Re: Death of a hypothesis
Elaine Morgan (elaine@desco.demon.co.uk)
Thu, 4 Jul 1996 13:24:01 GMT
In article <4q0krp$nnc@maze.dpo.uab.edu> karen@uab.edu wrote...
> Elaine Morgan <elaine@desco.demon.co.uk> wrote:
>
> >:In article <DsG080.DC6@inter.NL.net> ghanenbu@inter.NL.net wrote...
>
> >:> karen@uab.edu (Karen Harper) wrote:
> >:>
>
> >:> Maybe the human chin is not an adaptive feature at all,but simply the result of
> >:> a difference in rate of shortening between the base of the corpus of the
> >:> mandible and the toothbearing pars alveolaris during evolution.
> >:> In that sense the human chin has not enlarged,but became prominent because it
> >:> hasn't shortened at the same rate as the alveolar part.
> >:>
> >:This sounds reasonable. Only thing that worries me is people say the
> >:same thing about the nose;
> it wasn't that our nose started to stick
> >:out, but everything around it shrank back. And I don't buy that one.
> >:I believe the stickingoutness was functional. Otherwise we would not
> >:have evolved a nasal spine - a skeletal adaptation to support it and
> >:ensure that it continued to stick out. No other primate AFAIK has a
> >:nasal spine. The chin and the nasal spine do not predate Homo proper -
> >:around the time of habilis I think. But it is probable that some
> >:soft-tissue development (analagous to proboscis in several species?)
> >:had begun earlier; or else there would have been nothing for the nasal spine
> >:to support.
> >:>
> >:Elaine
>
. The nasal
> spine is actually part of what makes up the palatine bone that is
> inside the skull. Maybe you are confusing it with the nasal bones.
Quite right. I was.
>
> I am sorry that you have misunderstood the concept of how the changes
> in facial and cranial morphology taking place would effect other areas
> of the face and cranium. I will attempt to explain it in a way that
> is hopefully more understandable.. T he projection of the mandible in
> humans probably has several factors that lead to its' shape. There
> are a lot of muscles that are attached to the human mandible and some
> of these are major muscle attachments that originate in the area of
> the chin, some on the ventral region and some on the anterior region.
> The ones on the ventral region deal mostly with the movements of the
> lower lip. The muscles on the anterior region have functions that
> deal with the hyoid and neck. Some of the muscle attachments to the
> "chin" area of the mandible are rather large.
Compared to what? Larger than in apes? If so, why? We would need less
hefty muscles controlling orientation with neck muscles because our
heads balamce so neatly, The only item on your list that really
imoresses me is control of the lower lip, because of the needs of
speech to refine its movements.
It is likely that these
> muscles play a role in the size of the chin in humans. But it is
> probably not that simple. There are many things to be considered.
> Human teeth have gotten smaller and smaller throughout our evolution .
> This would be one of the factors that would make the mandible appear
> larger.
It doesn't only appear larger. It is larger, surely?
If you set up a row of skulls from the earliest fossil
> hominids and put them in a chronological sequence, you would see that
> over time, the entire facial skeleton shifted from a slant like this:\
> (in profile) to a nearly vertical position like this: l . The
> mandible hasn't changed as much as some other features on the face and
> yet it appears more prominent because of the change in its' position.
> Cultural factors such as the food we eat play a role in tooth size and
> morphology. These affect the morphology of the mandible which holds
> half of them. In the mammalian lineages, it is quite common for the
> mandible to undergo change during times of masticatory transformation.
Yes, but these changes do not include a chin.
> For example, the fusion of the symphysis of the mandible. Various
> species have undergone this change at times of masticatory change.
> The fusion of the mandibular symphysis in primates could be considered
> a more significant change in morphology than the projection of the
> mandible. But it happens because it is more adaptive in support,
> muscle attaachments, etc. So there are a number of factors that figure
> into the present morphology of human mandibles.
Yes, I can see that's multifactorial. But among the "number of factors"
how many are there that would not apply to an ape? And how do you sort
out cause from effect? You say that the shift to a vertical position may
have been a factor in producing the chin. I coulod argue that the
evolution of the chin was a major factor in making the profile look
more vertical.
> Also, the nasal spine is found in the other primates too and doesn't
> have the function of supporting a projecting nose. Nasion morphology
> is fairly clearly affected by the environment that populations have
> lived and evolved in for long periods of time. A projecting nose
> probably has several functions and is not a result of a lone factor,
> just as most phenotypical traits usually are resultant from more than
> one factor.
> Longer nasal passages help regulate air temperatures that are
> breathed in and I am not positing that this was a feature that was
> adaptive enough to cause the human nose to project.
It's not a bad idea though. Bipedalism and the descended larynx
had seriously eroded the capacity to regulate the temperature of the
air reaching the lungs. Maybe you've got something there.
Another
> function of the projecting nose is that it helps hold spectacles to
> our eyes. I don't think that is the reason we have a projecting nose
> either. But it is functional for that. My point is that the
> projecting nose may have many functions but that does not mean that it
> projects because it was selected for for any of those functions that
> are extant. Keep in mind that many evolutionary changes are
> non-adaptive. Not every single trait that humans or any other animal
> have are a result of some adaptive benefit. Many are simply a result
> of other evolutionary changes that WERE adaptive. Some genes affect
> more than one trait.
Yes, I know about that. I do not claim that all our species-specific
traits are adaptive. \I would say it's always worth exploring the
possibility that they are. The only thing I strongly claim is that
traits which are maladaptive would have been wiped out
unless there was some good reason. It is too easy to write them all
off as conqequences of linked genes which have never been identifie
protecting them from some other putative harmful agent which has likewise
not been identified.
Elaine.
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