Re: An alternative to ST and AAT
Paul Crowley (Paul@crowleyp.demon.co.uk)
Mon, 11 Nov 96 23:01:43 GMT
In article <561muk$b25@grootstal.nijmegen.inter.nl.net>
G.Hanenburg@inter.nl.net "Gerrit Hanenburg" writes:
> Paul@crowleyp.demon.co.uk (Paul Crowley) wrote:
>
> >This raises a very interesting point. For some 2.5 million years
> >(until about 2.5 mya) hominids could not stride and probably could
> >not run; if they could, they certainly did not do it well. The
> >australopithecines had short legs, long arms and no waist.
>
> Australopithecines,as represented by "Lucy" (AL 288-1),did not have
> long arms. The "intermediate" humerofemoral index (85) is the result
> of a relatively short hindlimb.
Lucy did have long arms. The A. afarensis humerus was about the
same length as in both H.s.s. and chimps. But the humero-femoral
index is misleading as the difference in arm length comes from the
ulna; and the male A.afarensis ulna is almost the length of the
male chimp's -- much longer than in H.s.s. The H/F index is also
misleading in that it ignores body size. Australo total leg
length was much too small in relation to body size for what we
would regard as effective bipedalism. Also they had relatively
large feet. IOW, Lucy waddled.
> >So why did they maintain this very strange morphology for so long?
>
> Why very strange? It's basically hominoid.
It is very strange for a bipedal animal; we're constantly told
that the whole point of bipedalism was to enable a more effective
progression on the ground. As you know, I don't buy this, but it
is surprising that, once bipedalism had been achieved, the selective
pressures towards a good running and striding capability were
restrained for some 2.5 Myr.
> You acknowledge that the anatomy of Australopithecines could have had
> something to do with selection for arboreality? That gives me a lump
> in my throat,although I do think you're actually borrowing this piece
> of your theory from others who conceived of the possibillity of
> arboreality quite some time ago. There's nothing new here.
(Elaine's theory about lumps in the throat must have made a deep
impression on you.)
I am ruling out the standard primate arboreality - that which
requires four prehensile limbs and enables sleeping in trees.
(The retention of such limbs would prevent bipedalism.)
I am suggesting that one particular ability - that of climbing
_vertical_ poles where there are no side branches and minimal hand-
and foot-holds. This kind of ability is shown by the Dayaks of
Borneo when they climb up to the canopy in dense equatorial forest,
using a rope going around the tree which enables them to lean back
from the tree and "walk up" it. (Your local telephone repair
person does something similar.) For a hominid to achieve this
without aids, it would need long arms and short legs, which is what
we have in the australopithecines. Large feet, some of curvature
in the manual and pedal phalanges, and strong wrists would also
help.
I thought I had made this clear by emphasising _palm-trees_.
Obviously I hadn't. My apologies.
Rather than palm-trees, I have begun to think that the most likely
probability is a dense equatorial forest (adjacent to a non-tidal
littoral, of course). The australos could then consume a partial
diet like that of orangs. They would not usually sleep in the
trees, nor take their young into the canopy.
Perhaps it could be thought of a sort of adaptation by a quasi-
chimp to an orang niche. However, without the littoral aspect,
this would not have enabled the full bipedalism that we know they
achieved.
The drying-out climatic phase that's supposed to have occurred
about 2.5 mya could have eliminated the equatorial forests,
thus terminating that particular form of adaptation. (I hate
suggesting anything "forced by climatic change", but as the
hominids would still have had the shellfish, it might just be
possible.)
> A longer leg has its advantages in a setting were you have to cover a
> larger range (it makes bipedalism more efficient by increasing stride
> length and decreasing stride frequency)
So why was a shorter leg maintained for 2.5 Myr?
> As I already pointed out in an earlier post,shellfish does not exactly
> constitute a rich foodsource. Shellfish is particularly low in fat and
> carbohydrates and not exceptionally high in protein.
Protein can substitute for both carbohydrates and fat. Protein
is an essential part of the diet and it's glaringly missing from
inland theories of hominid evolution. How could Lucy have waddled
in the direction of the circling vultures and expect to get to the
kill before all the other predators doing the same?
Paul.
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