Re: AAT Theory

David L Burkhead (
29 Sep 1995 21:12:15 GMT

In article <44e7jk$> writes:
>In article <44bccj$> (David L
>Burkhead ) writes:
[ 8< ]
>If I may jump in, could you amplify your picture of the series of
>gradual improvesments that lead from the mode of locomotion of the
>common ancestor (presumably ape-like) to true bipedalism?

Okay. Here's my scenario. Remember, though that this is _not_ a
theory. I am not calling it a theory. It is only one way I think
things _might_ have happened. IOW, it's a "just-so" story.

Start with a mostly arboreal ape, an animal that hangs from
branches and feeds on fruits, nuts, leaves, whatever. Since it's
primary feeding mode is to use its forelimb to transfer food to its
mouth, its forelimbs are starting to specialize in manipulation rather
than locomotion--"hands" rather than "feet."

Situation changes slightly. The arboreal ape has to spend more
time on the ground. For reason, perhaps the forests are thinning and
it has to drop to the ground to get to the next tree or perhaps a
stronger competitor has come along to force it either down to the
ground or out to less densely forested regions. On the ground it does
some walking on all fours and some walking bipedally (like modern
apes). Both modes are clumsy, since it's adapted to neither.

At this point we have an ape with hands preadapted to being used
to manipulate objects fumbling around on the ground at least part of
the time. They could, like chimps, use extremely primitive tools
(strip the leaves off a branch and stick them into a termite nest to
draw out a high-protein snack). This tool use could have quickly
extended to defense against predation (Chimpanzees have been seen
doing this--using sticks and rocks to drive off predators). The
arrival of more capable predators would drive this trend farther,
favoring those tribes that develop better tools and better

Extensive tool use, then, as one of this animal's major survival
adaptations, could be favored at the expense of other traits. Those
groups more capable of having tools in hand when need strikes (which
means using at least _one_ of their limbs for carrying it and thus not
for locomotion) would be more secure than those that less so capable.

Thus, the developing species would gradually spend less time on
all fours, and more time standing with stick or rock in hand.

Note that to this point I have postulated _nothing_ that is not
seen to some extent in other primates, all that differs is the
emphasis. However, once bipedalism is established, other traits (such
as Wheeler's results wrt heat rejection) can come into play. These
traits allow access to niches that were not available before and the
new niches would tend to push for greater improvement in the traits.

By this this point we're not terribly far from Lucy, if we
haven't passed her already.

Anyway, that's purely an "off the cuff" story. I made it up as I
went along, and most certainly would not dignify it by even calling it
a hypothesis.

>I am one of those who likes the AAT because I think it provides
>a means for making the transition which is adaptive, providing
>increased fitness, every step of the way. I have difficulty
>seeing how a mosaic (or savannah) enviroment provides the same
>path of increasing fitness. If it did, then it seems to me
>the world would have many more bipedal apes species.

"Argument from incredulity" is a logical fallacy. That you have
difficulty seeing it does not mean it's not so.

As for seeing more bipedal apes, does not that argument work both
ways? If an aquatic existance provides such a nice path for a
quadruped to become bipedal would we not have more bipedal aquatic
mammals? At least something more than . . . none?

[ 8< ]

>> Also, wading is not just how the so-called aquatic ape "got
>> started." It's supposed to have been their primary activity for a
>> million+ years. It's supposed to be a critical factor in why we are
>> bipedal, and you just can't get that from swimming--the adaptations
>> are all wrong. Of course, they're all wrong for wading too, but
>> that's another story.
>Ok, I'm game. Why are the adaptations needed for bipedalism
>maladaptive for wading?

Let's look at the big one: Next time you're driving down the
interstate stick your hand out the window. Hold the hand vertically
in the same posture as a wading human. Feel the wind drag? Now turn
your hand horizontally. Feel how much less the drag is?

_Every_ wading animal (habitual waders, not those who just
occassionally venture into the water) has to deal with water drag.
Wading birds do it by developing long skinny legs that they can lift
essentially straight up and down (and thus not have to drag them
through the water--at tremendous energy cost). Wading mammals (again,
habitual waders rather than those who happen to venture into the water
occassionally) tend to take a more horizontal, lower drag, posture.

Then there's the muscle and bone structure needed to support a
bipedal terrestrial animal (i.e. humans). While it may not be
"maladaptive" to wading, in the sense that it actually hurts a wader
to have it (I don't know enough to hazard a guess as to whether it's
actively harmful), but it is not helpful. Since it is not helpful to
be able to support one's full weight while standing in chest deep
water, there's no pressure to _develop_ the capability.

David L. Burkhead

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