Re: An alternative to ST and AAT

Phillip Bigelow (
Sat, 09 Nov 1996 22:33:00 -0800

Thomas Clarke wrote:
> But a kangaroo rat is more alike in saltation to a kangaroo,
> than either is to a human's bipedal locomotion.

That was my point, also. A human's locomotion is very closely similar
terrestrial locomotion to that of a brachiating primate on the ground.
Similarly, chimpanzeees are (in general) quite similar in locomotion
(occasionally using bipedal locomotion) to that of a human. There is no
*real* "unique-ness" to human bipedalism. It is just a matter of style
degree, that's all.

Tom, closely related taxa have closer similarities than any of them
do to distantly-related taxa. This phenomenon is well-known, and it
forms the basis for the science of cladistic phylogeny.
You are (unknowingly) making my point! Thanks!

> >> > There were lots of bipedal (tripedal?) dinosaurs.
> >Were they bipedal in EXACTLY the same way?

> Again. Tyrannosaurus moved more like a velociraptor, than either resembled
> a hominid in locomotion.

And again, you are proving the point that I was making.
(Tyrannosaurids + dromaeosaurs) have closer similarities to each other
either of them do to (humans + other primates). Similarly, (humans +
primates have closer similarities to each other (such as locomotion
than either of them do to (tyrannosaurids + dromaeosaurs).

> > Brachiating arboreal primates "walk like a
> >man" when they find themselves (infrequently) on the ground.
> >The only difference is that, morphologically, they don't
> >locomote EXACTLY in a human style gait. But that is to be
> >expected. After all, Tom, they ARE different species.

> Do they have locking knees?

No, but they still walk bipedally. Remember that we are talking about
terrestrial bipedalism in general. Locking knees is an apomorphy of
hominids (a matter of degree away from the more primitive condition in
other anthropoids).

> This type of discussion is, of course, a matter of degree.
> Can a brachiating arboreal primate walk twenty miles?

Hell, Tom. We don't even know if A. AFARENSIS could walk 20 miles!
You are damn-right it is all a matter of degree! That was my point.

> I just perceive a larger difference than you do, I guess.

You apparently haven't done a lot of comparative anatomy then.

> Actually, as I think about this, I will concentrate on the
> knee. I think the human (australopithicene?) knee is unique.

You think wrong.
This knee-joint style isn't unique in the animal kingdom. It is
just an apomorphy in primates. Curiously, though, that in the other
animals that have a graviportal, lock-knee style posture, these
animals are all terrestrial. Not an aquatic one in the bunch! :-)

You want a REAL example of a "unique" morphology? The maniraptoran
The feature is not shared by any other member of the animal kingdom, yet
members of the clade have a *roughly* similar hand.

> Now my question can be more precise. Do ostrich's and storks
> have unique anatomical features related to their style of walking?

Of course. But the plesiomorphy in this situation is that, in spite
of their different locomotion styles, they all had an ancestor that
was bipedal. Only their *specific* style of walking is unique
to each bird.

> >Example: Rheas and ostriches both shared a common adaptive
> >circumstance that led to their loss of flight (geographic
> >isolation; abundant ground food; loss of major predation).
> >Yet, these runner-specialized birds have a different pedal
> >count (three for the rhea; two for the ostrich), and a different
> >stride pattern and different stride length.

> I would argue that number of toes and stride length/pattern
> are not essential differences.

And I would argue that primate stride style is similarly not
that different enough to propose that one of their members (hominids)
has a "special" origin.

> >Similar environmental pressures for each of them; yet two different
> >morphological outcomes.

> As I said, I don't see the significance of the morphological
> differences.

And, similarly, I don't see any need to make a special pleading
for hominids, either.

> Even the human brain can be seen as just an enlarged
> primate brain.
> Howeever, the human foot and human knee are a lot
> different from the corresponging primate features.

Only in degree. As I noted above, you apparently haven't
done a lot of comparative anatomy. The differences between
the two are large IN CERTAIN ways, but it's only a matter of degree.
In gross morphology, a human foot still roughly approximates a
foot. Remember what I wrote about cladistic analysis (above)?
Similar differences can be found between other closely-related species.
There are significant differences in hyal morphology between two
species of sloth. The sloths in question are (Bradypus and Cloepus).
Countless other examples can be given.

> If the circumstances leading to the unique species of which I am
> a member were not unique, then why did it not happen before?

How can you be so *sure* that it hasn't happened in other fossil primate
taxa before? How many more Miocene and Pliocene primate fossils
remain to be discovered?