I’m currently reviewing a Khan Academy video on “Evidence for Evolution” that pushes outdated science including, as we saw yesterday, Haeckel’s phony embryo drawings.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 evidence for common ancestry: “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. In Icons of Evolution, Jonathan Wells explains:
[N]ext to the Darwinian tree of life, homology in vertebrate limbs is probably the most common icon of evolution in biology textbooks. But the icon conceals two serious problems: First, if homology is defined as similarity due to common descent, then it is circular reasoning to use it as evidence for common descent. Second, biologists have known for decades that homologous features are not due to similar genes, so the mechanism that produces them remains unknown.pp. 61-62
Khan Academy is committing the exact error that Wells identifies. As for the second common problem Wells mentions with arguments for homology, this goes completely unacknowledged in the video. Instead, Khan Academy relies upon dysteleological arguments against the “independent” creation of vertebrate limbs. Here’s what the video says:
And if you were independently trying to create structures for what each of these different species were doing, it’s not obvious that you would have such homologous structures actually be involved.
How does Khan know what might happen if you “were independently trying to create” these structures? How do they know that there aren’t good functional reasons for these similar bone structures?
Two Simplified Structures
The textbook Explore Evolution doesn’t argue for special or independent creation, but it does make a compelling case that functional requirements are a better explanation than common ancestry for the similar bone structure in vertebrate limbs. It compares two such simplified structures, illustrated here:
The textbook explains:
Look at the hypothetical limbs depicted…. Consider the “two-to-one” limb, for instance. It has two bones nearest the trunk, and one in the next limb segment — reversing the usual one-to-two pattern.
Now, right off the top of your head: can you think of any functional difficulties that might arise from this two-to-one arrangement? Do you see any practical problems this creature might face that wouldn’t arise with the familiar one-to-two pattern?
Let’s think about the range of motion in the “two-to-one” limb. Would it be lesser or greater than it would be with the one-two pattern? You can answer this question for yourself. Attach a pair of equal-length straws to a lump of clay. Tape their free ends together. Now, rotate the point where you’ve connected them, and note how far it can move without either straw detaching from the clay.
Now connect a single straw of the same length to the clay. Notice how far its free end can move without the embedded end detaching from the clay.
Do you see the functional problems with the two-to-one pattern? Clearly, it limits the range of motion. There are obvious functional advantages to the one-to-two arrangement. But why do so many creatures have this same configuration?
Many scientists are unhappy with the “it’s just history” explanation of homologous skeletal patterns. R. D. K. Thomas and W. E. Reif, for example, have developed an idea they call “the skeleton space.” This sounds like a Halloween costume shop, but it’s actually a cool way of saying that there are only a limited number of ways that geometric shapes and growing materials (like bones) can go together and still work well.
In their view, it’s not “just history” or common descent that helps us understand why organisms have similar limbs. They claim that there are only a limited number of skeletal patterns because of the functional requirements of organisms. These are the limits imposed by geometry, and the characteristics of bones and the way they grow.pp. 47-48
Because there are potential functional explanations for why vertebrate limbs have the same basic pattern of bones, functional requirements could explain their similarities as well as common descent. Though Explore Evolution does not go this far, these similarities could be explained by common design just as well as by common descent. Khan Academy argues against design, but doesn’t recognize the arguments in favor of it.
Next time I’ll consider another bogus evolutionary icon in the video: fossil horses.