Re: Aquatic Ape Theory

Thu, 28 Apr 94 06:33:58 GMT

> rh@ishmael.UUCP (Richard Harter) writes:

>>: >Have you looked? Are you familiar with the work of biologist
>>: >Pete Wheeler, for example? Wheeler's "stand tall and stay cool"
>>: >hypothesis explains the origin of bipedalism, the loss of hair
>>: >, the increase in body fat AND sweat glands.
>.... tedious text by RH deleted
>might have said, or rather what I suspect it did not say.

>Let me see if I can put this simply. Arboreal mammals will indeed have a
>certain amount of pelvis rotation and "pre-adaptation to bipedalism".
>Selection for this is a natural consequence of the mechanics of living
>in trees. There are many instances of arboreal mammals walking more or
>less upright when they are on the ground.

Not arboreal mammals in general. Arboreal apes which are hind-limb
dominant to begin with.

>This means less than it might seem. Walking upright is an expensive
>proposition for animals which are not fully adapted to it for at least
>two reasons. The skeletal structure is not load bearing (which implies
>that the load is taken up by muscle power) and it is mechanically unstable
>(balance must be maintained dynamically). It is cheaper for an arboreal
>animal which is moving to an non-arboreal niche to revert to a quadruped
>posture than it is to move to an upright posture.

How do you know it is cheaper? Is this based on data or is it an
assumption on your part?

>Why then do sundry arboreals walk upright? The answer is simple. They
>don't do very much of it. Given the existing skeletal structure and
>adaptations it is cheaper to walk upright and pay the associated cost.
>However the selection pressures for an aboreal animal which is invading
>a land niche is to lose the "bipedal" adaptations and revert to the energy
>efficient quadruped posture. And this is in fact what we see, with one
>notable exception. [The pan/homo common ancestor is not the only mammal
>that had to deal with a transition from trees back to the ground.]

Both Gorilla's and Chimpanzees knuckle-walk. The hominid-pongid ancestor
did not knuckle-walk. Additionally, chimpanzees and gorillas remain in
the forest and do not as a rule venture out onto open grasslands.

>Note also that the full upright posture is not adaptive for arboreal animals.

According to who? What are you basing this on?

>The question is then, what happened in this exceptional case. I don't
>know. What I guess is that it was a combination of several things. The first
>is the relevant ancestral primate became effectively an obligate biped due
>to limb specialization. It was small (energy considerations). And it
>moved into an unusual physical environment (but not aquatic!).

Circular. Limbs become specialized due to bipedalism. Apes become bipedal
due to limb specialization. I agree with the last sentence.

>It seems to me to be clear that the ancestor must have already been an
>upright biped before it invaded the Savannah environment proper. Nicholls
>cites Wheeler's work on cooling as a selection pressure for bipedalism.
>IMHO this is surely wrong -- the energy costs of being a quasi biped far
>outweigh the cooling advantages (to say nothing of there being alternatives).
>What is true is that a creature which was already an upright biped would
>(and did) have advantages in moving into the Savannah.

Again, do you have data or is this all just supposition?

I have data. A study done by Rodman and McHenry in 1980 measured the
energy costs (measured as amount of oxygen consumed per unit of body
weight per distance traveled) and found that bipedalism (walking) and
quadrupedalism in chimpanzees and that at walking speeds the chimpanzee
consumes 150% more energy than an equal sized quadrupedal mammal. Since
chimpanzees are knuckle-walkers, they are probably better quadrupeds
than the common ancestor.

So if you actually look at the available data, instead of making up facts
as you go along, you discover that there is little difference in the
energy costs. Therefore, any advantage provided by the ability to
withstand heat stress will push selection toward bipedalism, since
(a) a standing bipedal ape exposes less surface area to the sun and
(b) standing promotes evaporative cooling.

Wheeler has measured heat absorption and cooling rate on ape shaped
models and found this to be the case. Stand tall and stay cool.
Exploit an a new environment that is rapidly replacing closed forest.

>There are a couple of reasons for believing that an unusual intermediate
>environment was involved. The normal outcome for an obligate arboreal
>biped would be to become extinct if the arboreal environment disappeared.

The arboreal environment didn't disappear.

>[Most species go extinct if the environment changes.] If our current
>understanding is correct, pan/homo split with the rift valley separation,
>with the homo ancestor being in the Eastern side which became arid.
>Aridification is a very common event in geological history. Since the
>upright posture is novel and aridification is common we have to look beyond
>the normal environmental changes consequent to aridification.

The paleoclimatology points to a warmer, drier climate but there was plenty
of forest and standing water.
>That said, the "unusual" character of the niche need not have been
>superficially spectactular. It may have been something as simple as a bush
>with plentiful fruit that could only be reached by standing up.

Several anthropologists have suggested this may also have been a reason for
bipedalism. There is an article in the Journal of Human Evolution last
month on this topic.

>In the course of thinking about this I did come up with an interesting
>alternative which I submit for your consideration -- The Rock Climbing
>Ape Theory. When you climb vertical rock faces it is important to keep
>your body flat against the rock, i.e. to assume the full upright posture.
>If we assume that the ancestor lived in caves in the faces of cliffs there
>would have been a very strong selection pressure for upright bipedalism.
>As evidence for this hypothesis, I point out that primates are used to
>climbing things -- when trees disappear rock climbing is the only thing
>left. As further evidence, note the popularity of rock climbing in humans
>today. This is clearly a remnant of a phase in our history when we were
>cliff dwellers.

Australopithecus afarensis, the females at least, retained some tree
climbing ability. As I have pointed out, the forests didn't disappear
and there is no evidence that early hominids slept in caves. It is
much more likely that they slept in trees.

>I have fond hopes of converting Philip to the rock climbing ape theory.
>IMHO it is fully as sound as the aquatic ape theory, and is as equally
>well supported by the evidence. I expect that he will disagree with the
>arguments I have advanced. I will apologize in advance for not continuing
>the argument in detail -- there are constraints on the amount of time that
>I can spend on usenet postings.

One one point we agree. The Rock Climbing Ape is about as sound as the
aquatic ape theory and is equally supported by the evidence, which is to
say, not at all.

Philip Nicholls "To ask a question,
Department of Anthropology you must first know
SUNY Albany most of the answer."