Re: AAT Theory

David L Burkhead (
2 Oct 1995 01:42:23 GMT

In article <44ndmb$> (Clara N. Fitzgerald) writes:
> (David L Burkhead ) writes:
>>In article <44e7jk$> writes:
[ 8< ]
> 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.

"Grasping" yes, but not _manipulating_. Those are not the same
tasks. Again, look at what we actually find. Monkeys and apes
generally use their forlimbs to bring food to their mouths. The
forelimbs are specialized for manipulating even though all four limbs
(and in many cases a tail) are used for grasping.

[ 8< ]

>> 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).

Actually, bipedalism _would_ be favored. Going from a quadruped,
which already has bipedalism as one of its modes, to fully bipedal is
a _much_ smaller step than going to a triped with a single
manipulative limb. We _already_ had bilateral symmetry and changing
that would require _much_ deeper structural changes.

>> 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.

Since we _already_ see this kind of activity in modern primates
(standing for tool use--including driving off predators), I fail to
see how it can _not_ be justified. All I've done is change _emphasis_
very slightly.

>> 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

A "catastrophic environmental change" would have occured when the
original arboreal ape was forced out of its trees. There are a number
of reasons that could have happened, as I detailed before. Also,
species stability is dependent on many factors, part of that is how
much advantage the smaller, incremental changes produce. For
instance, a predator that's already "king of the hill" isn't going to
change much. The _prey_, on the other hand, is going to be in a
different situation (I don't have to outrun that smileodon, I just
have to outrun _you_.) The particular adaptations of humans
(particularly tool use and group cooperation), provide _enormous_
benefit for small changes. Thus, changes would rapidly come to
dominate a community. Isolate communities from each other and they
will quickly grow apart, producing competing species (until one
becomes dominant and widespread, and just out-competes the others).
This seems to match what we find in hominid evolution.

David L. Burkhead

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