Re: Bipedalism and theorizing... was Re: Morgan and creationists
Paul Crowley (Paul@crowleyp.demon.co.uk)
Sat, 27 Jul 96 22:25:57 GMT
In article <Dv5rL2.C9x@inter.NL.net>
ghanenbu@inter.nl.net "Gerrit Hanenburg" writes:
> >In spite of the fear inspired in them by modern hss, chimps have
> >snatched babies in the Gombe area in recent times.
>
> Goodall reports only two cases prior to her arrival as a researcher at
> Gombe.That's not significant number from the point of view of
> population dynamics.
The significance is that they will snatch Hss babies given the
chance. Chimpanzees are now so few and so well segregated, and
they are so (justifiably) afraid of modern Hss, that occurrences
will be rare. Such conditions would not have applied 5 mya WRT
hominids.
> If the infant has a delayed development of the neuromuscular system it
> is likely that it cannot hold on to its mother shortly after birth.
> Simply stating that human infants take longer to learn certain
> motorbehaviours only touches the surface.
As I said, you've got cause and effect reversed. Early neuromuscular
development would be disadvantageous to Hss infants in the same way
as (in my scenario) it was to early hominid infants. Once the mother
puts them down, it suits both adults and infants if they're immobile.
> It may have been impossible because its brain was not ready for it at
> an early age.Nevertheless this disadvantage was apparently more than
> compensated for by the advantages of a big brain later in
> life,otherwise natural selection would not have let it pass.
This argument, although traditional, is without merit. The amount
of brain power needed to enable the grasping reflex is tiny -
numerous small primates possess it; even Hss infants have it for
a few hours after birth. Not a great amount of brain power is
needed even for advanced muscular control, as all the precocial
bovids show. That secondary altriciality is a necessary pre-
condition for developing a big brain is a paleoanthropological
myth. It's an unexamined assumption.
The primary reason for Hss altriciality is that it was and is
selectively advantageous - the mother has no natural means for
carrying the infant beyond short distances and she has to put it
down in a safe place. Altriciality ensures that it stays there.
Less altricial infants were, and are, selected against.
I have clear and simple account for the development of bipedalism
and altriciality (and for most other hominid features). They
were established at the point of hominid speciation, when the
niche required the putting down of the infant.
I would like to see your account. What exactly lead to the putting
down of the child? . . his extra large head? . . the descent
from the trees? . . the loss of grasping feet? Was there a
change of habitat or niche at the time? What were the benefits
and costs involved in this major behavioral modification?
(Remember it was a break with a 60 Myr (or more) history.)
> >You've picked a case that makes my point. Why were those bare
> >patches on Spindle's loins unusual? -- Because he was male.
>
> Nevertheless is shows that the infant holds on to the hair.It is
> possible that the sexual difference was selected for,since male
> chimpanzees do not normally carry infants.
No, it does not. If infants held onto hair, chimp mothers would
show bare patches where hair had been pulled out. They don't.
> >Do you really think that there is the remotest possibility that
> >Spindle would adopt a bipedal stance while carrying Mel? -- Or
> >decide to carry him while walking bipedally for a few miles?
>
> Not for a few miles,but that's because he is a modern knuckle-walking
> chimp,not an ancestral hominid.
We have a pretty good idea of the anatomy of ancestral hominids
(Lucy & Co) at a point where you maintain they were walking
bipedally with infants holding on in the ventral position, --
i.e. an upright Spindle with a Mel attached.
> But just turn the picture 90 deg. and imagine Spindle climbing up a
> treetrunk as they so often do while carrying an infant. Why should it
> suddenly become physically impossible for the infant to hold on?
I doubt this. Remember that chimp infants are good climbers when a
few months old. Spindle would lift Mel onto a branch at head height
and later give him a hand if necessary. Small infants probably stay
in the ventral position, but their mothers thighs would often be
horizontal and she can use her hands for additional support. Why
would she ever need to climb fast? Females don't hunt and a
threatening male would always be faster anyway.
> >All other primates use the ventral position and it was, of course,
> >the ancestral one. At the point of hominid speciation either one
> >of two things happened: (a) the proto-hominid female did what you
> >suggest; OR (b) the proto-hominid female first put the child down
> >and then straightened up her back. Which is most likely?
>
> If there are no physical objections,retention of the ancestral
> condition until secondary altriciality evolves.
The physical objections are overwhelming and, I have to say, in
this context the "evolution of secondary altriciality" sucks.
Paul.
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