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

HARRY R. ERWIN (herwin@mason2.gmu.edu)
31 Jul 1996 13:25:40 GMT

Paul Crowley (Paul@crowleyp.demon.co.uk) wrote:
: In article <4tibn9$8gl@portal.gmu.edu>
: herwin@mason2.gmu.edu "HARRY R. ERWIN" writes:

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

: From Schultz: The Life of Primates (1969) p. 233 "Infants of such
: daring brachiators as all gibbons, usually continue to cling to
: the mother's abdomen while being cradled on the flexed thighs of
: the latter when hanging by her arms."

: > 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
: > [..]
: > Now bring an orthograde suspensory primate down to the ground.

: Why do you propose a suspensory stage?

Because we've got good evidence for it (Dryopithecus).

: It requires two major changes
: in locomotor adaptation. If the suspensory adaptation for most
: hominoids was never much more than the bipedal capability of, say,
: chimps, and the (formerly pronograde) territorial adaptations were
: never lost, then there are no major changes of locomotor adaptations.
: The only hominoids with a major change would be the gibbons, but as
: they are the only ones to brachiate and as their distribution is
: peculiarly limited, they can be regarded as highly specialised.

: I suspect that far too much arboreality is being read into the
: bones. If a fossil chimp or gorilla was dug up, I'm sure the general
: conclusion would be that the animal was 100% arboreal. There is
: unquestionably a strong bias in this direction. I wonder why?

Alex Duncan could discuss this, since his dissertation is in this area.

: > 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.

: I might accept this is we were 100% sure that the LCA was close to
: a gibbon. How such an animal might "evolve" if it was "forced" onto
: the ground is a moot point; however, it's an absurd proposition; it
: would need to be artificially kept alive for some tens of thousands
: of years in enclosures where it was well protected from competition,
: predators and parasites.

Evolution isn't discrete, and biomes are not discrete. All hominoids spend
some time on the ground, even gibbons. If you change the distribution of
local biomes that an animal has to deal with, you change the fixed point
corresponding to maximum fitness or even eliminate it. We have good
evidence for continuous and discontinuous environmental changes in Africa
during the last 10 MYr. That necessarily implies both speciation and
evolution within lineages. Two major changes in the environment in East
Africa during that period were the early replacement of a continuous
closed forest with a more patchy environment, including open woodlands and
scrublands, and the later appearence of the open savannah. The earlier of
these changes is associated with the appearence of hominids, while the
later of these changes is associated with the appearence of the ancestor
of H. ergaster.

: Your scheme of things puts huge obstacles in the way of hominoid
: evolution. Other primates have it much easier: no adaptation to
: suspensory climbing; no re-adaptation to the ground. Are we really
: sure our ancestors weren't baboons or monkeys? (Only joking).

We're orthograde, not pronograde, in our anatomy.

: Paul.

--

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++)