Re: AAT Theory

Tom Clarke (clarke@longwood.cs.ucf.edu)
9 Oct 1995 22:50:00 -0400

ghanenbu@inter.nl.net (Gerrit Hanenburg) writes:

>clarke@acme.ist.ucf.edu (Thomas Clarke) wrote:

>>If the general forest/savannah paleo-environment of Africa favored
>>bipedalism, then where are the other bipedal animals?

>This environment did not favor bipedalism in particular.It favored
>cursorial locomotion.And an animal can become cursorial in different ways.
>Quadrupedal digitigrade(cheetah),quadrupedal unguligrade(wildebeest),

The environment favored running? Perhaps so, but the animals you
cite are all fast runners. In the short run they can outrun man,
so the selection factors which led to man the long distance runner
may have been different from those that led to the sprinters.

>Bipedal locomotion in humans is more efficient then quadrupedalism in
>apes.Apes are bad quadrupeds especially on the long distance.
>However the study of Rodman and McHenry(1980) has shown that for a
>chimpanzee walking bipedally is no more and no less energetically expensive
>than walking quadrupedally.

This is not what I have read. Are they perhaps distinguishing between
quadrapedal locomotion and knuckle walking?

>If selection for cursorial locomotion was
>significant then any slight variation in the direction of more efficient
>bipedalism (compared to quadrupedalism) could have tipped the balance in
>the direction of a bipedal cursorial ape,no matter how inefficient it was
>initially compared to modern humans (we are virtuosi of bipedalism).

Well if the chimp is the starting point and it is equally bad
bipedal and quadrapedal, then it is a roll of the dice whether evolution
will drive it toward better quad- or bipedalism.

The starting point is supposed to have been an arboreal ape for which
we have no data about relative quad- and bi- efficiencies.

I still have trouble with the arboreal - non obligated quad as you say -
being forced to live on the ground by loss of forest and finding the
bipedal route better than the quadrapedal route. If the environment
forces them to run, they will be pretty poor runners and not survive
very well.

I still like the
idea of a transition environment, but perhaps it was just a toss of the
dice. The first random mutation that made a better runner was in the
bi- direction.

>The
>anatomical changes a chimpanzee-like ape has to make in order to become a
>bipedal ape are not as astronomical as is often suggested (see Aiello and
>Dean,1990,p.247)(the major modifications required for bipedalism in
>chimpanzees that they mention are: a greater angle between the ilium and
>ischium and the lumbar curve of the spine)

As Diamond argues we can be considered a third species of chimpanzee, we
are so close. However, the starting point wasn't the chimp, but an
arboreal ape. One branch of descent became the knuckle walking chimp
and the other world class bipeds. Why one and not the other?
Why the speciation? If bipedalism is so efficient, then how did the
chimps survive as a seperate species? Because they would be such
bad runners at first they couldn't ventur very far out into the cursory
(spelling joke) environment at first. As they got to be better runners
they could range farther etc. But meanwhile what kept them from
breeding with their forest cousins, bring the benefits of bipedalism to
all? After all, the speciation event took only
about two million years we now know.

>The main reason why no other animals became bipedal was probably the fact
>that the other animals were already dedicated quadrupeds who couldn't
>afford a switch to bipedalism (it was easier for them to become a cursorial
>quadruped).Apart from that,as far I know their is no evolutionary law that
>states that a "good trick" discovered by one species will also be hit upon
>by another.

No law of course, only the observation of the convergent evolution
of similar traits to fill similar niches in vastly different species
and locales.

Tom Clarke

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