tetrapods all to have pentadactyl limbs.
Figure: all tetrapods have a basic pentadactyl (five-digit) limb structure. The forelimbs of a frog, lizard and bird are all constructed from the same bones even though they perform different functions.