Re: AAT Theory

Elaine Morgan (
Sun, 03 Sep 1995 11:28:24 GMT

Reply to Alex Duncan: Afarensis/ larynx.

I promised to get back to you after I'd read Dean's "Comparative
Myology of the Hominid Cranial Base" which you recommended to me.

This analysis is based on the presence, in the skulls of modern
anthropoids and humans and eight different hominids, of bony
ridges and processes indicating where various muscles are/were
attached. So it represents a kind of interface between skeletal anatomy
and soft-tissue anatomy. This is clearly a more solid and practical
approach to examining the evolution of the upper respiratory
tract than deductions drawn from basicranial flexion alone.

Without claiming to have a total grasp of the detailed 3-dimensional
description of the muscles that is presented here, I would accept that
any conclusions reached in this study would have to be taken very
seriously indeed. After reading the first couple of pages I was
preparing to have to do a climb-down on this issue as I had to do on the
ecrine glands of the patas monkey, if the evidence suggested I must be

What the paper concludes is that that there are some primitive
features in the musculature of the upper pharyngeal region in A.
africanus that are closer to thwe physiology of the apes tan to that of
modern humans or any of the later hominids.

Thius suggests that some of the changes in this respect between apes
and humans did not take place until later than A africanus, hence
appreciably later than the proposed aquatic period, and therefore
cannot have been caused by it.

You appear to make the assumption that one of these changes was the
descent of the larynx. But Dean does not say this, and as far as I can
make out does not imply this.

Some of the differences seem to be a consequence of the contined
forward migration of the foramen magnum - i.e. they are a result of
bipedalism. Others are ascribed to differences in "the weight of the
skull in front of the axis of rotation of the occipital condyles".
e.g. the fact that robust hominids have a more massive face, despite
less obvious prognathism, than gracile ones and that alters the task to
be performed by the muscles. Some changes may have resulted indirectly
by changes in diet and dentition.

In short, I can see nothing at all that relates to the descent of the
larynx or helps us to guess when it took place. If I am missing
something please tell me. Otherwise I conclude that no climb-down
is necessary, and the idea that the descent of the larynx may have
had an aquatic origin remains on the cards.