Re: Alex's gibbon-like CA
David Froehlich (eohippus@curly.cc.utexas.edu)
Sun, 12 Nov 1995 14:37:40 -0600
On Sun, 12 Nov 1995, Paul Crowley wrote:
> In article <482psm$s6e@news.ycc.yale.edu>
> bdiebold@minerva.cis.yale.edu "Benjamin H. Diebold" writes:
>
> > Paul Crowley (Paul@crowleyp.demon.co.uk) wrote:
> > [snip]
> > : occurred. Environmental change would have had no bearing on such
> > : small groups. I'm sure that PA thought emphasises the climatic change
> > : largely because of a complete dearth of better ideas.
> >
> > This seems an astonishing claim. Why on earth should you believe that
> > environmental change would have no bearing on small groups?
>
> My point is that billions of species have been created. Only a
> very small proportion could be thought to be the result of major
> climatic change.
This seems to me a pretty astonish claim in itself. How do you "know"
that only a small proportion are due to climate change? Do you have some
source of information unavailable to us mere mortals? ;-
> Yet that explanation is always the first to be
> pulled out of the hat - in particular for hominid speciation.
> So, in principle, we have to be suspicious whenever we see it.
> Almost invariably it's a case of "Well, I can't think of anything
> else so it must have been the result of a change in climate".
This seems to be just the problem you have "I cannot see how bipedalism
might have occured any other way, therefore AAS is correct" Is this a
comment on you ability? AAS? your knowledge?
> Secondly, I was referring to the period of hominid evolution when
> brains expanded and language developed. How many beneficial
> mutations did that involve? 10,000? Each of them had to spread
> throughout the whole population. So it had to be small and
> localized, or as I said: a lot of choke points. Major climatic
> change would have had no significant effect on this process. Even
> in the broadest sense, it is hard to see how any environmental
> change could have had a substantial influence.
Reading your posts I always seem to get the flavor "I cannot understand
it therefore it could not have happened" Or "This is the way the world
works and therefore it could not have happened that way".
Why does brain expansion require 10,000 benficial mutations, why not
one? There are plenty of control genes that limit or effect multiple
systems, why not a simple change? (I do not know, however, your statement
that it must have occured by multiple steps is no more scientific than my
statement that it only required a single change). Furthermore, why is
mutation required? If there is variation in the population already then
that is all that is required for differential reproductive success and
thus evolution. If that success is mediated by climate then why not
describe the ultimate reason for the change as climate?
Let me give you an example of recent ideas of how a single change can
effect multiple systems. The synapsid lineage has the post-dentary bones
attached to the lower jaw. Their descendants, the mammals have these
post-dentary bones co-opted for use in the ear (completely unattached to
the lower jaw). It was always though that there must have been a massive
reorientation of all of these bones (and the standard critiscism leveled
was "what do the use to chew while their jaws are being reorganized").
However, it has recently been demonstrated that what actually happens is
a developmental shift. Primitively the brain was small and did not
continue to grow throughout development so the post dentary bones wound
up being the back of the jaw at the same time as they were used for
hearing (Hubey, FYI, we have the fossils, this is not just
"bone-gazing"). In Mammals, the brain has expanded markedly, and there
is a developmental shift that continues brain development long after the
lower jaw has stopped growing. What is the effect of this? The
post-dentary bones get "ripped" off the back of the jaw and totally
incorporated into the ear (making the malleus and incus (hammer and
anvil). You can even demonstrate that it will occur in modern mammals.
They have delayed the brain development of possum embryos and where do
you think the malleus and incus end up? On theback of the jaw. A single
developmental shift caused a wide range of changes. Why is this
fundamentally impossible for humans?
David J. Froehlich Phone: 512-471-6088
Vertebrate Paleontology Laboratory Fax: 512-471-5973
J.J. Pickle Research Campus
The University of Texas, Austin, Texas 78712
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