Re: An alternative to ST and AAT
Gerrit Hanenburg (G.Hanenburg@inter.nl.net)
Mon, 11 Nov 1996 09:22:55 GMT
Paul@crowleyp.demon.co.uk (Paul Crowley) wrote:
>The male A. afarensis humerus was about the
>same length as in both H.s.s. and chimps. 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 ulna/humerus length index of A.afarensis of ~91% (chimpanzee
mean=95%,r=88-101% and modern human mean=80%,r=74-88%) is indeed
closer to the chimpanzee mean and outside the human range,but the
estimate is based on the AL 137-50 humerus (incomplete) and the AL
438-1 ulna. These do not belong to the same individual. Thus
reliability is in question. Nevertheless I stand corrected.
>The humero-femoral index is further 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. And the relatively large feet don't belong. IOW
>Lucy waddled.
Lucy's relative leglength is comparable to that of bonobo's (which btw
are relatively longlegged compared to common chimpanzees) and the
relative length of the pedal phalanges of A.afarensis falls outside
the range of Pan and is closer to the human mean.
But this gives little information about the exact form of bipedalism.
Waddling is mainly determined by the distance which the center of
gravity has to move laterally in order to balance over the stance leg.
This distance decreases if you have a high bicondylar angle,which is
about 1 deg. in chimpanzees but up to 15 deg. in A.afarensis (AL
129-1a)
Even if Lucy waddled that doesn't mean her form of bipedalism was not
effective in relation to her goals (efficiency is something different
and you shouldn't confuse the two).
>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.
I wonder how you arrived at the figure of 2.5Myr. It seems a little
long. But anyway,certain features may be maintained because they are
adaptive,while others don't develop because they aren't needed.
Compromise anatomy can be better than specialization.
Chimps are a good example.
>I'm 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'm suggesting that one particular ability - that of climbing
>_vertical_ poles where there are no side branches and minimal
>hand- and foot-holds.
>For a hominid to achieve this without aids, it would need long arms,
>short legs and large feet, which is exactly what we have in
>the australos. A degree of curvature in the phalanges might
>also be helpful.
If you can climb a vertical pole then you can climb any tree with a
similar diameter. Side branches make it even easier because they
provide hand and footholds.
And once an animal like Lucy was in the tree it could build a nest and
sleep in it.
>> 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?
Because long legs get in the way when climbing trees and A.afarensis
didn't have to rely on very efficient long distance travel like Homo
erectus (with its larger home range) did.
Ecological circumstances didn't favor a longer leg.
>Protein can substitute for both carbohydrates and fat.
Metabolic efficiency of protein is less than that of fat and
carbohydrate. If the diet is low in fat and carbohydrates,proteins are
used for energy. That means you can no longer use them for structural
or enzymatic purposes.
But there are other problems when you have to rely on protein for
energy. Before conversion to glucose or fat aminoacids become
deaminated in the liver. Since the liver has only a limited ability
the convert ammonia to urea a high protein intake will lead to toxic
levels of ammonia in the blood. Second,you will need a considerable
amount of fresh water to excrete urea.
You can't life for extended periods on a diet with a protein content
that exceeds 50% of total calories. Even Alaskan Eskimos,who have the
highest protein intake,have a diet in which protein contributes only
22-25% of total calories.
>Protein is an essential part of the diet and it's glaringly missing from
>inland theories of hominid evolution.
Many nuts have a high protein content and are generally also rich in
fat and carbohydrates. The protein of mongongo nuts,for
example,accounts for ar least 10% of total calories of the !Kung. This
is adequate to meet the minimum daily protein requirement of an adult.
Fruits,eggs,insects and small mammals can add to that.
>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?
I suspect Lucy didn't do that at all.
>Why was waddling selected for anyway?
Apart from the fact that we don't know exactly how Lucy
walked,waddling to get somewhere is probably better than not getting
there at all.
Gerrit
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