Jawbone Length… Proves Creationism?

The DpSUs for 2012 are proving quite strange (but then I always think that, for some reason). According to Human Jawbone Size Reflects Diet, Not Just Lineage a paper in PNASGlobal human mandibular variation reflects differences in agricultural and hunter-gatherer subsistence strategies – …well, I’ll let Mr Thomas explain:

Human Mandible - Left view

Some scientists believe that jaw size is inherited, and many have factored this presumption into their speculations about human evolution. But a new study has confirmed that human jaw sizes and shapes vary according to diet, not just ancestry.

Though there was much overlap, the data clearly showed that the hunter-gatherer groups—including populations of the Alaskan Inuit, Australians, and Central Africans—tended to have more room in their mandibles (lower jaws). The diets of these populations consist primarily of unprocessed foods that require more chewing. Agriculturalist groups, including Italians and Japanese, had “relatively short and broad mandibles.”

Many of those who had to chew their food more vigorously developed larger mandibles and thus had enough tooth space. And that clearly implies that all human populations can grow different-size mandibles in response to chewing needs. Dr. von Cramon-Taubadel checked her results against other possible causes of the jaw differences and ruled out genetic ancestry. If changing jaw sizes is not directly inherited, then it cannot be ascribed to evolution, because evolution requires new traits to be heritable.

I can’t read the PNAS paper in full, but judging from what I can read (the abstract, plus various news articles about it) Mr Thomas has it wrong. The ruling out of genetic ancestry thing appears to be Dr von Cramon-Taubadel making sure that it wasn’t a case of all the groups that had the longer jaw being closely related, or similar – that was apparently the case with cranial size. She hasn’t shown that jaw size isn’t inherited, at least so far as I can tell.

The study is described by von Cramon-Taubadel as “supporting notions that a decrease in masticatory stress among agriculturalists causes the mandible to grow and develop differently.” Now, this could simply mean that kids’ jaws grow more if they have to chew, but I don’t get that impression from other information. For instance, this B.T. paragraph, from immediately what has already been quoted:

Nevertheless, the study author referred to the groups’ different diets as different “selective pressures.” She wrote, “Masticatory [chewing] pressure acts preferentially on the mandible rather than the maxillary [upper jaw] region, [and] the results presented here suggest that the mandible can evolve independently.”

That suggests to me that what she is saying is that the change in diet in populations caused jaws to shorten, via the various processes of evolution. If so then there is no contradiction or any other problem here – the lack of genetic relatedness merely shows that it happened in each population. (Or something like that – the numbers are hardly clean, but the relationship is there and that’s what matters.)

With that settled it should be added that the study in fact contradicts the biblical account. It shows that jaw size is, in fact, caused by diet – changing from a hunter-gatherer population to an agricultural population caused jaws to get smaller, crowding out teeth. But, if you will remember from the DpSU Population Growth, “there was never a time when humans weren’t engaged in agriculture.” Hence, the only way out for the creationists is to claim that we were designed to be hunter gatherers but only some of us became them. Which would be a little odd for them to do.


3 thoughts on “Jawbone Length… Proves Creationism?

  1. Well, I have access to the full article (honk for universities). What it’s essentially saying is that recent mandible variation is not evolved. That’s not to say the mandible isn’t evolved, just that the difference between the shape of mine or yours isn’t the product of genetics.

    It’s a lot like human height. Since the middle ages, people have gotten taller. This is likely the result of an improved diet, not evolution. Does this mean that height, in general, is independent from evolution? Nope.

    To go from recent mandible variation isn’t an evolved trait (or recent height growth) to the mandible isn’t evolved is completely unfounded.

    • Interesting… So what’s the context of the “Masticatory [chewing] pressure acts preferentially on the mandible rather than the maxillary [upper jaw] region, [and] the results presented here suggest that the mandible can evolve independently” line?

  2. Well I’m working on a post on the issue, but it shouldn’t be giving too much away to put the quote in context. The full quote is:
    “the palatomaxilla region, which is tightly integrated with the mandible, follows a similar pattern as the mandible, although the relationship with masticatory behavior is weaker overall. Importantly, what this suggests is that masticatory pressure acts preferentially on the mandible rather than the maxillary region, with the maxilla altering in relation to the mandible to retain effective dental occlusion. Therefore, despite considerable integration between the mandible and the skull (25, 26), the results presented here suggest that the mandible can evolve independently (e.g., 27).”

    Essentially the author is noting different pressures are exerted on different parts of the jaw and these different parts can change independently (the mandible changes and the maxilla has to keep up). Thus these different parts can evolve independently.

    The ICR takes from this is that she’s asserting natural selection played a role. Not at all, she’s merely saying its still possible evolution had an influence. Because she hasn’t ruled it out.

    As far as I can tell she’s only ruled out neutral mutations/genetic drift being responsible.


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