Re: An alternative to ST and AAT
John Waters (jdwaters@dircon.co.uk)
20 Nov 1996 10:14:04 GMT
Rohinton Collins <rohinton@collins.prestel.co.uk> wrote in
article <01bbd0f4$a5d7c6c0$LocalHost@dan-pc>...
> John Waters <jdwaters@dircon.co.uk> wrote in article
> <01bbcf19$51756240$2d2270c2@default>...
>
> The human brain is 3 times bigger than an ape's brain
would be, given the
> same body size. The need to grow such a large brain has
distorted several
> basic life-history characteristics. For instance, the
adult ape brain is
> about 2.3 times bigger than the brain of a neonate; in
humans the
> difference is 3.5 times. More dramatic, however, is the
size of the human
> neonate compared with ape newborns. Even though humans
are of similar body
> size to apes and have a similar gestation period, human
neonates are
> approximately twice as big and have brains twice as big
as ape newborns. In
> apes, who have precocial young, brain growth proceeds
rapidly until birth,
> whereupon a slower phase ensues for about a year. In
humans, the prenatal
> phase of rapid brain growth continues until well after
birth, a pattern
> that is seen in all altricial species. This rapid
postnatal phase continues
> for a further 12 months. The effect is to give humans the
equivalent of a
> 21 month gestation period (9 months in-utero, 12 months
outside). This
> occurs because of the size limitation of the pelvic
outlet. One important
> consequence of this is that human infants are far more
helpless and for a
> much longer time than the young of the great apes.
>
> It is theoretically possible that no change in infant
care would be
> needed until after the adult human brain size exceeded
873cc, which is the
> transition size between H. habilis and H. erectus:
Suppose that hominids
> had been able to make all the other changes in foetal
development -
> speeding up body and brain growth - but then reverted to
the basic primate
> pattern in the neonate. This pattern would have allowed
for an adult brain
> size of 873cc since the brain size of human infants is
384cc (384 x 2.3).
> This theoretical calculation depends on the assumptions
that the birth
> canal in the pelvis of H. habilis or early H. erectus
females would be able
> to accommodate a neonate's head the size of a modern
infant's. From the
> fossil evidence available thus far, it is clear that the
hominid birth
> canal was smaller than the modern female's at this point
in our history.
> Which means that a shift to humanlike postnatal brain
growth patterns would
> have had to have occurred already in H. habilis,
presumably with the
> concomitant impact on social organisation. The
australopithecines, having
> an adult brain size of 400cc would have comfortably fit
into the ape
> pattern of neonatal/postnatal brain growth and would have
therefore also
> had precocious young.
>
> (Refs: Roger Lewin "Human Evolution"; figures by Robert
Martin)
JW: Roh, let me congratulate you on the above exposition,
which you had little time to research. It is amazing how
many people forget about the importance of the pelvic
outlet. (I usually call this the Pelvic Canal. Is my
nomenclature incorrect here?).
The pelvic canal of A. afarensis is (relatively speaking)
as wide as that of a modern female. However, it is much
narrower from front to rear. Because of this, a number of
authorities believe that the alfricial developments in the
hominid species were already present in these
Australopithicenes. I accept that you disagree with this,
and you put the development in the H. habilis period.
In common with many people, you also seem to believe that
the size of the brain is the only factor to be considered
in these matters. Of course the brain size is important,
but it is only part of the head; and it is the whole of the
infant's head which has to pass through the pelvic canal.
In this regard, it should be noted that most authorities
consider than the early hominids were megadonts. So the
size of the jaw is important. Last but not least, the shape
of the skull may have some bearing on ease of birth.
One final point, you state that the hominoids have
preconscious young from birth. Primatological evidence
suggests otherwise. Apparently most apes give birth to
alfricial young. The newborns are only alfricial for a
short time. 24 hours is a typical maximum. Then they become
preconscious. So the human species is not fundamentally
different, it's just that human babies are alfricial for a
longer time than ape babies.
In your last but one sentance, you use the phrase _
presumably with the
concomitant impact on social organisation._ Would you care
to expand on that please?
The alfricial young of hominid species raises questions
about likely maternal behaviour. In this context, most
species with alfricial young secure them in nests, either
in trees or in underground burrows. The Apes do not do
this. Instead, the nursing females wait in their birthing
place until the ape infant has entered its preconscious
stage.
So the question arises as to whether the hominid females
would build nests in trees or burrows in the ground. Or
would they follow the pre-established hominoidal pattern of
simply waiting for the infants to move into the
preconscious stage? What do you think, Roh.
I take the view that evolution tends to be a relatively
slow process. Because of this I would expect any increase
in alfricial development to be quite small. For example, if
the anthropoidal maximum of alfricial development is 24
hours, the first hominid extension would be no more than
four hours. Is this a reasonable assumption?
Your exposition implies a relationship between alfricial
development and brain size, as far as the hominids are
concerned. Is this a fair interpretation?
> To begin with, you make it sound as if the in fact was
carried by the
> mother most of the time. I think that if you study chimp
infants (the
> comparison may be made if we assume an equivalent level
of precociousness)
> you will find that they spend very little time being
carried by their
> mothers. Even if the australopithecines were more
nomadic, they would have
> stayed in the same area for weeks or even months,
precluding any special
> requirement for infants to be carried at these times. And
so what if they
> were carried, or if they hung on to their mother's neck
(or both)? What is
> wrong with proximity to the mothers body? This time
please just show a
> clear flow of logic.
JW: I am not saying there is anything wrong with proximity
to the mother's body. Quite the reverse. It seems to me
that it would be vital from the point of view of good
balance and locomotary dynamics that the infant should be
carried as close to the female body as possible.
When I use the phrase_being continually carried by the
mother_, this is not meant to imply that mother's carry
their infants all the time. However, when mothers are
traversing a relatively long distance, then a young baby
will be carried continually by its mother.
Jane Goodalls studies in Gombe show that the daily travel
range is relatively small, particularly for females.
However, in more arid regions such as Senegal, the daily
range can be much greater. Furthermore, the range tends to
increase when food is scarce, as you would expect.
Although the early hominids may have had a similar pattern
of daily ranging as Chimpanzees, most authorities seem to
consider that H. erectus was much more nomadic. Do you
agree with this?
> > This would
> > favour carriage on the hips, as far as older infants
were
> > concerned.
>
> Rubbish. Have YOU ever tried walking with a baby on your
hip? It is much
> easier to carry them in your arms or on your back -
closer to the midline.
JW: In a word, no. But I wasn't talking of babies. If you
read the sentence carefully, you will see I was referring
to older infants. Contrary to what you seem to believe, it
is quite common for mothers (and sisters) to carry infants
on their hips. Of course, very young babies are usually
carried in a mother's arms.
> >
> > JW: Oh Roh. I would have expected more from a Bsc.
>
> Meaningless, look at Milford Wolpoff.
JW: I'd rather not, thank you. Is this what is known as an
*in* joke?
>
> I was trying to point out how fallacious your argument
is. You base
> assumption upon assumption. That is no way to build
hypotheses. No other
> animal has such an relatively over-developed brain (snip)
JW: Over-developed brain? How do you know it is
over-developed? Or is this an assumption?
John.
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