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

29 Jul 1996 12:44:25 GMT

1. Hominoid infants whose mothers use an orthograde suspensory locomotor
pattern hang on very well.

2. The issue of motor programs is a lot more important than
non-specialists are aware. I model cognitive processes in the central
nervous system (CNS), but as part of my coursework I had to take Avis
Cohen's year-long course in cognitive and computational modeling, which
spends a semester on motor programs. (I had to design a motor program for
the final. I modeled finger-gliding.) The spinal cord operates
autonomously of the CNS to a large extent. The CNS proposes a speed of
movement and a general direction, and the spinal cord handles choice of
gait, modulation of reflexes, and responding to small irregularities and
obstacles, using somatosensory and tactile afference. When a gibbon is
swinging along at speed, almost all of the computation is going on in the
spinal cord with practically no input from the head. If you've ever seen a
decorticated cat preparation walk, you know what I mean.

The implication is that a major change in locomotor adaptation involves
changing those motor programs. That's almost certainly why early apes were
pronograde rather than orthograde--the physical and behavioral changes to
become orthograde were relatively minor, but the changes to the motor
programs in the spinal column were major. (Synapsids needed at least 100
MYr after the hindlimbs were tucked under the body to go from a sprawling
tripedal stance to a narrow-based one with the elbows pointed rearward.)
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. The
'reflexes' involving the hands are wrong, particularly when running, since
rapid tactile feedback and response is what protects against damage. If a
suspensory primate wants to move fast, it either needs to keep the hands
off things (bipedalism), or it needs to protect the parts that damage too
easily (knuckle-walking). Which one is followed up is a function of
details of the biome.

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.

Harry Erwin, Internet:, Web Page:
49 year old PhD student in computational neuroscience ("how bats do it" 8)
and lecturer for CS 211 (data structures and advanced C++)