Re: Bipedalism and theorizing... was Re: Morgan and creationists
Gerrit Hanenburg (ghanenbu@inter.nl.net)
Wed, 24 Jul 1996 22:16:29 GMT
Paul Crowley <Paul@crowleyp.demon.co.uk> wrote:
>You'd accept, I'm sure, that early hominids were more ground-based
>than chimps - which spend (I believe) about 90% of their waking
>time on the ground. So climbing for a hominid female would be
>an occasional activity - mainly for collecting fruit.
I wouldn't be so sure about this.Clark and Tobias in their article
about the Sterkfontein footbones (Stw 573) write:"It is becoming clear
that Australopithecus was likely not an obligate biped,but rather a
facultative biped and climber.The exact proportion of its activities
spent on the ground and in the trees is at present indeterminate."
(Clarke,R.J. and P.V.Tobias(1995),Sterkfontein Member 2 Foot Bones of
the Oldest South African Hominid.,Science 269:521-524)
I subscribe to that.
>They'd have been able to cope with diurnal predators, without needing to climb.
>If the predators were chimps (which is not at all unlikely) then
>a tree would be about the worst place to be.
One of the advantages of maintaining an arboreal adaptive complex may
have been precisely the escape from terrestrial predators.
Your suggestion that chimps posed a serious pressure in the form of
predation on early hominid populations does not seem very realistic
since meat forms only a minor component of the diet of chimps.
Predation by chimps is relatively rare,and if they hunt they prefer
small mammals (such as Red Colobus).
Predation pressure by chimps also implies extensive sympatry,which may
not have been the case.
>> Frankly,I do not have much faith in this ascii method of judging
>> physical phenomena.
>> Just presenting such an image without any further physical explanation
>> (or perhaps even some experimental data) is handwaving.
>There comes a point where one just says "Look at it" and if the
>other says "I still can't see it" then one can say no more.
Yes,but in science one should not resort to this approach as a first
means.
(If I were unable to see the rabbit in the duck-rabbit image wouldn't
you try everything to make me see it?
Perhaps you would point to certain aspects of it and say "this is its
eye,these are its ears and this is its snout",or maybe you would draw
the rest of its body in connection to the head.)
Anyway,*you* are already convinced that vertical clinging in a biped
is virtually impossible,that this is almost self-evident and that an
ascii-image clearly shows why.But it doesn't.
Simple logic may lead you astray here.
If you do the calculating,and the experimenting it may turn out that
your logic was wrong.
That's why we make prototypes before sending the "real thing" into the
air.
>I am making the simple points: (a) hominids have vertical trunks
>in normal terrestrial locomotion; (b) other primates maintain
>horizontal trunks; (c) gravity acts vertically; ergo (d) hominid
>infants can't hold on whereas other primates can.
ad (c):it does so in both the vertical and the horizontal
positions.The pull of gravity does not become stronger in the vertical
case. 2kg remains 2kg in both situations,it doesn't change from 2 to
3kg if you change from horizontal to vertical.
ad (d) this conclusion does not necessarily follow.
If the infant can get a hold (as it can in horizontal cases) and pull
its body close to the carrier it may be able to hang on like in other
orthograde positions.
>It is not possible (in any real terms) for an infant that weighs
>around 4 kilos to become adapted to holding onto a moving, bouncing
>vertical structure, which has not provided good foot- and hand-holds.
The vertical excursion of the body in normal bipedal walking is only
minimal.
>There are two possibilities (a) bipedalism first
>with the continued carrying of the infant; this is IMHO effectively
>impossible - and that was the point of my ascii drawing; and
>(b) putting the infant down, with bipedalism following; this is
>entirely plausible. We know that the hominid line adopted this
>behaviour at some point; if we posit that it was at_the_point_of_
>speciation of the hominind line then bipedalism has relatively few
>problems; all we have to do is identify the niche that required
>and enabled the putting down of the child.
That is the logic,...now the evidence.
>> >The cause of secondary altriciality in infant hominids is their
>> >inability to learn normal locomotory behaviour within a reasonable
>> >period.
>>
>> But what causes this inability?
>> Why are they different from apes in this respect?
>Hominid infants have to learn to walk bipedally. It does not come
>easy. Infant apes don't have to learn this; they can clamber around
>trees with all four limbs a few weeks after birth.
That's not enough.You have to think further,i.e.in terms of
development of the neuromuscular system.
We have to answer questions like "why does motor-coordination develop
so much faster in the chimpanzee?","what is the neurological substrate
(if any) that's responsible for this difference?",etc.
>> Their is no reason to think that if they,like other apes,were able to
>> carry their infants during orthograde positions in the trees,they
>> suddenly had to put them down when descending to the ground and move
>> bipedally.
>If (as seems likely) the LCA was a proto-chimp, then there was
>not a lot of being "in the trees".
White et al.(1994) write:"...the modern African apes are distinct in
many dental features from both Aramis and middle to late Miocene
hominoids,and thus probably do not represent the ancestral condition."
(White,T.D.et al.(1994),Australopithecus ramidus,a new species of
hominid from Aramis,Ethiopia.Nature 371:p.306-312)
If the same applies to the postcranial material then you possibly have
a problem.
>> There's seems to be nothing physically or anatomically that
>> makes clinging to an orthograde bipedal body more difficult than to an
>> orthograde arboreal body,as long as the mother has hair and the infant
>> can grasp.
>Chimp, gorilla, orang and gibbon infants do *not* hold onto hair.
They most certainly do.They grab the hair on the flanks and on the
sides of the ribcage somewhat below the armpits.
For a lateral view see the photograph in Goodall's "Through a window"
where Spindle carries Mel (it's in the last photosection of the dutch
translation,so I expect it is also there in the english version).For a
dorsal view see page 34 in "Chimpanzee politics" by Frans de Waal
(Johns Hopkins University Press,1989).
>The claim that you are making - that hominid infants could grasp
>their mothers in the same way as ape infants - appears to be quite
>unusual. I'll look up the references you give -- do they also make
>this claim?
No,Stanley proposes an alternative way of carrying.Just read it,you
will like it.
Falk only draws conclusions about the ape-like character of the Taung
endocast,but in my opinion (with less certainty) this can be related
to motordevelopment (probably also ape-like)
>It is not a "commonsense" view.
So what,if it isn't? Does science make progress through "common
sense"? "Common sense" once made people believe that the earth was
flat. :-)
I do not claim that early hominids carried their infants as
suggested,but I do not want to exclude the possibility,and I think we
should explore it before discarding it.
Gerrit.
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