Developing from the same or a similar part of a remote ancestor.
Example:
Arms and wings are homologous structures that reveal our ancient relationship to the birds.
In his discussion of the panda's thumb, Stephen Jay Gould carefully explains how this thumb is not homologous to the human thumb. Although in function the two digits are similar, the panda's thumb developed from a bone in its wrist and is an addition to the five "fingers" of its paw. The panda's thumb is indispensable for stripping bamboo of its tasty leaves, the staple of the panda's diet; but it did not develop homologously with our thumb. The tiny stirrup and anvil bones of our inner ear, however, do seem to be homologous with the bones that allow a garter snake to swallow a frog whole.