Re: Bipedalism and theorizing... was Re: Morgan and creationists

HARRY R. ERWIN (herwin@mason2.gmu.edu)
3 Aug 1996 17:49:37 GMT

Phillip Bigelow (bh162@scn.org) wrote:
: herwin@mason2.gmu.edu (HARRY R. ERWIN) wrote:
: >
: >2. The issue of motor programs is a lot more important than
: >non-specialists are aware.

: Undoubtably. But without fossilizable motor program evidence, we are left to
: speculate
: on what phylogenetic route(s) motor program evolution in Primates went. In other
: words, was motor progam evolution a "leader" in biped evolution, was it a "follower"
: in biped evolution, or was it simply a "co-evolver" in biped evolution?
: From your perspective as a neurologic specialist, can you provide us with
: any concrete ways in which your discipline can actually productively contribute to
: this phylogeny discussion?
:

Motor programs can provide character data for inclusion in a cladistic
analysis. Unfortunately, we are only just beginning to understand how they
are constructed and can evolve. My 'gut-feel' is that they evolve very
slowly, based on the similarity between the motor program in lampreys and
the motor program in much more advanced vertebrates. (The motor program,
by the way, is not just a simple adaptation to environmental constraints.
There are some weird aspects to how lampreys swim that make no sense
except as primitive retained features.)

: >Now bring an orthograde suspensory primate down to the ground. The motor
: >program they use in the trees is suspensory climbing, and that does not
: >convert easily into either terrestrial quadrupedalism or bipedalism.

: Perhaps not. But it appears to be somewhat of a non-issue here. If arboreal
: suspensory movement is considered to be
: a primitive trait for Primates, then at least some ground-anthropoids seem to
: have conquered this "motor program roadblock to quadrupedalism
: and bipedalism" fairly well; e.g., those being the gorillas, chimpanzees, and
: hominids.

Arboreal suspensory movement is not ancestral. Arboreal skittering about
on the tops of branches with the aid of claws is. It's not that much
different from the motor program that marsupials use. As the primates got
bigger, we then get evidence for vertical clinging and leaping (lemurs,
etc., emphasizing the jumping part of the skittering) and arboreal
pronograde quadrupedalism (where the animal holds on as he/she walks on
the top of branches). About the time of Proconsul, we see some lines head
off towards Oreopithecus and Hylobates, but most apes still moved around
like monkeys. 10 MYr ago, we finally encounter Dryopithecus laietanus and
Sivapithecus indicus (closely related), which have general locomotor
anatomy similar to that of Pongo. The Dryopithecus skeleton is
particularly interesting, since it's clearly adapted to slow below-branch
suspensory locomotion. That entails a major reorganization of the
locomotor program and seems to be a prerequisite for bipedalism.

: The corrolary to this fact is that, PERHAPS, just perhaps, the evolutionary
: PATHWAY of motor programming in the hominoid brain is not as complicated
: as is, say, the influences of a mosaic of niche and ecological changes
: that probably affected most anthropoids around 5 mya.

Ne zhnayo.

: >
: >Note that if you put a orthograde suspensory primate into deep water, it
: >will swim with its arms--that's what the motor program is biased towards.
: >Bipedal locomotion in water is wading.

: An experiment involving an orthograde suspensory primate placed in deep water is
: considerably different than a researcher observing a biped in shallow water.
: Bad comparison, Harry. What do bipeds do when placed in deep water? What do
: suspensory primates do when placed in very shallow water? Is there any overlap in
: locomotor motions between the two morphotypes when placed in similar water depths?
: How "plastic" is the adaptation levels of individuals in such circumstances?
: <pb>

Very plastic...

--

Harry Erwin, Internet: herwin@gmu.edu, Web Page: http://osf1.gmu.edu/~herwin 

49 year old PhD student in computational neuroscience ("how bats do it" 8)

and lecturer for CS 211 (data structures and advanced C++)