Re: An alternative to ST and AAT
Rohinton Collins (rohinton@collins.prestel.co.uk)
22 Nov 1996 18:28:13 GMT
John Waters <jdwaters@dircon.co.uk> wrote in article
<01bbd6cb$5dfa23c0$1b2470c2@default>...
> 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?).
Either is correct.
> 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.
Are the newborn crania of A. Afarensis and the chimpanzee of a similar
size? If so it would be instructive to compare the pelves of these species
in order to deduce A. afarensis' brain growth pattern, both prenatal and
neonatal. Has anyone read any papers on this?
> 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.
The jaw would have little bearing on the size of the pelvic outlet. In all
hominoid species (including extinct taxa) the size of the newborn's jaw is
tiny in relation to the cranium, in comparison with that of an adult skull.
> 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.
I'm not sure whether you understood me here John. To quote my previous
post:
## 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. ##
Clear now?
> 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?
No problem John. A newborn ape can look after itself to a certain extent
shortly after birth. Of course it needs its mother for food, warmth and
protection, but it can move around by itself and begin to feed itself
within weeks (? any refs) of birth. This determines the kind of social
organisations we see in extant apes. Human newborns are totally helpless,
they rely on their parents for everything, including getting from one place
to another (at least until they can crawl) for many months, and it is years
before they can feed themselves. Such intensive care for a newborn human
requires a very different social organisation. So a hominid species whose
newborns were altricial as opposed to precocial (as seen in its ancestral
species), would be expected to adopt a different social organisation than
its ancestral species.
> 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.
This is because apes have precocious young.
> 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.
Since ape newborns are precocious, and human newborns altricial, the latter
being due to postnatal brain growth continuing at the prenatal rate,
reaching the ape newborn stage at about 1 year, I would expect intermediate
species (assuming the LCA to have an ape brain growth pattern -
parsimonious) to have a varying length in the postnatal, fast-growth phase,
ranging from a few weeks to 1 year (the present human condition). This
would be reflected in the way in which the young are reared, and whether a
long-term intensive care strategy would be required, as is for human young.
Hominids certainly did not make nests in trees. If early australopithecine
species had precocious young, because of their bipedal adaptations they
would not be able to grip branches (or their mum) with their feet, a
pre-requisite for arboreal-nesting primate. This would also be dependent
upon life history patterns - were they nomadic or did they have home bases?
If the latter is true, how long did they stay in one place? Long enough for
a newborn to grow beyond its 'helpless' stage?
> 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?
It is an irrelevant assumption. Anyway, punctuated equilibrium is more
important given a reasonably sized gene pool. The fossil record may show
some increase in adult brain size during the lifetime of a species, but we
only see a marked and definite increase after a speciation event.
> Your exposition implies a relationship between alfricial
> development and brain size, as far as the hominids are
> concerned. Is this a fair interpretation?
Yes. There is a physical limit - the size of the pelvic outlet - which
restricts the size of a newborn hominid's brain. Obviously the
biomechanical cost of further enlarging the pelvic outlet (in any altricial
hominid species) evolutionarily outweighed (and outweighs) the cost of
having 'helpless' young.
> 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.
This doesn't help your argument John. All this tells us is that chimpanzees
are elastic - as you would expect from a higher primate - when it comes to
foraging patterns and new environments. Oh and be careful, that 'can' makes
you sound like a statistician ;-) (If you aren't aware, statisticians are
famous for making data 'fit' their arguments, even if just by the
application of one three letter word - 'can')
> 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?
Of course. He would have to have been in order to colonise most of the Old
World.
> 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.
When did we stop talking about newborns and babies and start talking about
older infants? How is this relevant to our arguments? To whom are you
referring when you talk about '... to carry infants on their hips'?
Australopithecines? Apes? Modern humans? How is this relevant to our
arguments?
> > 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?
I said relatively over-developed John. Humans are hominoids, they are
cladistically grouped with apes. Some would go as far as to group hominids
and the African apes within a clade. Many palaeoanthropologists would say
that we are phylogenetically closer related to the chimpanzee than either
of us are to the gorilla.
Given the above, it is reasonable to remark that hominids, and modern
humans in particular, have a RELATIVELY over-developed brain in comparison
to the apes, being contemporaneous taxa.
Regards,
Roh
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