Re: An alternative to ST and AAT

Paul Crowley (
Sun, 08 Dec 96 22:41:41 GMT

In article <> "T&B Schmal" writes:

> wrote:>
> > (The first hominids') bipedalism must necessarily have been less
> > effective at ordinary progression than chimps' quadrupedalism.
> Paul, you are driving me crazy when you keep repeating this. Are you
> stating some law of mechanics, or quoting a well-known study, or what?
> This is the most counterintuitive statement I have heard from you and I
> have just got to know what is the reasoning behind it.

Sorry Tom, I thought we had been through this. I really can't
see what the problem is, or why you think it's counter-intuitive.

We start with a fully adapted quadrupedal animal; it changes
to a completely different form of locomotion; once completed,
it may (or may not) be better (faster, more energy efficient,
whatever) than its initial quadrupedal state, but surely its
intermediate state has to be worse than either?

In the first form of locomotion its trunk is horizontal; in
the second it's vertical. It cannot have a trunk at 45 degrees
or at angles other than ~0 or ~90 degrees. It cannot move
gradually from one to the other. So at the initial stage
we have to have a quadrupedal animal adopting a bipedal stance
-- as we see with chimps. The only way it can progress is
to adopt this stance more and more often, going from 10% of
locomotion time to 20% to 30% and so on. As it does this,
it will get better and (over generations) adapt. Its spine
will adapt to the vertical posture; its pelvis will re-
orientate; its legs will get straighter and its knees will
develop locking mechanisms. But each of these changes will
reduce its ability to progress quadrupedally.

As soon as it starts to make adaptations towards bipedal
locomotion, its quadrupedal adaptations deteriorate.

Taking modern chimps as our model, let's say a doubling of
their bipedal capability (measured any way you like) results
in a 10% reduction in their quadrupedal ability. Their
method of getting about is still primarily quadrupedal, so
they endure an overall deterioration.

What's counter-intuitive about this? Try drawing or
describing a chimp that's achieved a stable 50% bipedalism
(instead of 10% -- as current ones are).