Re: Dissecting the Aquatic Ape: Bipedalism
HARRY R. ERWIN (herwin@mason2.gmu.edu)
22 Jul 1996 19:42:48 GMT
Paul Crowley (Paul@crowleyp.demon.co.uk) wrote:
: 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.
He's actually correct. Knuckle-walking involves walking on the ->backs of
the hands<-. It's only seen in Pan, Gorilla, Pongo (as fist-walking) and
some chalicotheres.
: > 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:
It's explicitly correct. We have fossil evidence for an order of
development: pronograde (above-branch) quadrupedalism --> orthograde
(suspensory) climbing --> whatever the earliest hominoids did on the
ground.
: / - - - - - 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.
Last I checked (February), the split was still a bush. I did read a good
article (November) that argued (based on sequencing data) that the three
lineages diverged within a couple of hundreds of thousands of years,
hence effectively simultaneously.
: 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.
These have not been ruled out as ancestral. These are mostly behavioral
and hence very labial.
: 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.
Pan troglodytes and Pan paniscus are knucklewalkers for more than 95% of
the time they move on the ground. Pongo is more of a knucklewalker than
Pan sp. are bipeds.
: 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.
The original reason for any large ape to move down to the ground is to
get out of the tree it's in and move to another tree. Pan troglodytes
_never_ bridges (moves arboreally) between trees. Pan paniscus does, but
only rarely.
: In other words, a non-knuckle-walking LCA multiplies complexities.
: Occam's razor should rule it out.
: Paul.
--
Harry Erwin, Internet: herwin@gmu.edu, Web Page: http://osf1.gmu.edu/~herwin
49 year old PhD student in computational neuroscience ("how bats do it" 8)
and lecturer for CS 211 (data structures and advanced C++)
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