Re: An alternative to ST and AAT
Rohinton Collins (rohinton@collins.prestel.co.uk)
13 Nov 1996 00:04:11 GMT
John Waters <jdwaters@dircon.co.uk> wrote in article
<01bbcf19$51756240$2d2270c2@default>...
> Rohinton Collins <rohinton@collins.prestel.co.uk> wrote in
> article <01bbcdb9$208a50c0$424698c2@dan-pc>...
> > John Waters <jdwaters@dircon.co.uk> wrote in article
> > <01bbcd2f$8f445e00$3a2270c2@default>...
[snip]
> This morphology enables bipedalism, but is
> not as efficient for long distance walking. In this sense,
> the australopithicene bipedalism was less efficient than
> modern Hss.
This is a point of contention; opinion is by no means unanimous.
> >Australopithecine babies were likely no less capricious than chimpanzee
> >babies, as the level of encephalisation was not dramatic in these
species. If
> >you do not know why human babies are born helpless and therefore cannot
> >gauge when this condition may have emerged in hominid evolution, please
> >say and I will explain.
> JW: Please do.
Fair enough.
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)
> Of course, the primary reason for infantile overheating may
> have been disease. A viral infection could lead to a fever.
> But whereas a chimpanzee baby would survive because it
> could dissipate this extra heat, an early australopithicene
> baby carried in its mother's arms could not. Infantile
> diseases of this kind are quite common.
This is where your theory falls down in a major way John. On my above
logic, an australopithecine baby would have been as precocious as a chimp
baby, and therefore would not be carried any more in its mother's arms than
the chimp would be. Perhaps if we were talking H. habilis...
But aside from this your theory is illogical with no precedence. After all,
how many other animals do you know off that have lost their fur due to
being carried by their mother? The kangaroo, according to your theory,
should be hairless. And conversely, how many other animals wear clothes?
Regards,
Roh
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