Straight Dope Message Board

[twickster]

I can understand why bringing the mouth forward into a muzzle would help a carnivore (dog, fox, bear) -- but why a more or less flat face with a chin?

[WhyNot]

The chin serves as an attachment point for several facial muscles. I'm not sure I would have included it if I were designing a species, but these things aren't well discussed in Research and Development, y'know...

[kidchameleon]

So we can use straws.

[Wee Bairn]

Without chins, millions of pillows would be without pillowcases.

[Squink]

You got to hang your teeth on something, which unless you're thinking of going Remora style, is going to be a jaw. Once you have a jaw, you need some sort of mechanism to keep it from breaking when you wham it into something hard. A chin'll do for that.

[twickster]

Originally Posted by Squink

You got to hang your teeth on something, which unless you're thinking of going Remora style, is going to be a jaw. Once you have a jaw, you need some sort of mechanism to keep it from breaking when you wham it into something hard. A chin'll do for that.

Crocodiles don't have chins.

[Squink]

Originally Posted by twickster

Crocodiles don't have chins.

Ah, but they do have the snouts, which you've already covered. Besides, what are the chances of a tetrapedal croc slipping on a banana, and bashing its chin against a rock? This upright stance we humans affect requires special attention to the strength of the jaw.

[CalMeacham]

Chins aren't needed for strong primate jaw muscles -- all other apes do quite well without them. Gorillas have a high bony riidge atop their skulls for muscle attachment, but chimps and other apes get along fine without any such long-range bases for muscles, and have quite powerful jaws, thank you very much.


I've long had a belief that this is another case of sexual signalling and duplication of sex organs.


really.


Humans also have everted lips, which other apes don't. We favor red lips on women and they often enhance this with lipstick and lip gloss, suggesting the vaginal lips. In a similar way, the human chin suggests the testicles. I find it significant that large chins are not thought attractive in women, but are in men, and that a "cleft" chin (which more closely approximates the double construction of the scrotum) is considered especially attractive in men.

I'm not alone in this. Desmond Morris has suggested it (and I know his name is greeted with derision, but the guy has the appropriate background in primate behavior, and makes some excellent suggestions, along with some absurdities), and one of his illustrated books reproduces a painting which makes the chin < -- > scrotum analogy perfectly.

[WhyNot]

Originally Posted by twickster

Crocodiles don't have chins.

They also have much longer levers working in those long jaws - good ol' Archimedes, wasn't it he who said with a lever long enough and somewhere to stand, he could move the Earth?

That being said, IIRC from anatomy class (which I may not), most of the hard working chewing muscles attach in other places along the jaw and the temple and cheekbone - the human replacement for the skull ridge that CalMeacham mentions in gorillas. The chin muscles are more for mouth dexterity and talking, which might be why gorillas don't have chins.

[Napier]

>a "cleft" chin (which more closely approximates the double construction of the scrotum) is considered especially attractive in men

Well, at least noses make more sense now.


As an alternative theory, consider that other apes have more of a snout than we do. Perhaps something about our facial development or diet favored less of a snout, particularly in the mid face, and while both our upper and lower teeth are further back today perhaps the point of the chin has had less evolutionary pressure to move back and so has done so slowly. After all, it's more in relation to our teeth and maxilla that chins seem to protrude more than in other animals, than it is in relation to our eyes and forheads.

[John Mace]

Originally Posted by CalMeacham

Chins aren't needed for strong primate jaw muscles -- all other apes do quite well without them. Gorillas have a high bony riidge atop their skulls for muscle attachment, but chimps and other apes get along fine without any such long-range bases for muscles, and have quite powerful jaws, thank you very much.

Not to mention that our most immediate ancestors didn't have chins either. Nor did our cousins, the Neanderthals. The chin appears to be unique to our species, even within the genus Homo.

[twickster]

Originally Posted by John Mace

Not to mention that our most immediate ancestors didn't have chins either. Nor did our cousins, the Neanderthals. The chin appears to be unique to our species, even within the genus Homo.

Okay -- so why do we have them?

[Lumpy]

I read that the prominence of our chin isn't due to the point of our jaw sticking out more, but that the rest of the jaw has receded compared to our ancestors.

[WF Tomba]

Originally Posted by twickster

Okay -- so why do we have them?

There might not be any reason. It could be the result of a "founder effect" or a genetic side effect of some other trait. Do we even know what genes cause a chin?

[John Mace]

Originally Posted by twickster

Okay -- so why do we have them?

There is no factual answer to that question. As others have said, it could be sexual selection, a founder effect, or dumb luck. Not every trait has some purpose-- as long as it doesn't reduce reproductive success, it won't necessarily be selected against.

[Sal Ammoniac]

Originally Posted by John Mace

Not to mention that our most immediate ancestors didn't have chins either. Nor did our cousins, the Neanderthals. The chin appears to be unique to our species, even within the genus Homo.

Neanderthals didn't have chins? What did they have instead?

[CalMeacham]

Neanderthals didn't have chins? What did they have instead?

Their lower jaw slopes away without our pointed, right-angle chin. Apes don't have chins, either, as I pointed out above. Look at a chimp sometime.

[Sal Ammoniac]

In other words, they have receding chins, I suppose. I would still call that a chin, myself, but I guess you (and maybe the OP) are talking specifically about that thing that's called the mental protuberance.

Well, if I had to hazard a guess, I would say that the purpose of the chin is to protect your jaw from being shattered when your four-year-old unexpectedly jumps up while you're hovering over her. That's what mine is for, anyway.

[John Mace]

Originally Posted by Sal Ammoniac

Neanderthals didn't have chins? What did they have instead?

A lower jaw w/o a chin. Diagran showing Neanderthal vs modern Human skulls.

[smiling bandit]

Dare I say the entire question is rather a mountain out of a molehill (almost literally)? The distinction isn't hugely pronounced, and may well be a trivial point of genetic history, of course, but even if it had meaning it's likely not a huge factor.

Does the placement of the chin have some efect on the lips and their muscles? This could have some small implication for speech?

[Dunderman]

Originally Posted by John Mace

A lower jaw w/o a chin. Diagran showing Neanderthal vs modern Human skulls.

The chin part shows a rather small difference. What definition of "chin" are we working with here? I would certainly say chimps have chins, for example, but I'm clearly in the minority.

[twickster]

Originally Posted by Priceguy

The chin part shows a rather small difference. What definition of "chin" are we working with here? I would certainly say chimps have chins, for example, but I'm clearly in the minority.

Looks somewhere between a chin and a muzzle, to me.

[Dunderman]

Originally Posted by twickster

Looks somewhere between a chin and a muzzle, to me.

Looks like a chin, to me. What is a chin anyway?

[John Mace]

Originally Posted by Priceguy

The chin part shows a rather small difference. What definition of "chin" are we working with here? I would certainly say chimps have chins, for example, but I'm clearly in the minority.

The definition the the lower jaw protrudes outward, not back inwards. It's a key indicator for our species, and if you didn't have one you might very well visit a plastic surgeon to get one.

[Sal Ammoniac]

Right, and make sure you get the model that makes your face look like it's sprouted a pair of testicles!

[Darwin's Finch]

Anatomically, what is being referred to here is the "mental protuberance" of the mandible.

And, if George B. Schaller is to be believed,

The mental protuberance...evolved as a purely cosmetic feature that serves as a "badge of recognition and as a lure and stimulant to mating. In males, particularly, the pointed chin also accentuates gestures of defiance, and in females lends eloquence to expressions of haughtiness or petulance."

I don't buy it, however. This is one of the main reasons why I tend to stay far away from human evolution questions: too many "just so" stories with little actual evidence to back them up.

[Spoke]

Originally Posted by twickster

Okay -- so why do we have them?

Secondary sex characteristic? Women like the chiseled-jaw types, don't they?

[mikews99]

Originally Posted by CalMeacham

In a similar way, the human chin suggests the testicles.

Then why do men have beards? Male facial hair very effectively covers the chin and effectively hides any details of it's structure (and please, no jokes about hairy balls). We developed beards long before we started shaving, so the idea of prominent chin developing because it became a secondary sexual characteristic is fairly weak. I do agree that there is some sexual signaling from the chin (mainly from the enhancement of the cheekbones and chin from the testosterone surge in male puberty), but this doesn't provide a satisfactory explanation as to the evolutionary development of the chin.

The most often overlooked reason for many of our seemingly superfluous physical traits is that they're needed to support our greater intelligence. A prominent chin gives more space for the tongue muscle to root, thus giving it finer control and the ability to make a greater variation of sounds -- in other words, language. A chimpanzee can basically scream softly or scream loudly but they really can't create phonemes. There are some human languages with over 80 different phonemes and these are almost all controlled by the tongue (only a comparative handful are controlled by the lips). Without that extra space in the jaw, our tongue muscle would have a much coarser movement and consequently we wouldn't have the ability to produce as many phonemes and our ability to communicate via language would be severly reduced.

[CalMeacham]

Then why do men have beards? Male facial hair very effectively covers the chin and effectively hides any details of it's structure (and please, no jokes about hairy balls).

No jokes needed - the analogy is pretty good, especially if mating takes place when young.







And not everybody has extensive beard growth -- look at the American Indians. We don't know what the hair pattern in early man was.


To me it's a perfectly satisfactory exsplanation for the development of the chin.


"Greater Intelligence", on the other hand, doesn't satisfy me at all.

[ultrafilter]

Originally Posted by spoke-

Secondary sex characteristic? Women like the chiseled-jaw types, don't they?

Selection on secondary sex characteristics can explain why sex-specific differences become more pronounced, but is it sufficient to explain why they showed up in the first place? I doubt it.

[racer72]

There are some folks, myself included, that can honestly say we were shortchanged in the chin department. My sole reason for having a beard is to hide the fact I don't have a much of a chin.

[mikews99]

Originally Posted by CalMeacham

To me it's a perfectly satisfactory exsplanation for the development of the chin. "Greater Intelligence", on the other hand, doesn't satisfy me at all.

Then what would be the indicator of greater fitness to a female Homo antecessor that they would select a male with a bigger chin? To me, the result precedes the effect, which means she wouldn't make the selection because any deviation from the mean looks less attractive (less healthy). Whereas if jaw development was a mutation that allowed greater communication skills which directly increased survivability, that would be intrinsically self selecting: "Well, he's got a funny looking jaw, but damn, he's outlived his brothers. I think I'll mate with him."

[Civil Guy]

...The chin probably came along before Norelco, so I'd have to say that it sounds like a chin is a good place to hang a beard - which for us relatively hairless apes, is itself sort of a defining sexual characteristic. Yes, we have sort of messed around with that one, but it's also true that a clean-shaven man makes another kind of statement about his socio-economic status - generally, a good statement.

I'd also join in the speculation that a long jaw allows for a well-anchored tongue suitable for talking - but in thinking about it, it doesn't seem likely that the human tongue is much more agile than other mammalian tongues.

Or, as others have said, maybe there is no good reason.

[mikews99]

Originally Posted by Civil Guy

it doesn't seem likely that the human tongue is much more agile than other mammalian tongues.

Agility and precision are two different things. A giraffe's tongue is extremely agile, which helps it strip leaves. A human tongue can make very precise, small movements, whereas most other primates can't.

[mikews99]

Originally Posted by racer72

My sole reason for having a beard is to hide the fact I don't have a much of a chin.

And that, my friend, is precisely the reason men have evolved the ability to grow facial hair: Anthony Edwards, Anthony Edwards with facial hair.

[Darwin's Finch]

Originally Posted by mikews99

Whereas if jaw development was a mutation that allowed greater communication skills which directly increased survivability, that would be intrinsically self selecting: "Well, he's got a funny looking jaw, but damn, he's outlived his brothers. I think I'll mate with him."

Unfortunately for your theory, the mental protuberance has nothing to do with tongue attachments or communication: it's on the outside of the mandible! Meaning, it's either for exterior jaw muscle attachments, or it's cosmetic, or it's just a neutral mutation that has become more or less fixed. But what it is almost certainly not is "related to language".

[twickster]

Originally Posted by Darwin's Finch

... it's either for exterior jaw muscle attachments, or it's cosmetic, or it's just a neutral mutation that has become more or less fixed.

I'll take that as as much of an answer as is possible -- thanks to you, and to everyone else who's taken a whack at it. Interesting stuff!

[Schuyler]

I think it's possible, based on comparing the pictures of humans, Neanderthals, and chimps, that the chin in humans isn't something that exists in us but not in other primates, but rather that it's jaw growth that hasn't shrunk in size at the same rate that our teeth (and their supporting bony structure) have gotten smaller. If I imagine a chimp with much smaller teeth, and whose front teeth are more nearly vertical, but which retains the same jaw length, then the chimp would start to have a pronounced chin as well. Or not.

[WF Tomba]

Originally Posted by Schuyler

I think it's possible, based on comparing the pictures of humans, Neanderthals, and chimps, that the chin in humans isn't something that exists in us but not in other primates, but rather that it's jaw growth that hasn't shrunk in size at the same rate that our teeth (and their supporting bony structure) have gotten smaller. If I imagine a chimp with much smaller teeth, and whose front teeth are more nearly vertical, but which retains the same jaw length, then the chimp would start to have a pronounced chin as well. Or not.

This is an area where genome research on living organisms can answer questions about the past. With enough study, we could presumably figure out what genetic difference leads to this difference in the shape of human and chimp jaws, and then determine whether the expression of that genetic difference leads to human chins as a side effect or whether there is actually a separate "chin gene". Until recently, theories about the evolution of particular traits had to be made in total ignorance of the specific mechanisms linking genes to expressed traits; not surprisingly, this resulted in a lot of speculative just-so stories and little certainty.

[ultrafilter]

Originally Posted by Schuyler

I think it's possible, based on comparing the pictures of humans, Neanderthals, and chimps, that the chin in humans isn't something that exists in us but not in other primates, but rather that it's jaw growth that hasn't shrunk in size at the same rate that our teeth (and their supporting bony structure) have gotten smaller. If I imagine a chimp with much smaller teeth, and whose front teeth are more nearly vertical, but which retains the same jaw length, then the chimp would start to have a pronounced chin as well. Or not.

Take a look at the image John Mace linked to in post #19. Modern humans and neanderthals have differently shaped jawbones, and that's why we have chins and they didn't (or at least not to the same degree).

[KneadToKnow]

Originally Posted by racer72

My sole reason for having a beard is to hide the fact I don't have a much of a chin.

I can sympathize. It wasn't until I had shaved my full beard down to a circle beard 3 weeks ago that I remembered that one of the reasons I decided to grow a beard was that I don't have much in the way of a jawline any more.

Needless to say, I now have about 3 weeks worth of full beard growth.

[Waenara]

I googled around and managed to find this incredibly detailed paper (43-page, 6.6MB PDF!) titled The Human "Chin", with a detailed analysis of the variation in chins in humans and Pleistocene hominids (including Neandertals).

I don't have time right now to read it in depth, but basically I came away with the idea that the "chin" is not a well-defined technical term, and is often used to describe different morphological characteristics. There is a broad range of normal variation in human chins, and the characteristics of human or hominid "chins" can change as they mature (juveniles to adults).

Starting on page 36:

Morphologically, the primary implication of our observations is that a bulging symphyseal region is not equivalent to a chin. When humans and elephants can both be described as having chins (e.g., Enlow, 1982), it is probably time to reconsider the applicability of the term. Consequently, we recommend that the term ‘‘chin’’ no longer be used in discussions of mandibular morphology and its phylogenetic and systematic significance. If, however, one feels the need to retain this term, it should be restricted in usage only to extant H. sapiens and those fossils displaying the constellation of symphyseal features of this species. Since it is obvious that bulges and swellings as well as subalveolar depressions of differing degrees of expression and different morphologies have come to adorn the symphyseal regions of a diversity of hominids, we further recommend that the terms (and the synonyms of) ‘‘mentum osseum,’’ ‘‘incurvatio mandibularis,’’ ‘‘tuberculum laterale,’’ and even ‘‘trigonum mentale’’ also be limited, or even dropped from usage. As anatomical terms, they are used to refer both to sometimes superficially similar and to sometimes markedly different structures in a diversity of adult specimens that may have arisen from totally different juvenile configurations. As for children, the sometime presence of a gomphotic scar especially low down along the midline of the mandible is a naturally occurring result of symphyseal fusion. Unless it is distinctly raised and part of an everted, inverted ‘‘T’’-shaped structure that is associated with mental fossae, it is not a ‘‘hint’’ or a ‘‘trace’’ of a mental trigon as seen in H. sapiens. Thus, these terms have become charged with a scenario of ‘‘chin’’ evolution which should itself be discarded.

Throughout this paper, we have attempted to keep our description of morphology apart from a phylogenetic and, especially, functional interpretation of it. We have, however, stressed the importance of understanding the morphology of the adult by starting first with an appreciation of the details of juveniles precisely because this is the most fundamental way in which to appreciate the similarities or differences that exist among individuals and between the average morphologies of taxa.

Basically, however, the facts are that H. sapiens has a distinctive symphyseal configuration from its ontogenetic outset and that many other hominids, especially Neanderthals, do not share any version of these apomorphies. From this it would seem reasonable to conclude that Neanderthals and those specimens that are morphologically similar to them in symphyseal morphology are not members of the species H. sapiens. But if one was committed to the interpretation that there was only a single species, which encompassed a diversity of morphologically distinctive specimens— such as all or most of those we discuss here—then one would be obliged to explain how members of the same species could have come to be morphologically different from one another. We, however, are trying to understand as best we can what the details of difference and similarity are from the beginning. Only afterward can we bring this information to bear on our phylogenetic and systematic hypotheses, and, if we so choose, to offer explanations for how and even perhaps why certain features came to be the way they are. Our appreciation of development also differs significantly from the more prevalent one in the literature: essentially, if development is taken into consideration at all, it is done secondarily, after specimens of adult individuals have been sorted into a Haeckelian scale naturae that is regarded as reflecting a transformation sequence. This license is prevalent in paleoanthropology, as is indicated in the oft-published diagrams of fossil adult crania arranged in a presumed phylogenetic sequence.

Clearly symphyseal morphology, especially as considered within a developmental framework, lends support to previous suggestions of notable taxic diversity within the genus Homo (e.g., Howell, 1994; Santa Luca, 1978; Schwartz & Tattersall, 1996b; Tattersall, 1986; Zollikofer et al., 1995): a diversity that is at present substantially underestimated.

[furryman]

Originally Posted by WF Tomba

There might not be any reason. It could be the result of a "founder effect" or a genetic side effect of some other trait. Do we even know what genes cause a chin?

The "Jay Leno/Quentin Tarintino" genes.