Re: Alex's gibbon-like CA
Alex Duncan (aduncan@mail.utexas.edu)
25 Oct 1995 03:26:50 GMT
In article <814581738snz@crowleyp.demon.co.uk> Paul Crowley,
Paul@crowleyp.demon.co.uk writes:
>(a) A forest-mosaic of the right "patchiness" which forced regular
>walking between the patches is too remote a possibility. Each patch
>would have to be a certain size, say, bigger than 10 hectares and
>smaller than 100 hectares. If they were too small they would not
>contain enough food to justify a dangerous walk; if they were too
>big the primates would continue to brachiate in them for a long
>time. Also the distance between patches must not be too large or
>too small for similar reasons. But this is not how geography works.
>You get fairly random patterns, through which all kinds of pathways
>are possible. And trees follow linear features such as rivers. You
>don't get the almost perfectly patterned mosaic that this scenario
>needs. This can be seen by looking at almost any map. Areas which
>are accessible with no walking, or minimal walking, should be marked
>out and contrasted with areas of reasonable size attainable only with
>reasonable amount of walking. The sum of the second group of areas
>will be seen to form a very small proportion of the whole, quite
>insufficient for any purpose, let alone the justification of a new
>species.
Hey, where do you get off, bringing in a post from another group?
Once again, I should have thought a little more clearly about what I was
trying to say before I said it. Let me reorganize.
First off, you claim that if the patches were too small, they would not
contain enough food to justify a dangerous walk. This depends upon two
things: 1) how far separated are the patches? and 2) how dangerous is the
walk? I personally don't have any problem w/ the idea of gibbon-like
protohominids traversing 50 - 100 m of open space to get from one patch
of trees to the next. Especially if they are group living primates, and
could cross from one patch of trees to the next en masse.
However -- back to an absence of clear thinking: I hacked the original
post off in a minute or two, without thinking too clearly about what I
was saying. I'm not backing off the "patches" idea, as I think it's
plausible. But I think a more likely explanation is a thinning of
previously continuous forests. In such a situation, a creature who
previously had been able to move from tree to tree without ever coming to
ground might have been required to come down and move from tree to tree
terrestrially. In such a situation, a selective premium would have been
placed on whatever form of terrestrial locomotion the animal was already
capable of. If the animal was largely a quadruped, then we should expect
an increase in the efficiency of terrestrial quadrupedalism. However, if
the animal was an obligate biped (as in modern gibbons) then we should
expect to see the efficiency of bipedalism increase.
-snip-
>(c) "Gradually becoming more arid" means nothing over the time that
>is necessary for speciation. In the hundreds of thousands of years
>required for it, forests would have "ebbed and flowed" innumerable
>times (probably thousands). However this scenario requires the
>almost perfectly patterned mosaic of "patchiness" to be *constantly*
>maintained in the same area throughout the period of speciation;
>otherwise the species would revert to ordinary brachiation.
I urge you to read some of the material about the terminal Miocene
climatic event and its probable effect on the African paleoenvironment.
I don't have any references handy, but if you'll pester me, I'll get them
for you.
The terminal Miocene event would have effected hominoid evolution in
several ways: 1) Previously continuous forest would be reduced to forest
refugia. Similarly, previously continuous populations of Miocene apes
would have been isolated in small groups, thus generating conditions for
speciation. And 2) there would have been an expanse in arid country
flora. Savanna, desert and woodland would all have increased in areal
extent at the expense of closed forest. The survival capabilities of
some primate lineages may have required a capacity to live in more open
habitats than were previously occupied.
>(d) A purely arboreal existence does not favor the development of a
>complex social structure. Gibbons can gather their fruit without
>danger and do not need to band together to fight off predators as
>do chimps. This proposal appears to require the simultaneous
>development of bipedalism AND a social structure. Most unlikely.
Gibbons aren't the only example, and are probably a bad example for
proto-hominid social stucture. Remember, when we're using gibbons as a
model, we're not saying hominids evolved FROM gibbons, but that hominids
evolved from primates that had some gibbon-like characteristics. Gibbons
are monogamous, and as is expected for monogamous primates, they are
sexually monomorphic. However, everything we know about African hominoid
evolution indicates that substantial sexual dimorphism is the rule. If
we look to spider monkeys, which are another example of an obligate biped
suspensory species, we see a very complex social structure.
>(e) The "hominid-gibbons" would have to adapt to life spent
>predominantly on the ground. This would imply a voluntary change
>of diet. So it's bipedalism AND a social structure AND change of
>diet.
Are you suggesting such a change is unlikely because of the NUMBER of
features that would have had to change? Compare to AAH: protohominids
entered the water and the following changes happened: 1) they became
bipedal, 2) they lost their hair, 3) they got sweaty, 4) they changed
their diet, 5) they developed a tearing mechanism that excreted sodium,
6) they developed thick layers of subcutaneous fat, 7) their laryngeal
structure changed, 8) they developed webbing between their digits, etc.
etc. I'm sure I'm leaving out some things.
We also need to look at the proposed magnitude of these changes.
Bipedalism: the "gibbon model" suggests that the protohomininds were
already bipedal due to their suspensory adaptations. The change is an
increase in frequency. Social structure: a big change is implied if we
assume the protohominids had a gibbon-like social organization. I don't
assume that, for reasons mentioned above. "Voluntary" dietary change:
most apes have extremely variable diets. Foley (I think) recently
documented over 200 different kinds of plant foods that are eaten by
chimps. Chimp diet varies depending upon the environment they live in.
A protohominid faced with thinning forest may have had to change its
dietary emphasis, but it's unlikely that it would have had to discover
entirely new kinds of food. (Again, compare to AAH.)
>(f) If such an adaption was so readily achieved and favorable for
>gibbon/hominids, it should have happened many, many times thoughout
>the history of evolution - to all kinds of brachiating primates.
>But no other such form of life exists today, nor is one present
>in the fossil record.
This argument is baseless. We've dealt with this again and again, and
yet you continue to bring it up. This time I'll deal with the specifics.
You say the change should have happened again and again, in all kinds of
brachiating primates. Well, how many kinds of brachiating primates are
there? This depends on how you define the term. Most physical
anthropologists restrict the use of the term to hylobatids. The other
hominoids practice "underbranch suspension", but are not capable of the
kind of high-speed "richochetal" brachiation that is characteristic of
gibbons. We're sort of getting bogged down in semantics here though.
Let me re-ask the question: how many taxa of primates are there that are
so specialized for suspensory positional behaviors that they are largely
restricted to bipedalism when they come to the ground? The answer is
two: gibbons and spider monkeys. Using modern primates as an analogy, it
doesn't look as if this is a form of positional behavior that arises
frequently, and so we shouldn't expect to see lots of gibbon-like
protohominids in the fossil record.
>(g) Any adaptions towards bipedalism must necessarily imply a
>deterioration in tree-climbing ability. The selective benefits
>obtained must outweigh the serious concomitant disadvantages.
>This scenario does not attempt to weigh them.
Actually, the muscles and movements used in bipedalism are very similar
to those used in tree-climbing. Check the following reference:
Fleagle JG et al. (1981) Climbing: a biomechanical link with brachiation
and with bipedalism. In: (Day MH, ed.) Vertebrate Locomotion: Symposium
of the Zoological Society of London, 48:359-375.
>(h) At some point the hominid adult foot lost the ability to
>grasp branches. At some point the hominid infant foot lost the
>ability to cling to its mother. This was the drastic change
>and *this* is the only bipedalism worth talking about. From
>this point on, life in the trees was impossible. Life on the
>ground was rendered much more difficult as mothers had to use
>one or both arms to carry infants. The benefits the hominid
>obtained to justify these disadvantages must have been enormous.
>They were not those achieved by walking more easily to the next
>patch.
As far as I'm concerned, we're dealing with a separate issue here. I'm
talking about the origins of bipedalism, and you're talking about it's
enhancement at a later stage of hominid evolution. I agree with you that
the loss of substantial arboreal capacities was a drastic change. This
is an issue that the fossil record speaks clearly about. The early
hominids retained tree climbing abilities. Much of this capacity had
disappeared by the time of the origin of H. erectus. We can speculate
about why this change happened all we want, but the fossil record is
clear: the change did happen, and there are no indications that any kind
of aquatic interlude was involved. It is with H. erectus that
discussions of "savanna adaptations" really become relevant, as it is
with this species that we see evidence for a major expansion of the
hominid niche. (Note: not change, but expansion -- H. erectus was also
probably still taking advantage of the resources available in more closed
habitat. There was probably a lot of inter-population variability, much
like we see in modern humans.)
>It is a comment on the level of thought on human evolution, and
>on bipedalism in particular, that this gibbon/hominid theory is
>"state of the art".
The gibbon/hominid hypothesis is not "state-of-the-art". I know of few
people besides myself who favor this model. The fact is that most
paleoanthropologists don't sit around trying to develop models for the
origin of bipedalism. A lot more time is spent trying to understand the
adaptations of the hominids for which we have a fossil record. Someday,
we'll have a fossils that document the origin of bipedalism. When that
happens, we'll see a lot more speculation about the adaptive
circumstances the earliest protohominids were dealing with.
Thanks for the thoughtful post.
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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