Re: An alternative to ST and AAT

Rohinton Collins (
2 Nov 1996 19:55:30 GMT

pete <VINCENT@reg.Triumf.CA> wrote in article
> Rohinton Collins ( sez:
> `Fair enough Pete, I may have not explained myself well enough. What I
> `by an arboreal quadruped is an ape which uses all four limbs for
> `locomotion, but being arboreal, has much flexibility of locomotion and
> `posture (the orang-utan would probably be a good model). The chimpanzee
> ` a rather restricted mode of locomotion, since it is a fully adapted
> `terrestrial quadruped (N.B. small valgus angle). So, what I am saying is
> `that my LCA would have been more predisposed to the transformation into
> `terrestrial biped than would be a proto-chimp LCA. And no, the gibbon is
> `brachiator. I would say that a gibbon-like LCA (in its mode of
> `would be unlikely because brachiation is specialised. What it comes down
> `is that a more generalised LCA (in all ways, as well as locomotion)
> `be more predisposed to take advantage of a new (or a forced) mode of
> `locomotion than a more specialised ape would be.
> Ok, but the orangutan has a pretty unique method of locomotion, because
> of its size. One must suspect from the size of extant a-pith fossils
> that the lca was a smaller creature. When one considers the gibbon
> style of locomotion, which is divided between brachiation with the
> fore limbs and running along branches with the hind limbs, as well as
> having an erect-posture pelvis resulting from its brachiating niche,
> it doesn't seem unreasonable to postulate an LCA which used a similar
> repertoire of motive strategies, but had not evolved so far along that
> line, so that for instance its forelimbs were not so elongated, might
> be well positioned to transfer to a bipedal terrestrial niche when
> (not yet specified) pressures forced it in that direction. ...I wonder
> if the gibbon family was fully developed to its current fore/hind limb
> ratio 6 to 8 Mya...

Okay, Pete. The orang-utan metaphor a bad idea, but so is the gibbon. The
rest of what I said I still stand by though. Any extant hominoid would make
a misleading model for the LCA. But remember Pete, hominids and the African
apes are considered to form a clade, so you could expect to find
symplesiomorphic characters amongst these hominoids which would not be
found in gibbons or the orang-utan, or their antecedants. Such characters
could possibly point to the mode of locomotion of the LCA. All modern apes
are specalised in their modes of locomotion when compared to many miocene
apes. The gibbon family had become dedicated brachiators some 20 mya. A
specialisation like this would happen very quickly, once evolution had
started in this direction (punctuated equilibria). It is unlikely that the
LCA was evolving towards brachiation simply because it was most probably
too big to be a brachiator (gibbons are far smaller than the greater apes).