Re: Dissecting the Aquatic Ape: Bipedalism
Paul Crowley (Paul@crowleyp.demon.co.uk)
Tue, 16 Jul 96 16:55:49 GMT
In article <4scc02$cbe@newsbf02.news.aol.com>
promethius@aol.com "Promethius" writes:
> Knuckle walking is an very specialized form of locomotor adaptation,
> every bit as much so as bipedialism.
This is a wild over-statement. Knuckle-walking is a form of quadru-
pedalism, the almost universal mode of locomotion for terrestrial
mammals.
> It therefore stands to reason that
> both bipedialism and knuckle walking evolved from a common ancestor that
> did neither.
This does not follow at all. There are two main possibilities:
/ - - - - - Knuckle-walker(s)
{a} Arboreal Ape <
\_ _ _ _ _ _ Bipedal hominid
{b} Arboreal Ape - - -> Proto-chimpanzee - - - -> Chimpanzee
\_ _ _ _ _ _ _ > Bipedal Hominid
The arguments in favour of {b} are:
1. We are genetically closely related to chimps. The date of the
split from them gets later and later and certainly post-dates the
split from gorillas which also knuckle-walk. So if {a} were true,
it would mean that knuckle-walking had developed independently at
least twice.
2. Likewise we share many behavioural features with chimps, such
as: multi-male groups, female exogamy, tool-use, weapon use, sophis-
ticated social abilities, war-like tendencies among males, and strong
family ties primarily based around older females. If {a} were true,
all these would have had to evolve independently at least twice.
3. Chimps regularly adopt a bipedal stance. The niche now occupied
by chimps has existed for tens of millions of years. Consequently we
have no reason to believe that they have changed significantly from
the time they first adapted to it. All that is required for theory {b}
above is an account of the niche that enabled a small population of
(proto-)chimps to adopt bipedalism permanently. This is no small
task, but it's much easier than the alternative.
4. The non-knuckle-walking LCA (theory {a} above) requires the
identification of a (non-chimp) terrestrial niche occupied by a
largely arboreal ape when it first moved onto the ground. We have
no model for such an animal. It also requires the identification
of the niche that enabled this animal to later develop bipedalism.
Again, here, we don't have the beginnings of a model (as we do for
"partially bipedal" chimps). These are different niches because
effective bipedalism would take some time to develop and could not
be the original reason for the ape moving down to the ground.
In other words, a non-knuckle-walking LCA multiplies complexities.
Occam's razor should rule it out.
Paul.
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