Your Legs are Poorly Designed

You’ve heard of exoskeletons (‘bones outside’) and you’ve heard of endoskeletons (the opposite). The obvious question to ask is “which is better,” and while it’s probably impossible to give a definitive and overarching answer to such a question a newish paper can at least go some way to resolving the matter. By modelling bones as hollow cylinders with a radius and thickness the researchers were able to calculate optimal r/t ratios for resisting certain stresses. According to the abstract, locust tibias primarily deal with bending and is “optimized for this loading mode.” Crabs, meanwhile, endure both bending and compression and their ratio is thus an “ideal compromise to resist these two types of loading.” But because their leg bones are within the body, the tibias of vertebrates such as us have relatively smaller radii and greater wall thickness – according to this research, this situation is not optimal.

As is only to be expected, Brian Thomas runs with the fallacious “optimal insect = designed insect” argument in Optimized Engineering in Locust Legs, opening:

People instantly recognize intelligent engineering in a structure that has optimized size or shape. Optimum parameters don’t just happen. So when two mechanical engineers recently discovered optimum sizing in locust legs, to what did they attribute that high level of engineering?

Allow me to demonstrate an obvious problem with the idea that begins this article with an example: Sand under a current of water, such as at the bottom of a stream, is moulded by the action of the fluid above which optimises the shape of the sand in light of aerodynamics. There is no need for either intelligence or design here – optimum parameters can happen by themselves (while we’re on the subject of sand, slope angle is also “optimum”). The Giant's causeway: Teach the controversy!It would be a little more accurate to say that people “instantly recognise” whether or not a design is optimal, excluding the part about determining that the structure is the result of design in the first place. Place names like the “Giant’s causeway” should show you how good we really are at correctly detecting design in the wild.

It goes without saying that the researchers attributed their results to the processes of evolution. It’s not a hard thing to grasp, at least for non-creationists, that something like this is easily mouldible by natural selection. If you have variation in r/t ratios, then the closest to optimum will have an advantage and will be more successful. Because if this the population moves ever onwards towards said optimum.

You can predict, therefore, the gist of his next couple of paragraphs talking about the locust and crab examples. But what about the non-optimal human leg bones?For those of us who are unable to remember which is which

The same study showed, however, that the human tibia is far from the ideal shape to resist either compression or bending. However, the study, which treated the bones as hollow round tubes, ignored the usefulness of skin, the additional strength provided by muscle, and the multifunctional uses of bone. Plus, people would just look weird as giant arthropods. A separate study found that the human tibia is ideally shaped when it factored in the support added by bone’s interior trabecular structure.

“Plus, people would just look weird as giant arthropods”? I don’t think Brian has thought his ‘made in God’s image’ theology through – and regardless whatever it is that is considered normal can’t also be ‘weird’ basically by definition. As for the rest of his objections it’s nice to see him focusing on the full picture, but it’s worth noting that starting off at the optima probably gives a starting advantage that muscles and the like can build on.

This other study is made out to be a more recent and more up-to-date than the one being talked about here, but in reality it’s actually from 1985. In its abstract the more recent study acknowledges that it has drawbacks and that other studies got different results in this aspect:

Vertebrate long bones were found to be far from optimal, having much lower r/t values than predicted, and in this respect our conclusions differ from those of previous workers. We conclude that our theoretical model, though it has some limitations, is useful for investigating evolutionary development of skeletal form in exoskeletons and endoskeletons.

Unable to read beyond the abstract of either paper I can’t determine which has truly looked at more elements, so I’ll leave that up in the air. But why could vertebrate bones be suboptimal if they too are subject to the processes of evolution? The answer is simply that you can’t get there from here – though of course you can still get to either place starting from another point. A fundamental redesign of the kind that would turn a endoskeleton into an exoskeleton – and that is indeed what would be required to change the ratio to such an extent – is beyond the abilities of evolution. Not for design, however, but that’s a different matter entirely. So if vertebrate bones are not at the optimum ratio, then they will still be at a local optimum factoring in the other aspects that Brian so kindly listed for us earlier.

Beyond this point in his article, Thomas rants about how the authors “substitute natural processes for a real engineer,” how nature can’t “engineer” anyway (of course it can’t, that’s not what’s happening here), and concludes by telling us that “it looks like people can trust their intuition.” These paragraphs too can most likely be envisioned without reading. You don’t have to if you don’t want to though.

2 thoughts on “Your Legs are Poorly Designed

  1. When endoskeletons first appeared we were still living in the sea, and it would certainly be interesting to see whether they would’ve given an advantage in those environments.


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