The Ten Myths of Evolution Refuted, Part 2

The 9th myth in the Answers in Genesis list of 10 is a particularly stupid one. They claim that homologies do not prove evolution, and therefore it is a myth.

The idea behind their claim is that homologies provide just as much evidence for a common "designer" as they do for evolution. This is silly.

This ties to another claim that is often made by creationists (falsely, I might add). They like to point out that homology is defined as similarity due to common ancestry, and so the claim that it is also evidence for common ancestry is a circular argument.

The evidence for common ancestry does not come from homologies, but rather the patterns of similar traits we see in nature. These similarities group themselves naturally, and form what appears to be a tree-like structure with the many branches representing the different groups (the tree of life).

So, let's take what I just said and make it visual. This image will help greatly.

The evidence for common ancestry comes from the groupings of organisms into different domains, phyla, classes, genus', et cetera. To make it easy, we'll just look at deuterostomes, chordates, and vertebrates.

A deuterostome is an organism whose embryonic blastopore forms the anus, and not the mouth. On the other hand, a protostome is an organism whose blastopore forms the mouth and not the anus. If you trace the tree down those two paths (you can't see the protostome path in that image, but it's there), you will notice that the two paths do not converge. No protostome or descendants of a protostome will have the blastopore form the anus, and no deuterostome or their descendants will have the blastopore form the mouth. You absolutely will not find that anywhere in nature.

The same applies for the chordates, who are characterized mostly by the jelly-like notocord that the embryos develop in the early spine (the spinal column eventually replaces this). No chordate or non-chordate will ever have the blastopore form the mouth, ever, because they do not share ancestry with the protostomes. You will also never find a descendant of a non-chordate that has a notocord, nor will you find a chordate that doesn't have a notocord. You see where I'm going with this? The same goes for vertebrates. No vertebrate will ever be a non-vertebrate, protostome, or non-chordate, and no non-vertebrate will ever be a vertebrate, protostome, or non-chordate. The reason being they do not share ancestry with these organisms, plain and simple.

A homology is what we use to describe the similarities between organisms in a particular group that come about due to common ancestry. That term is not applied to a feature until the evidence for common ancestry is presented.

To put it this way, homologies are not evidence for common ancestry, but rather common ancestry is evidence for homologies.