Casey Luskin talks about the circular arguments for common ancestry:
I’m currently reviewing a Khan Academy video on “Evidence for Evolution” that pushes outdated science including, as we saw yesterday, Haeckel’s phony embryo drawings. The popular and influential Khan Academy shows that what biologist Jonathan Wells called the “icons of evolution” — long refuted lines of evidence — are alive and well and being served up to students, teachers, and other on the Internet. The video promotes another fallacious icon: homology in vertebrate limbs.
Circular Arguments for Common Ancestry
In fact, that is the very first line of evidence for common ancestry cited by the video. It defines homology as “things which have similar structures, similar position, similar ancestry, but not necessarily the exact same function.” Note that cases of homology are defined as features that have “similar ancestry.” The video says that the “bone structures” in the limbs of humans, dogs, birds, and whales are “eerily similar” and offers the punchline that “this is a very strong hint that maybe humans, dogs, birds, and whales share a common ancestor more recently in the past than say other animals or organisms whose structures aren’t as homologous.”
But there’s a problem with this argument. If you define homology as resulting from common ancestry, you can’t then turn around and use that as an argument for common ancestry. That’s circular reasoning.
Casey Luskin, “Khan Academy Video, “Evidence for Evolution,” Gives Circular Arguments for Common Ancestry” at Evolution News and Science Today (February 8, 2022)
One obvious problem with resorting to circular reasoning is that there are many examples of convergent evolution where unrelated life forms end up with very similar adaptations.
Because of widespread convergent evolution, claims about common ancestry can’t be based on similarity of form alone — any more than we can assume that two people who look quite similar (body doubles) must be closely related. Life is more complex than that.
You may also wish to read: The Khan Academy markets 1980s Darwinism From back when all official “evolution” claims were expected to be reverently accepted by everyone. Luskin: “But in the famed series, the horse fossils don’t evolve in a straight line, nor are they found in the same place, nor do they show a continuous direction of change.”
and
Evolution appears to converge on goals—but in Darwinian terms, is that possible?