Why we have chins: Our chin comes from evolution, not mechanical forces

Apr 17, 2015

Credit: Tim Schoon, University of Iowa

By Science Daily

Look at a primate or a Neanderthal skull and compare it with a modern human’s. Notice anything missing? We have one feature that primates, Neanderthals, archaic humans — any species, for that matter — don’t possess: a chin.

“In some way, it seems trivial, but a reason why chins are so interesting is we’re the only ones who have them,” says Nathan Holton, who studies craniofacial features and mechanics at the University of Iowa. “It’s unique to us.”

New research led by Holton and colleagues at the UI posits that our chins don’t come from mechanical forces such as chewing, but instead results from an evolutionary adaptation involving face size and shape — possibly linked to changes in hormone levels as we became more societally domesticated.

The finding, if true, may help settle a debate that’s gone on intermittently for more than a century why modern humans have chins and how they came to be.


Read the full article by clicking the name of the source located below.

21 comments on “Why we have chins: Our chin comes from evolution, not mechanical forces

  • I don’t know about the skulls in the image above^^… they both seem to have chins. For comparison, look at a gorilla skull ( HERE ), where the teeth are proclined but there is no MENTAL PROTUBERANCE.

    Could the evolution of the chin not be an outcome of sexual selection?

    Steve (DDS)



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  • Interesting headline. Just how could “our chins” have come from mechanical forces, rather than evolution, anyway? Maybe I am missing something, but this whole article seems to be about nothing except that chins aren’t required for chewing. Instead, it’s proposed, they might have evolved! To be precise, they might have evolved due to “spatial dynamics during development.”

    Sounds legit.



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  • If there is anyone out there who knows biology and can help I’d appreciate some feedback on this.

    As I understand it:

    The original story is about a bunch of dentists testing modern day humans to see what role the chin plays in the mechanics of chewing – because (see link to studies on jaw evolution next to the original story) others have been positing that the chin evolved to help us chew certain kinds of food?

    Finding, if anything, that chins are a bit of a loss as far as helping with the chewing goes the dentists have speculated on the reason that modern humans have marginally more prominent chins. Simply put: We can see that chins are not for chewing so why did we evolve with chins?

    Without, as far as I could see, linking to any other research the dentists concluded that social and/or hormonal changes were in some way connected to the evolution of chins that are more prominent than our ancient evolutionary ancestors (and therefore, also, closely related species today)?

    It seems like they pulled a rabbit from their proverbial hat there.

    If I understood that correctly (the original story seemed to be a bit of a mash-up to me), then the dentists are over-reaching by quite a long way.

    Granted our jaws, on average, are slightly more prominent at the front than other related species – this has been noted before and was probably an incentive for the dentists to investigate. The dentists appear to have made a good study of the mechanics of chewing in modern humans and this does appear to challenge the notion that our jaws thus evolved to enable some form of chewing. They get full marks for this work.

    Also, the dentists are correct; in order to have evolved there was probably some form of selection pressure that favoured the spread of chins across the gene pool. But probably is not definitely.

    Speculating further is just that – speculation, but this does not seem to have held the dentists back: Where did the hormone / social interaction idea come from? It seems to me that the dentists should have stopped there and just said: Further study into human jaw evolution is clearly required to understand chins.

    Also, it could be that chins are an accident – an allele that got a free ride because it arose alongside other alleles that favoured survival and reproduction and that (chins) while not being the most effective chewing jaw were not important enough to affect those survival and reproductive rates negatively … no?

    As I say, I would appreciate a lesson in biology here.

    Peace.



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  • Although I am not the biologist that Steven of Wimbleton requested, I will try to add some relevant material that is from a book that I have here on my shelf. The book is The Evolution of the Human Head by Daniel E. Lieberman. He is “a paleoanthropologist at Harvard University, where he is the Edwin M Lerner II Professor of Biological Sciences, and chair of the Department of Human Evolutionary Biology.” Wikipedia.

    An interesting point about certain features of the skull is that they could be formed by deposition of bone or by resorption of bone. In the case of the chin, both happen. Before I read this book I imagined that our skulls were created by creation of bone material here and there based on a genetic blueprint. Now I see that it’s accomplished by addition and subtraction of bone according to direction from the genetic blueprint but also dependent on dynamic forces as well. Here is a paragraph from the chapter titled Modular Growth of the Head, page 141:

    One last component of lower facial growth deserves mention: the chin or mental eminence. This unique derived feature of modern humans appears prenatally from a novel configuration of growth fields in the symphysis (Figures 4.5 and 4.14). The anterior margin of the symphysis is entirely a depository growth field in nonhuman primates and hominins, but in modern humans, the surface of the alveolar process below the anterior teeth is a resorptive field (Duterloo and Enlow, 1970; Bromage, 1990). Bone is thus resorbed on either side of the symphysis, hollowing out two mental fossae and leaving an upside-down T-shaped protuberance below, projecting out from the symphysis (Buschang et al., 1992; Schwartz and Tattersall, 2000).

    I can’t include the diagrams mentioned above, but if we look at the human skull in the picture above, on the left, Lieberman is describing the dip that we see just under the roots of the teeth.

    Here is another paragraph on page 262 that I thought was interesting:

    Moreover, computer modeling studies find that the presence or absence of a chin has little effect on the magnitude and pattern of strains that chewing generates (Ichim et al., 2006). In hindsight, such results should not be surprising: if the chin counteracts high strains, then one would expect its presence in other hominins, such as H. erectus, with similarly wide mandibles and who probably generated much high bite forces. An alternative hypothesis, in need of further scrutiny, is that resorption of the upper alveolar margin helps to orient the lower incisors more vertically in modern humans whose faces are particularly short and retracted (Enlow, 1990) (see Chapter 13). Alternatively or additionally, the chin may be an example of sexual selection. Testing this last hypothesis is especially difficult, but the reader is encouraged to think of appropriate experiments.



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  • From the article above:

    Males in particular became more tranquil during this period, less likely to fight over territory and belongings, and more willing to make alliances, evidenced by exchanging goods and ideas, that benefited each and all.
    The change in attitude was tied to reduced hormone levels, namely testosterone, resulting in noticeable changes to the male craniofacial region: One big shift was the face became smaller — retrenching in effect — a physiological departure that created a natural opportunity for the human chin to emerge.
    “What we’re arguing is that modern humans had an advantage at some point to have a well-connected social network, they can exchange information, and mates, more readily, there’s innovation,” says Franciscus, who was on the team that first laid out the theory in a paper published last August in the journal Current Anthropology and is a contributing author on the current paper, “and for that to happen, males have to tolerate each other. There had to be more curiosity and inquisitiveness than aggression, and the evidence of that lies in facial architecture.”

    This whole section from the article is the part that I really have the most problem with. Did males become more tranquil? Did it reduce testosterone levels?



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  • What about teeth, the amount of teeth we started out with has diminished. Less molars smaller canines. If you put the same amount and size of teeth that a primate has, on a human, their chin will be smaller.

    The same could apply to loss of hair. Once we were covered in hair, now not so much. That seems to be caused by evolution. The organism adapting to changing environments over time.

    Based on the types of foods we eat today, I bet that wee will end up toothless and hair less.
    My take on the protruding chin, to save your teeth if you fall on your face…



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  • Hi LaurieB,

    Thank you for those responses. I agree that his extract is the most troublesome. Given that the dentists were obviously conducting good science when they reported on their observations it seems likely that the problem lies with the Journalist at Science Daily.

    The dentists, assuming they continued to be the good scientists we see evidenced in the early part of the original story, would have linked there theories and observations to the existing body of peer-reviewed work.

    The dentists, when interviewed, would have assumed that the Jounalist would fact check their actual paper(s) for references. However, fact checking is one of those pesky time-consuming tasks that modern journalists can do without – as is apparent from the vast majority of their work.

    The more I think about it the more this seems to be another example of poor science reporting. That might be excusable in a tabloid given that – unlike the tabloids’ usual mode of operation – it is at least a story based on fact. But as this is Science Daily we deserve much better.

    Peace.



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  • 13
    Lorenzo says:

    The interest for the chin is, I suppose, because it is one of the most distinctive features of H. S. Sapiens. Not even H. S. Neanderthalensis had it.
    And, despite some razor manufacturers might strongly disagree, I doubt it is there just to provide an excuse for 254 blade razors that can shave you, skin you, strip your flesh away and peerlessly polish the bone underneath… All in one go!

    Anyhow, I’m not buying very readily the shrinking face after observing some mammals carrying a shrinking face mutation, namely bulldogs and persian cats: while it is true their face shrunk, their lower jaw did not. I’d bet on sexual selection, here.



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  • My take on the protruding chin, to save your teeth if you fall on your face…

    Along similar lines, I thought that any extra bone in the face or head area would serve as a welcome buttress against the disastrous effect of a well placed punch. Those facial bones are fragile. Jaws seem to break easily and a little extra bone on the chin might mean that you’ve still got a few teeth left in their sockets when you pick yourself up off the ground. But then I thought, if a rugged chin and mandible to go with it is so valuable for this purpose then why don’t you guys have those big brow ridges to go with the beefed up chin? Gain a chin and lose the brow ridges. Now I’m back to a whole lot of nothing with this.



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  • Alas, poor Yorick! I knew him, Horatio: a fellow
    of infinite jest, of most excellent fancy: he hath
    borne me on his back a thousand times; and now, how
    abhorred in my imagination it is! my gorge rims at
    it. Here hung those lips that I have kissed I know
    not how oft. Where be your gibes now? your
    gambols? your songs? your flashes of merriment,
    that were wont to set the table on a roar? Not one
    now, to mock your own grinning? quite chap-fallen?
    Now get you to my lady’s chamber, and tell her, let
    her paint an inch thick, to this favour she must
    come; make her laugh at that.



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  • Phil Rimmer seem to have the answer. It’s a place to put your hand when you’re going all pensive and existentialist. Compulsive beard stroking would be equally acceptable.

    I go with development of language and larynx. Or just some random mutation that caught on. We all come from roughly the same ancestors, yes?



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  • Obzen,

    Phil Rimmer seem to have the answer. It’s a place to put your hand when you’re going all pensive and existentialist.

    I congratulate you sir. With the evidence in front of us pretty much every day and you’re the one who put it all together in a cohesive theory. 😀



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  • There is a sadly neglected sub-species which dwells among the male members of the upper echelons of British society: namely, the “chinless wonder”.

    However, it has always fascinated me to observe that the females among the group possess prominent chins.

    I have long considered these to be adaptations which allow them to lock together during mating, thus helping to assure successful procreation.

    They do certainly seem to be awfully large in number.



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  • 21
    aroundtown says:

    I like your assertion on whether males became more tranquil and personally I don’t think we have. Your post got me thinking about the chin in regard to aggression and two things came from it. I will post a link below on the subject, it was broached here on RDFRS once upon a time if my memory serves me correctly on the human hand and its evolution respective to aggression. When I think about this it seems like the chin might have been an effective evolutionary feature to preclude your teeth being knocked out in your social group.

    Secondly and somewhat obscurely I had watched a documentary called your inner fish (second segment called your inner primate actually) that proposed the concept of gene reconfiguring in the lower jaw and one such distinction was that part of the lower jaw became the bones of our inner ear. Additionally they discussed the fact that we primates only receive one set of teeth for our lifetime and that seemed like a potential contributing factor as well as regards protection. The transition and selection for performance is an interesting concept.

    http://www.medicalnewstoday.com/articles/254424.php



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