Re: Bipedalism and theorizing... was Re: Morgan and creationists
Paul Crowley (Paul@crowleyp.demon.co.uk)
Wed, 24 Jul 96 00:26:01 GMT
In article <DuxzKE.Aw8@inter.NL.net>
ghanenbu@inter.nl.net "Gerrit Hanenburg" writes:
> If the infant was able to cling to the mother (either ventrally or
> dorsally) without additional support then both her arms could be used
> in climbing,and thus it would be much easier for an early hominid to
> climb.
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. 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.
So why would a female want to be encumbered with an infant when
up a tree? It would be inconvenient and dangerous for both.
When an early hominid female with a small child spotted some
desirable fruit, she'd have done exactly the same as a modern hss
female. She'd leave her child with her sister, her mother or its
sibling, and go get it. The level of co-operation needed is just
about what chimps can manage, maybe fractionally more -- which is
exactly what we would forecast for the hominid line.
> 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.
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.
> >> Neither have you made it clear why it easier to hang on underneath the
> >> mother with nothing to support the infant.
>
> >I shouldn't need to. It's basic physics/engineering. You can
> >hold onto a horizontal branch of a tree, wrapping your legs and
> >arms around it -- even if it moves around; but you can't hold
> >onto the vertical trunk of a tree unless you have good foot- and
> >hand-holds; and if the vertical trunk starts moving along,
> >bouncing up and down, you've got no chance of staying on.
>
> The point is that I do not have to cling to vertical trunks during any
> phase of my life,and are not thus adapted,but did you perform the
> experiment with apes and monkeys?
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.
> Singleton births are not required since the Callitrichidae (tamarins
> and marmosets) normally give birth to twins.
Yes -- but they are the only exceptions and they are very small
and have dense fur. Each twin can hold onto the mother's *fur*.
This is not possible with the larger simians, especially the apes.
> >The abandonment of this basic primate behaviour was undoubtedly
> >*the* distinctive hominid achievement. I'm sure it occurred prior
> >to bipedalism, and was a necessary condition for it.
>
> And what do you have to back up your confidence?
Simple logic. 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.
> >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.
> 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".
> 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.
It would not be strong or plentiful enough and they're much to heavy.
They hold onto their mothers' *bodies* using their long arms and legs.
You can see this in any zoo.
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? It is not a "commonsense" view. I've never seen it
illustrated. All the pictorial representations of early homo show
infants being held in the arms. Of course that's impossible for
running, climbing or doing any work; but it *is* about the only
way in which early hominid infants could have been carried. So the
conclusion has to be that they generally weren't. They remained
"at home" in a safe place, being looked after, exactly as today.
Paul.
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