Re: Alex's gibbon-like CA
Harry Erwin (herwin@gmu.edu)
Tue, 24 Oct 1995 21:52:53 -0400
In article <814581738snz@crowleyp.demon.co.uk>, Paul@crowleyp.demon.co.uk wrote:
> In article <45u12g$4pn@nntp5.u.washington.edu>
> aduncan@mail.utexas.edu "Alex Duncan" writes:
>
> > My personal favorite hypothesis for the origin of bipedalism is that
> > hominid ancestors were obligate bipeds due to their suspensory
> > adaptations. For modern analogies, see gibbons and spider monkeys. Both
> > creatures are so specialized for arboreal suspension that they are not
> > capable of "normal" quadrupedalism, and usually adopt bipedal positional
> > behaviors when walking on tops of branches, or on the ground. If such a
> > creature were faced with an environment that was gradually becoming more
> > arid (thus less tree cover), a selective premium might be placed on the
> > capability to locomote bipedally from one patch of trees to the next.
>
> (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.
All you need is a woodland biome. Without a closed canopy present, some
resources will be hard to get to by brachiation. That can make having
significant ground mobility selectively advantageous--it doesn't have to
be all or nothing. We have evidence of just that ecological shift in the
African Miocene. Not an argument.
By the way, are you assuming ->gibbons<- are being proposed as ancestral
in this paradigm???? They aren't. They go back a long way as a separate
lineage.
>
> (b) Large clumps of forest would have to be completely excluded from
> the scenario, otherwise the potential hominid-gibbons would interbreed
> with the ancestor-gibbons which would be maintaining their well-
> established and successful form of brachiating life.
Actually, what you need is sufficient isolation so that the gene flow is
interrupted. That occurs from time to time in woodland biomes simply by
accident. Not an argument.
>
> (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.
Speciation generally takes place much more rapidly than that. Remember
most mammalian species don't last for much longer than a million years. In
addition, forests 'ebb and flow' much more slowly than you indicate. Not
an argument.
>
> (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.
Primates are extremely variable in social structure. Not an argument.
>
> (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.
Actually the diet change is not required. The 'hominid-gibbons' would be
adapting in order to maintain the ->same<- diet. Not an argument.
>
> (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.
Is this argument by incredulity? Sorry, but the proposed adaptations
appear to have occured about as soon as the physical substrate evolved in
the anthropoid line. Not an argument.
>
> (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.
Yes, and you would expect the balance to be close to optimal for the
environment and behavioral repetoire. So? Definitely not an argument.
>
> (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.
Actually, the infant child shows the necessary reflexes. The adult and
child stages are likely to have evolved at separate rates. Not an
argument.
>
> 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".
>
> Paul.
Sorry, but your arguments don't hold water.
Cheers,
--
Harry Erwin
Internet: herwin@gmu.edu
Home Page: http://osf1.gmu.edu/~herwin (try a couple of times)
PhD student in comp neurosci: "Glitches happen" & "Meaning is emotional"
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