Alex's gibbon-like CA

Paul Crowley (
Wed, 25 Oct 95 00:42:19 GMT

In article <45u12g$> "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

(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.

(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.

(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.

(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

(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.

(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.

(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

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".