Re: AAT Theory

Gerrit Hanenburg (
Tue, 10 Oct 1995 15:03:59 GMT (Tom Clarke) wrote:

>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.

A cursorial animal is an animal that travels fast or far.I think in
hominids the accent is on long distance not speed.
I checked Jane Goodall(1986,p.210) on the daily travel distances of
The average for different individuals ranges from 0.91 to 5.0 kilometers
a day.Chimps are clearly not long distance travelers.
Hunter/gatherers cover much longer distances,but to be honest I'm not sure
if they are a good model for comparison.

>>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?

The only form of quadrupedalism in chimps is knuckle walking.
Compared to the quadrupedalism of (for example) a dog the knuckle walking
of chimpanzees is less efficient.Knuckle walking in chimps is a compromise
between being arboreal and moving on the ground.
The chimp lacks the adaptations that facilitate movement of the limbs in
the sagital plane.In chimps the clavicle attaches the shoulder to the
thorax.In dedicated quadrupeds such as the ungulates the clavicle is absent
so that it doesn't limit the movement of the shoulder.In chimps the thorax
is broad while in other quadrupeds it is deep and narrow.In chimps the
scapula is postioned dorsal while in other quadrupeds it's positioned
lateral,again facilitating movement of the limb in the sagital plane.

>>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.

Well,I think contingencies play a major role in evolution.A future
development may depend on a certain individual living at certain time at a
certain place.
(remember that,next time when you swap the butterfly of Lorenz :-)

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

I was using the chimp as a not entirely appropriate model.
For an arborial ape the transition to bipedalism may have even been easier
since it was not,like the chimp,already partly dedicated to quadrupedalism
in the form of knucklewalking.

>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.

As I said above,the accent is not on speed but distance.Cursors are able to
forage over large areas,seek new sources of food and water when familiar
supplies fail and they can take advantage of seasonal variation of climate
and food sources.
As I understand it,East Africa became much drier and more seasonal
(wet/dry) because it came to be situated in the rainshadow of the Rift.
This caused a more open mosaic environment complicated by a wet/dry
seasonality.(I'm simplifying here).It seems to me that cursoriality(with
the accent on covering long distances) can be of advantage here.

>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.

>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 I knew the answers to these questions I would be a happy man.
But again I believe in contingency.

>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.

Chimps survived as a separate species because they managed to "create" a
niche in the more forested areas.They don't have to become bipedal
cusorials because they don't have to cover long distances.Knuckle walking
seems sufficient for their kind of ranging behaviour in their kind of
I don't know what kept the incipient hominid from interbreeding with its
forest cousin.Why does the Eastern lowland-gorilla not interbreed with the
Mountain gorilla? How do populations become isolated from each other?
Geographically?Ecologically?Behaviourally?Take your pick.

>>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.

Well,the avian dinosaur Struthio camelus is our fellow biped on the African
continent. :-)

Reference:Goodall,J._The Chimpanzees of Gombe.Patterns of behaviour._
Harvard Univerity Press,Cambridge.1986.