Re: Bipedalism and Arboreality.

["Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>]

Nick Pharris wrote:
>Cladistically, though, the relationships among the living members of the 
>African ape clade most likely look like this: (gorillas + (chimps + humans)). 
> Thus, either humans are ancestrally knuckle walkers (which introduces a 
>whole new set of issues), or all three clades became terrestrial 
>independently, most likely deriving from a clambering form.
        This almost does it, except that terrestriality and knuckle walking
are separate (but perhaps linked) character states. Since terrestriality
(although not obligate terrestriality) is present in all three living
groups, it is most parsimonious to conclude that their common ancestor was
terrestrial (although this may be contradicted by fossil evidence).
        The mode of locomotion this animal employed when on the ground may
have been knuckle walking, or knuckle walking may have developed separately
in the two non-human groups separately. At this level of analysis either
solution is equally parsimonious, and the question becomes, do you prefer
paralellism in chimps and gorillas or reversal in humans?
        Now, I can see a decent argument for reversal, in that the
"reversal" is an artifact of binary characterization (indeed, this may be
quite common). A transition from "clambering" to knuckle-walking to
struttin' will give the appearance of a "reversal" if it is coded 0 animal
doesn't knuckle-walk, 1-animal knuckle-walks. However, I can also see an
argument that the top-heavy, brachiocentric locomotor aparatus of the
ancestral ape left its terrestrial descendants with a predeliction for
knuckle walking, arguing for convergence. However, in any case, homoplaisy
is homoplaisy.
        As an aside, if "chimps" are demonstrated to be paraphyletic, with
Bonobos being the closest extant outgroup to humans, than the situation
(ignoring fossil evidence, etc) suggests that human ancestors at one point
did indeed walk on their knuckles (two transformations to three).   

        Wagner

--------------------------------------------------------------------------------
     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi