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Re: Humans, 'droids, and convergent evolution



 > Well, is it all so really cut and dried?
 > 
 > I suppose that it isn't inenvitable that an intelligent dino would look
 > a lot like us.
 > 
 > But, I also know there is this thing called convergent evolution where
 > the same basic traits evolved independently several times.

True, but this happens mostly from organisms starting from a similar
state.
 > 
 > And, bipedalism and opposable thumbs seem (so far) to be distinctions that,
 > along with large brains, make _some creature_ likely to evolve into the
 > role we now hold.

Perhaps - but note that the deinonychosaurs and troodontids were
*already* bipedal, so they hardly needed to switch to such a
bizarre, unstable, form of bipedalism as we use.  Avian/dinosaurian
style bipedalism is much more stable.

That is the main problem with the 'roid.

The fact is that the troodontids already did have the relatively
largest brains among dinosaurs.  But this still was below even
the average brain size among mammals - so they had a *long* way
to go to evolve intelligence.

 > be I can't possibly speculate well, but I imagine comparative biologists 
 > would
 > look first at the extant models, because the one argument I hear over and
 > over again from biologists is that forms are a certain way for a good reason.

In the case of human bipedalism, it is one of two ways for a
long-armed, brachiating or semi-brachiating ape to walk about
on the ground, the other way being knuckle walking as used by
several living great apes.

Those apes, great and lesser, that do not use knuckle walking
use a crude form of bipedal walking on the ground (the "raised
arms" gait).  In apes, knuckle walking is correlated with size,
the larger forms being more prone to use knuckle walking, the
smaller forms more prone to use bipedal walking.

In short, absent a counterbalancing tail, which apes had long
since lost, and with arms too long to be positioned for a proper
quadrupedal gait, our ancestors had few alternatives as they
were forced out of the trees.  Since our ancestors were in the
small chimpanzee size range, we sort of defaulted to bipedal
walking, not being large enough to really gain much from knuckle
walking.


A dinosaur which still retained its counterbalancing tail
would never evolve to erect bipedalism - counterbalance bipedalism
is much more efficient.
 > 
 > It strikes me that there are many bipedal dinos who could have evolved in
 > our general way -- maybe the big difference would have been in the tail (not
 > clear we couldn't have had tails), ....

Yes it is, our arboreal ancestors had already lost the tail long
ago.  It was no longer available to us.  Even the typical 'monkey'
tail of the other primate lineages was too small and light to
function effectively as a counterbalance, so it is unlikely that
we could have evolved a bird-like posture even from tailed monkey
ancestors.

The tail was a *big* difference, as it is what *allowed* the
more balanced horizontal bipedalism of dinosaurs and birds.

 > but I think in many key aspects, the
 > creature would have ended up looking a lot like us in size, proportions, and
 > general layout, because bipedalism and opposable thumbs, like everything
 > else in biology, aren't disconnected from everything else (and, their
 > relative importance would possibly magnify their effects on the rest of
 > the body.

Since they were already bipedal, they only needed opposable thumbs,
and a *much* larger brain.  The latter is hard to see how they could
acquire, as it has taken birds a long time to reach their present
state from their dinosaurian ancestors.

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.