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.
Looks like a chin, to me. What is a chin anyway?
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.
Right, and make sure you get the model that makes your face look like it’s sprouted a pair of testicles!
Anatomically, what is being referred to here is the “mental protuberance” of the mandible.
And, if George B. Schaller is to be believed,
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.
Secondary sex characteristic? Women like the chiseled-jaw types, don’t they?
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.
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.
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.
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.
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.”
…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.
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.
And that, my friend, is precisely the reason men have evolved the ability to grow facial hair: Anthony Edwards, Anthony Edwards with facial hair.
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”.
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!
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.
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).