Re: Bipedalism and theorizing... was Re: Morgan and creationists
Gerrit Hanenburg (ghanenbu@inter.nl.net)
Fri, 04 Oct 1996 21:07:37 GMT
Paul@crowleyp.demon.co.uk (Paul Crowley) wrote:
>> That's why I think that reorganization of the pelvic region and lower
>> limbs is crucial,and happened early in hominid evolution.
>> It may have started during a stage when the main function of
>> bipedalism was postural and not yet locomotory.^^^^^^^^^^^^^
> ^^^^^^^^^^^^^^^^^^^^^^^
>What does "the main function of bipedalism was postural" mean?
It means that bipedalism is mainly used to stand upright,both in the
trees and on the ground,with or without stabilization by an upper limb
(semi-suspension).
>Such changes must have been very costly (in all kinds of ways)
>and probably seriously affected the existing form of locomotion.
>What scenario do you envisage that could have created such strong
>selective pressures?
I already mentioned the work of Kevin Hunt who found that 80 percent
of chimpanzee bipedalism was during feeding,95 percent of which was
postural.
The ecology of chimpanzee bipedalism turned out to be mainly
determined by diminutive understory trees with small fruits (2 cm or
less) which are evenly distributed among the terminal branches.
I imagine an early hominid environment in which such vegetation was
relatively more abundant,favoring adaptation to bipedalism.
>Origin of LCA / Chimp (knuckle-walker)
> O>--------[unknown locomotion]------ Gorilla (knuckle-walker)
>(20mya? 15mya? 10mya?) \ Hominid (bipedal - NEW)
>Given that the hominid bipedal locomotion was new, what is
>the most parsimonious suggestion for the "unknown locomotion"?
That can be determined by means of outgroup comparison,which is
defined as:"For a given character with 2 or more states within a
group,the state occuring in related groups is assumed to be the
plesiomorphic state." (Watrous,L.E.and Wheeler,Q.D.The outgroup
comparison method of character analysis. Systematic Zoology 30:p.5)
If there is a Homo/Pan clade Gorilla can be considered as
outgroup.This means that the LCA of humans and chimps was a
knucklewalker (knucklewalking is plesiomorphic).
If there is a Pan/Gorilla clade with Homo as the sistergroup then the
outgroup to the Pan/Gorilla/Homo clade is Pongo.Pongo is not a
knucklewalker but moves either by fistwalking or bipedally when on the
ground.Thus the LCA of Pan/Gorilla/Homo may have been a bipedalist on
the ground (bipedalism is plesiomorphic).
Gerrit.
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