From Seed Magazine:
Open any biology textbook with decent evolution coverage, and you’ll find a version of a familiar diagram—a bat’s wing, a dolphin’s fin, a horse’s leg, a human’s arm. These vertebrate-limb structures are homologous, their similarities the result of shared origins in an ancestral mammal with a general-purpose limb. Evolution has modified each, but all have a common internal structure, a common embryological origin, and a fossil history that reveals a shared phylogenetic origin.
Comparative biology seems to make it clear that the limbs of invertebrates such as insects have different origins. There is little correspondence to what we see in our own structure: Insect limbs lack bones altogether. Embryologically, insect limbs arise as repeated segmental bulges in the cuticle. The comparative evolutionary history of vertebrates and invertebrates is most telling. The ancient chordate ancestors of our finned or limbed modern vertebrates were completely limbless, little more than undulating ribbons of eel-like swimmers, and our appendages are evolutionary novelties, less than 500 million years old. The arthropods, on the other hand, have been flourishing elaborate limbs for as long as they appear in the fossil record, beginning with tracks laid down in the pre-Cambrian, easily 500 million years ago. The last common ancestor of insects and mammals was a legless worm, and each of our lineages independently worked out how to build limbs, so they can’t be homologous.