Re: AAT Theory

Gerrit Hanenburg (
Mon, 9 Oct 1995 16:02:16 GMT (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),
bipedal plantigrade(Homo erectus),bipedal digitigrade(Ostrich).
The kind of cursorial locomotion the animal comes to "prefer" depends on
the kind of animal it was before it became cursorial.If it was already a
dedicated quadruped then the investment to become bipedal is large.If it
was *not* a dedicated quadruped,like a chimpanzee,the investment to become
bipedal is much less.Hominids are in fact cursorial apes.

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.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).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)

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.

References;Rodman,P.S.and McHenry,H.M.1980,Bioenergetics and the origin of
hominid bipedalism. Am.Jour.Phys.Anth.52,p103-106.

Aiello,L.and Dean,C.1990.An Introduction to Human
Evolutionary Anatomy.Academic Press.