Re: An alternative to ST and AAT

Rohinton Collins (
30 Oct 1996 22:25:33 GMT

Paul Crowley <> wrote in article
> In article <>
> "T&B Schmal" writes:
> > wrote:>
> > > the first bipedal hominids would have been less
> > > effective at terrestrial progression than their quadrupedal
> > > cousins.
> >
> > This seems to me to be a difficult thing to assert. Now, if we knew
> > sequence of changes and how long it all took, maybe. But as things
> > stand I think you've got a bit of a stretch, here, Paul.
> I can't see the problem, Tom. Let's assume the LCA was a quasi-
> chimp. (I think this is most probable, but much the same argument
> would apply to other kinds of LCA.) Chimps can walk bipedally,
> although they are not good at it and revert to quadrupedalism
> whenever they need speed or endurance. If, for some reason, a
> population of chimps began to walk bipedally 90% of the time
> instead of, say, 10% and there was some selection for those that
> were best at it, there would be many, many generations where the
> regular chimps would still be far better at getting around, by
> comparison with the bipedal population.

This is a common mistake - using the chimpanzee as a model for the LCA.
The chimpanzee is a fully adapted terrestrial quadruped who occasionally
walks bipedally, if very inefficiently. The LCA, assuming it was an
arboreal quadruped, would be more predisposed to bipedality than the chimp,
since much of the time it would have stood on two feet, steadying itself by
grasping upper branches, or reaching for fruit. The valgus angle
(femur/tibia angle) would most likely more closely match hominids than the
African apes (note the orang-utan has a hominid-like valgus angle and is
largely arboreal) which would also aid the transformation to bipedality.