Re: An alternative to ST and AAT

Paul Crowley (
Sat, 02 Nov 96 12:17:59 GMT

In article <01bbc7cd$7d98a740$c92270c2@default> "John Waters" writes:

JW> Why does bipedalism need special justification? It happened
JW> in Dinosaurs, Birds, Kangaroos and Hominids. So what? What is
JW> the big deal, Paul?

No one knows how bipedalism started in dinosaurs, birds or
kangaroos. They were all probably quite small creatures at the
time with much more favourable power/weight ratios enabling an
enormous variety of highly plastic responses to innumerable
changes in niche. From the millions of species about the size
of a shrew or a squirrel, a few found it more convenient to hop
or use their upper limbs for other purposes, rather than run on
all fours. Without specific data, no special explanation is

The change from the LCA to the hominids is of an utterly
different nature: there was a fairly large ape with an
established method of locomotion, which it altered to become
bipedal. It was a particular event at a particular time that
calls for a particular explanation.

Firstly, size is a crucial issue. Secondly, we know who our
cousins are: gibbons, orangs, gorillas and chimps. Thirdly,
there are only a limited number of possibilites for the general
morphology of the LCA; we can make reasonable estimations and
more fossil evidence is becoming available all the time. On the
basis of Ardpithecus Ramidus and for many other reasons, my
preference is to assume that it was similar to the modern
chimpanzee. You may prefer another hypothesis, but you should
state it.

PC> But bipedalism and quadrupedalism are two completely different
PC> kinds of progression. If you get better at one, you necessarily
PC> get worse at the other. A change from one to the other must
PC> entail a deterioration (for at least ~100 Kyr).

JW> There would only be a deterioration if the LCA was either
JW> wholly bipedal or wholly quadrupedal. However, if the LCA had a
JW> mosaic of bipedal and quadrupedal characteristics, there would
JW> have been no deterioration.

These are empty words about a purely hypothetical animal. You
have to give us some account of the "mosaic LCA". There's no
such animal in existence now. Nor has any likely fossil been
found. What was its niche? What was its normal method of
progression? How did it relate to orangs, gorillas and chimps?
Where did it sleep at night? How did females carry their
infants during normal progression?