From: my...@netaxs.com (PZ Myers)
Subject: Re: AAT: Can Sherry address the argument w/out changing the subject?
Date: 1998/09/19
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In article <6tq5n0$se...@nnrp1.dejanews.com>, myke_reyno...@yahoo.com wrote:

[snip]

>
>I'd like to see an objective rebuttal of what I think
>is the most compelling argument for AAT, which is the
>morphology of pelvic bones in formerly terrestrial
>aquatic and semi-aquatic marine animals. (I should
>say formerly non-aquatic animals, but the statement
>"formerly non-aquatic aquatic and semi-aquatic marine
>animals" is ambiguous and difficult to parse.)
>
>This assertion is based on comparing the orientation of
>legs to torso. All large terrestrial limbed vertebrates
>(besides humans) have between a 45 degree and 90 degree
>angular separation between the orientation of the legs
>and torso, and the lower limbs are oriented along a
>vertical plane (in the belly down positioning of the body).
>
>By contrast, many formerly non-aquatic marine animals
>have legs that are oriented in the same direction as the
>torso, or they had an ancestor with such an aligned
>orientation before loosing their lower appendages. The
>fossil remains of the intermediate evolutionary stage of
>whales were discovered not too long ago, and they had legs
>that were oriented in the same direction as the torso, swept
>back behind the body, just like pinnipeds.
>
>Other aquatic animals have limbs directed straight out
>to the sides, but this is still similar to the straight
>leg-torso aligned marine animals, and distinct from the
>vertical limb orientation of most terrestrial animals, in
>that the limbs are oriented along a horizontal plane rather
>than a vertical plane the way ALL terrestrial *mammals*
>are.
>
>In semi-aquatic marine animals of this category, this
>results in extremely cumbersome terrestrial locomotion,
>where the animal has to crawl on it's belly because it
>can't get it's legs underneath itself in the 90-45 degree
>orientation of non-marine vertebrates. Penguins (and
>potentially humans) are the only exception to this prone
>posture in terrestrial locomotion for this class of
>animals. (Sea otters are also an exception, but this is
>due to cat like arching of the back to it's limits.)
>
>Some animals have their femur and humerus aligned along
>a horizontal plane, but the limbs are not straight, they
>are bent precipitously at the elbow and knee joints,
>entirely unlike the majority of marine animals who have
>fully horizontal limbs that are straight and unbent. This
>category of animals with a horizontally aligned femur
>and humerus, but precipitously bent knees and elbows
>consists mainly of small reptiles and amphibians. *No*
>mammals are in this category. The only large animals
>I can think of that fit this category are alligators
>and crocodiles.
>
>In summation, only marine adaptation is known to cause
>an animal's limbs to evolve from being aligned along
>a vertical plane relative to the torso, to being
>aligned along a horizontal plane. Two types of
>horizontal plane orientations exist, straight back,
>and laterally out to the sides. Only the straight
>back variety could potentially evolve into a fully
>upright biped. (Note that no bipedal bird or dinosaur
>has a fully upright posture. They conform to the
>terrestrial orientation of a 45 to 90 degree angular
>separation between legs and torso, entirely unlike
>marine animals with limbs aligned along a horizontal
>plane.)
>
>Here is a list of animals who have their legs oriented
>in the same direction as their torso, or who had an
>ancestor with such: all Cetacea, all Sirenea, all
>Pinnipedea, sea tortoises, pleisiosaurs, sea otters,
>penguins and humans.
>
>With the exception of humans, all other animals with
>legs that are oriented in the same direction as their
>torso, or who had an ancestor with such an aligned
>orientation, are all aquatic or semi-aquatic marine
>animals, with humans being supposed as the singular
>exception by the anti-AAT argument.
>
>Similarly, humans are the only supposedly non-marine
>animals with subcutaneous fat. With the exception of
>humans, there are no non-marine animals with subcutaneos
>fat or straight leg-torso alignment. Aquatic adaptation
>provides the only positively known means for producing
>selective pressures that induce the formation of
>subcutaneos fat and a straight leg-torso alignment.
>
>The alternative argument is that straight leg-torso
>orientation in hominids evolved from pressures that
>give gibbons a different pelvic structure. But gibbons
>do not have anything even remotely approaching a
>straight leg-torso alignment, nor has this ever been
>shown to occur in *ANY* non-marine animal. So in effect,
>the anti-AAT argument poses pure supposition without
>evidential support as being entirely superior to the
>only positively known mechanism for the development of
>subcutaneos fat and straight leg-torso alignment.
>

[snip]

Tsk, tsk. We went all over this a year or so ago, Mr. Reynolds.
This whole line of argument, that you can use the orientation of
the legs relative to the spine, as evidence for the aquatic ape
hypothesis is simply silly. It's an extremely superficial analogy
that requires a stubborn refusal to actually look at the limbs of
these animals.

You want to argue that the orientation of the legs is a functional
adaptation that is only seen in swimming animals. You also insist
that it has to be that way, because otherwise humans are the odd
mammal out, and for some reason, that bothers you. Unfortunately,
there is nothing in human pelvic and limb anatomy to suggest any
convergence on the aquatic mammal plan. There is also no logical reason
why one particular species ought not to be allowed to have a unique
evolutionary history.

There are a set of anatomical changes to the limbs of marine mammals
that seem to be reasonable functional adaptations. As you've noted,
hindlimbs tend to be modified to provide propulsive force along the long
axis of the animal, along the axis of movement, rather than perpendicular
to that axis, to provide support against the force of gravity. The fact
that humans also have hindlimbs oriented parallel to the body axis seems
to be the sole comparison you want to make. However, in our case the
pelvis exhibits a number of traits that represent adaptations to allow
the limbs to support us against the pull of gravity, not to propel us
through the water. And the adaptations found in marine mammals may enable
it to swim well, but they certainly aren't the beginnings of a useful
exaptation that would allow one to walk bipedally on land. When was the
last time you saw a seal hop up on its hind flippers and walk around?

Look at the bones of a seal or otter. They have a pelvis that is typical
of other Carnivora, elongate longitudinally and forming a kind of dorsal
plate. The main differences from a 'typical' mammalian pelvis are a trend
towards reduction of the ilium and the absence of a pubic symphysis -- the
whole bone is being minimized. It is no longer needed as an important
brace for supporting the weight of the animal, and isn't even providing
an attachment for powerful muscles needed in locomotion. It is fading
away, and you can see that in the bones. 

Now look at a human pelvis...there we see functional adaptations along a
completely different line. Compared to a cat or a chimp, there is a
change in orientation -- it's less of a dorsal plate, and is more compressed
longitudinally. The ilium is broad and wide, and is an important attachment 
point for muscles critical in maintaining our posture. The pubis and ischium
are robust and form a strong symphysis. The pelvis is a solid ring of
bone that cradles our guts and links legs to spinal column. 

Compare a pinniped and a human, and you can see that they've been going
in completely opposite directions! Our hindlimbs aren't adaptations for
better swimming. They don't exhibit any of the anatomical characteristics
that are common in aquatic mammals, but they do seem to function pretty
well in terrestrial locomotion. I would argue that if we *had* gone partway
down the road to an aquatic lifestyle, that would rather effectively 
cripple our ancestors and prevent them from returning to the land. How
successful do you think a seal or a sirenian would be if it decided to
give up on the sea and hunt or graze on the land?

-- 
PZ Myers