how do homologous structures provide evidence of evolution?

Answer 1

Homologous structures have enough in common to suggest that they have a common ancestor. However, the structures have changed in form and/or function to suit the organism's need. Anytime you can find evidence of change in a population over time, that's evidence for evolution.

Answer 2

A homologous structure is a structure in common between several organisms, but which has a different function in each.

For example, both humans and bats have the same bone structure in the arms ending in a five digit hand. However this bone structure serves *very* different functions in both organisms (grasping in humans, and flying in bats).

They provide evidence of evolution because they are evidence that the same structure has been adapted for two very different functions. For example, there is no reason either the bat wing or the human hand has to have five digits (not three, and not six). But the fact that they both have five digits is a piece of evidence that they had a common ancestor with five digits (the therapsid ancestors of early mammals).