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From: friesen@psivax.UUCP
Newsgroups: net.bio
Subject: Re: Bipedalism
Message-ID: <1196@psivax.UUCP>
Date: Mon, 19-May-86 11:05:40 EDT
Article-I.D.: psivax.1196
Posted: Mon May 19 11:05:40 1986
Date-Received: Fri, 23-May-86 06:56:41 EDT
References: <1058@h-sc1.UUCP> <1180@umcp-cs.UUCP> <533@bcsaic.UUCP>
Reply-To: friesen@psivax.UUCP (Stanley Friesen)
Organization: Pacesetter Systems Inc., Sylmar, CA
Lines: 44

In article <533@bcsaic.UUCP> michaelm@bcsaic.UUCP (michael maxwell) writes:
>
>If I may revert to my original question, I agree that there is a lot of
>"reason" for bipedalism.  In addition to the usefulness of hands as hands, I 
>had heard it alleged that bipedalism was "useful" to the dinosaurs as a means 
>of running faster, i.e. bipeds are (in general?) faster than quadrupeds.  The
>question then was, why aren't more mammals (outside of man and kangaroos, and
>chimps with polio) bipeds?  Bipedalism arose in two orders of dinosaurs (or at
>least so I thought--more on this in the next paragraph), while it is almost 
>nonexistent in mammals.  If bipedalism is so useful, why is it so rare today? 
>
	A large part of the answer comes from realizing that there is
more than one solution to any problem. For every problem that
bibedalism can solve there is also a solution based on a quadrupedal
gait. This multiplicity of solutions is so widespread that it is cited
as a principle of evolution by some authors.

>One poster (I've seen a quote of part of his article, but not the original)
>said that the dinosaurs arose (no pun) from ancestors which were already
>bipedal, whereas the mammals arose (declined?) from quadrupedal ancestors. 
>If this is true, then I was incorrect in thinking that bipedalism arose 
>(independently) in two orders of dinosaurs, and this is probably all the 
>explanation I need.

	This is indeed the case. But the question still exists. Why
did the ancestors of the dinosaurs develope bipedalism and the
ancestors of the placental mammals go for walking on thier toes! I
believe that the principle of "preadaptation" is important here. That
is a transitional form must itself be advantageous to the organism.
The shift to bipedalism requires a differentiation between the fore
and hind limbs. The early archosaurs were large aquatic reptiles
similar to modern crocodiles(they are called Proterosuchians or "first
crocodiles"). They evolved enlarged hind limbs for power in swimming,
thus making the shift to bipedalism simple and straightforward. The
primate ancestors of humanity were arboreal forms in which the fore
limbs were adapted for brachiating, thus making the shift away from
a pure quadrupedal gait. The next step, true bipedalism, was thus made
possible.
-- 

				Sarima (Stanley Friesen)

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