Re: Dissecting the Aquatic Ape: Bipedalism

HARRY R. ERWIN (herwin@mason2.gmu.edu)
26 Jul 1996 18:38:33 GMT

Paul Crowley (Paul@crowleyp.demon.co.uk) wrote:

---trimmed---

: Knuckle-walking may be unusual; that does not mean it's "very
: specialized". A Ford Edsel may be unusual without being
: specialized; a refrigerated truck may be specialized without
: being unusual. The number of morphological and behavioural
: changes necessary to convert a quadrupedal palm-walker (such as
: a baboon, or a monkey) into a knuckle-walker are far less than the
: number it would need to become a biped. A palm-walker could well
: occupy the same niche as a knuckle-walker (chimps are not *that*
: different from baboons). OTOH no one has begun to outline the
: niche occupied by the bipedal hominid.

Well, monkeys are pronograde arboreal quadrupeds, so they palm-walk on the
ground in the same way they palm-walk arboreally. Advanced hominoids are
orthograde suspensory types, so their natural posture on the ground would
be erect or semi-erect hand-aided bipedalism (fist-walking). For reasons
almost certainly involving constraints posed by how motor programs work in
the CNS, fist-walking can't be very fast (you can't gallop or run unless
the limbs involved can work autonomously and with low probability of being
damaged in the process). Hence for speed on the ground, you'll abandon the
fist-walking.

The niche is large-bodied suspensory feeding in open-canopy forest. This
differs from large-bodied suspensory feeding in closed-canopy forest in
that you can't bridge from tree to tree (as can Pongo or Hylobates).
Instead, you have to climb down out of the tree to move to another tree.
In relatively closed open-canopy forest, a bipedal stance doesn't give you
useful early warning of attack. That set of selective pressures leads to
Pan and Gorilla. In more open forest, or in close forest if the understory
is not overgrown, the height above ground of the sensory organs in a
bipedal posture is useful in giving early warning of attack, and you get
various hominids.

: > 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.

: Is this true? You're assuming that (large?) apes originally were
: 100% arboreal - like gibbons.

The Fayum primates appear to be (nearly) 100% arboreal. The early
hominoids (Proconsul and its relatives) are fruit-eating pronograde
arboreal quadrupeds from closed-canopy forest biomes. It is suggestive
that the brachiator, Oreopithecus, appears to trace back in ancestry
directly to close relatives of Proconsul. Hylobates appears to stem from
the same group. Dryopithecus, the earliest orthograde suspensory hominoid
that we have good evidence for, had no significant adaptation to
terrestrial movement.

: Why? I can see no reason why a
: substantially ground-based existence, especially for a larger
: animal, should be ruled out. Many primates (e.g. baboons) are
: mainly ground-based today. Why should the general pattern have
: been different at any time during the past 65 Myr? The ancestral
: ape certainly acquired a brachiating capability but that does not
: mean that that was its main activity.

The niche occupied by the earliest monkeys (catarrhines) appears to
involve food collection on the ground but taking refuge in the trees to
chew it.

: > : 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.

: I strongly disagree with you that behavioural features are labile.
: They are always intimately tied into niche, and IMHO in evolution
: niche is everything.

Behavioral features can change much more quickly than phenotype.
Behaviorally based speciation can take as little as 1000-2000 years.
(Cichlids in Lake Nabunago, fruit flies in Hawaii.) Why this occurs is
interesting, involving how learning and selection interact in defining an
ESS. I have an old paper on the subject from back when I was doing ESS
theory (1980-1990). 

--

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++)