Re: AAH update (was: Bipe

Alex Duncan (
6 Jul 1995 19:45:15 GMT

In article <3t7ar6$> Pat Dooley, writes:

> According to the DNA evidence, Chimps and Bonobos are
> our closest relatives, and we are more closely related to them
> than they are to Gorillas. You would never believe that was
> the case based on comparative anatomy.

Have you ever taken an anatomy course? The most astonishing thing about
the anatomy of chimps, gorillas, and humans is how similar they are to
one another. I can think of far more aspects of African hominoid anatomy
in which we are nearly identical to chimps and gorillas than aspects that
are different. As far as humans being more closely related to chimps
than to gorillas -- 1) this notion is controversial, and not as well
supported as you seem to think, and 2) there are a number of
synapomorphies (e.g. early fusion of premaxillary suture) that we share
with chimps and not with gorillas. Are you familiar with the distinction
between apomorphies and plesiomorphies?

> What is even more surprising is the short time frame in which those differences
> emerged. Most of the skeletal transformation occurred in the
> interval between the initial separation from the ape-line, say
> 7.5 mya, and the appearance of fully bipedal Australopithecus,
> say 4 mya.

What exactly do you mean by saying Australopithecus was fully bipedal?
Yes, they were bipeds, WHEN THEY WERE ON THE GROUND (and not in the
trees). However, their bipedalism seems to have differed in some pretty
substantial ways from that of modern humans. Most of the differences
seem to be related to a "compromise anatomy" that enabled terrestrial
bipedalism coupled with tree climbing capacities.

> That is extremely rapid evolution, and could only
> have come about due to a major environmental change; a change
> that somehow seems to have bypassed just about every other
> mammal group in Africa over the same time scale.

How can you say how rapid it is until you've quantified it? To my eyes
the most astonishing thing about the australopithecine skeleton is the
similarity to those of the African apes. Contrary to what you've placed
in other posts, the reorganization of the ape hind limb skeleton to
arrive at an australopithecine hindlimb skeleton is not "major". Fairly
minor changes involving the cranio-caudal length of the pelvis, the
bicondylar angle of the knee, and some details of the foot seem to be the
only real changes that occured. Relatively minor shifts in muscle
placement and size probably accompanied these shifts in skeletal anatomy.
Also, contrary to what you might think, Leakey & Lewin (and Johanson)
are not authorities on functional anatomy. I suggest you read papers by
Stern & Susman and their colleagues.
Finally, this change did not bypass every other mammal group in
Africa. Bovids are arguably among the most successful creatures in
Africa today, and they began their major radiation about the same time we
see evidence for hominid divergence from apes.

Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086