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Re: Palaeontology (1)
alex duncan (aduncan@mail.utexas.edu)
3 Aug 1995 05:47:46 GMT
In article <183866662wnr@desco.demon.co.uk> Elaine Morgan,
Elaine@desco.demon.co.uk writes:
>Look at these two statements in a paper by Susman, Stern, and Jungers
>(from : "Ancestors : the hard evidence.")
>
>(1) "In the wild, chimpanzees also spend a substantial portion of their
>time on the ground, as quadrupeds, but their manner of quadrupedalism
>is more costly than that of almost any other ground-dwelling
>quadruped. Compromise to increase the range of their habitat seems to
>be the best explanation for this apparently stable adaptation in
>chimpanzees."
>
>(2) "Bipedalism may have been a response to the need to travel to and
>among the ever shrinking forested areas."
>
>They seem to be saying: "The chimpanzee became quadrupedal because it
>needed to increase its range. The hominid became bipedal because it
>needed to increase its range."
You're absolutely right. However, as you note below, this is only a
problem if we assume the ancestor of the chimp and the ancestor of the
hominid were very similar (virtually identical) to start with. I've seen
suggestions that only necessary difference in morphology required to
account for these differences in terrestrial locomotion might be body
size. There seems to be a trend in the African apes: the smaller they
are, the more frequently they're bipedal. Well, what if we posit the
ancestor for chimps to be essentially like a modern bonobo, and the
ancestor for hominids to be the same thing, only perhaps 5 - 10 kg
smaller -- with the associated allometric differences? In African apes
(basing the observation on what we see in modern taxa), smaller size
might mean (most importantly) different intermembral proportions. In
modern Gorillas, the intermembral index is around 116, in chimps its
around 105, and in bonobos its around 100. In a creature otherwise
identical to a bonobo, but 5 - 10 kg lighter, the intermembral index
might be somewhere between 90 and 100. This falls within the range of
specialized terrestrial quadrupeds. But, there is a very important
difference between my proposed miniature bonobo and a critter like a
baboon with a similar intermembral index (both around 95). The baboon
doesn't have a thorax and upper limb stucture specialized for suspensory
behavior. These specializations probably make "normal" quadrupedalism
impossible. In the proposed mini-bonobo, the alternatives are some form
of modified quadrupedalism, which would be difficult given the relatively
short arms, or bipedalism, which wouldn't be all that difficult to
accomodate. Several features are already present that might make
bipedalism the easiest way to go. These include the exaptation for
partially erect posture resulting from a suspensory existence, and the
already existing features of the hindlimb musculature that are adaptive
for climbing (see EMG papers by either Stern or Susman).
Is there any fossil evidence for this? I'm sure you're familiar
with Zihlman et al's work noting similarities between A. afarensis and
bonobos. There are the persistent arboreal adaptations in the
australopithecines and earliest Homo. (The new foot is wonderful.
Hopefully it'll encourage someone to reexamine Latimer & Lovejoy's work
on the AL333-115 foot. Anyone can LOOK at it and see they were
incorrect. Rumor has it A. ramidus also had an opposable toe -- guess
we'll have to wait for publication to be sure though.) There's the trend
toward reduced body size the farther back in time we go. A. ramidus will
probably wind up having smaller average body size than A. afarensis.
Older hominid material like the Tabarin mandible also looks to have been
smaller than even Lucy.
>That is a perfectly tenable position, but only on the premise that the
>animals were aleady very different before either of them set a foot on
>the ground - eg maybe the prehominid ape was smaller-bodied, had a
>different intramembral index, etc. But this is pure speculations,
>isn't it? For all we know, at 4mybp all the African apes were
>small-bodied and resembled one another very closely. It has been
>suggested that the l.c.a. looked like a bonobo. The gorilla could have
>become larger much later - as late as 2mybp or even 1mybp. We have no
>fossils.
Well, its really not "pure" speculation. I'll admit I'd be a lot happier
with my ideas if they were better supported by the fossil record. But a
lot of the model is based upon observation of modern primates. So, yes,
it is speculation. But, I would defend it as reasonably well-informed
speculation.
>If they did resemble one another closely, we would need a very good
>reason why one lot would respond to the advent of savannah mosaic by
>becoming quadrupedal and the other by becoming bipedal.
see above
>There is also an unwarranted assumption that the trend towards
>bipedalism postdated and was a consequence of the opening up of the
>forests and the advent of s/mosaic. That too is pure speculation,
>isn't it? Susman et al make a point of stressing that bipedalism did
>not arrive overnight. : "Far from being abrupt and complete, its
>evolution can be seen as having taken place over a considerable time
>span". I'll drink to that, If in the beginning they spent say 90% of
>their time in the trees and only occasional intervals locomoting at
>ground level (whether across grass or through water) it could have
>evolved very slowly indeed. It could have been under way, gradually,
>since the onset of speciation. It could have been a response to the
>same ecological event that caused the speciation. That would be
>elegant.
Well, again, I'd have to say its not "pure" speculation. The oldest
bipedal hominids we know of are just pushing 4 Myr. Their adaptations to
bipedalism are no where near as all-encompassing as those of modern
humans, and to my eye it doesn't look like they've been spending time on
the ground for all that long. It certainly doesn't seem unreasonable
that we could go from a mini-bonobo precursor to A. afarensis in 1.5 -
2.0 Myr. All it might take would be some minor reorganization of the
hindlimb skeleton. The proposed aridification of Africa is thought to
have reached its climax at 6.0 - 5.5 Myr ago. That gives plenty of time
to arrive at A. afarensis.
>I'll come to Laetoli and all that jazz when more papers arrive.
I hope you're enjoying the reading.
Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086
512-471-4206
aduncan@mail.utexas.edu
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