In this second item in the series, Jonathan Wells discusses the similarity “in the bones in the human hand and the wing of a bat” (technically homology), which Darwin considered evidence for common descent. But, Wells notes,
Yet animals and plants possess many features that are similar in structure and position but are clearly not derived from a common ancestor with those features. The camera eye of a vertebrate and the camera eye of a squid or octopus are remarkably similar, but no one thinks they were inherited from a common ancestor that possessed a camera eye. The spines of South American echidnas and North American porcupines are remarkably similar, yet echidnas give birth by laying eggs, while porcupines give birth to live babies after nurturing them in a womb, like human beings. This fundamental difference means that echidnas and porcupines had very different origins, and they did not inherit their spines from a spiny common ancestor. The folds of skin between the forelimbs and hind limbs of Australian flying phalangers and North American flying squirrels are very similar. Yet the former give birth to fetuses that crawl into a pouch to complete development, like kangaroos, while the latter nurture their fetuses in a womb, like human beings. Again, they had very different origins.
Jonathan Wells, “Top Scientific Problems with Evolution: Homology” at Evolution News and Science Today (February 11, 2022)
So, Wells explains, “similarity of structure and position is evidence for common ancestry, except when it isn’t.”
It’s not clear, at that point, what problems the claim of common ancestry is supposed to address. If it’s true, it’s true but if we can’t use similarity of form as a reliable guide, how do we know it is true?
The whole series to date is here.
Note: The article is excerpted from Jonathan Wells’s chapter in The Comprehensive Guide to Science and Faith: Exploring the Ultimate Questions About Life and the Cosmos (2021)