Re: AAT Theory

Clara N. Fitzgerald (cfitzger@s.psych.uiuc.edu)
2 Oct 1995 01:02:03 GMT

r3dlb1@dax.cc.uakron.edu (David L Burkhead ) writes:

>In article <44e7jk$8qf@news.cc.ucf.edu> clarke@acme.ist.ucf.edu writes:

> Okay. Here's my scenario. ... 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."
It seems that a mostly arboreal ape would have all limbs specialized
for grasping (ie, all 'hands', as chimpanzee). The forelimbs would
develop more of a precision grip, possibly.

> 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
>... 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

> 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.
This does not favor bipedalism over running on three limbs and
carrying something on the fourth. Also, the fossil evidence doesn't
seem to show tools prepared for a future occurence until a couple of
million years after Lucy (usually associated with larger brains).

> Thus, the developing species would gradually spend less time on
>all fours, and more time standing with stick or rock in hand.
I don't think you've justified the 'standing' assumption, certainly
not before Lucy. Unless the proportions of the limbs made quadrupedal
walking near impossible, the problem of balance and the general body
layout (spine and muscles arranged to support body organs, as the
pelvis wouldn't be, and infants riding on backs) should favor four legs,
three if tools were actually being kept and not improvised for an
impending purpose.

> 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.
I have seen, a couple of places, the claim that most species in a
normal environment (no catastrophic environmental changes, etc) endure
for about 6 my. before they change sufficiently to be declared a different
species (for example, many of the creatures which coexisted with Lucy
are almost identical to moderns). What is the impetus that produced
3 (assuming Ramidus if justified) or more genus(es?), totalling at least
a dozen species, since Ramidus? Are the different nomenclatures
justified?

--

-Clara A. N. Fitzgerald     cfitzger@s.psych.uiuc.edu

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