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From: rengler@hvrunix.UUCP
Newsgroups: sci.bio
Subject: Re: Human asymmetry
Message-ID: <324@hvrunix.UUCP>
Date: Sun, 5-Apr-87 13:48:18 EST
Article-I.D.: hvrunix.324
Posted: Sun Apr  5 13:48:18 1987
Date-Received: Wed, 8-Apr-87 00:13:34 EST
References: <586@batcomputer.tn.cornell.edu>
Reply-To: rengler@hvrunix.UUCP (Rich Engler)
Distribution: sci
Organization: Haverford College, Haverford, PA
Lines: 50
Summary: Answer in evolution

In article <586@batcomputer.tn.cornell.edu> plimpton@batcomputer.UUCP (Steve Plimpton) writes:
>Humans (all animals?) are asymmetric right vs. left.  Namely, my heart is on
>my left side, my appendix on the right, liver someplace, etc., etc.  And a
>nurse told me virtually everyone (99%+) is the same.  My question is, given
>that we all start from a one-cell symmetric egg, how do we all develop the
>same asymmetry?  It doesn't puzzle me (as much) that we are asymmetric at
>all, but that we are all the same!  That is, why aren't 50% of the people
>around mirror images of me with their heart on the right side?  In physics
>lingo it seems like a violation of the law of parity.  I guess what I'm

You've made some very interesting points, especially about the law of parity.
I don't know why humans have evolved with this asymmetry, the fact is that we
have.  Since, somewhere in our evolutionary past, one of the forms began to
develop asymmetrically, this is where you should look for the answer.  After
that point, it makes perfect sense that we would continue to show the same
patterns of development, the ones developing as "mirror images" would be
mutations of a sort.    

You also have to consider that there has to be some inherent asymmetry, due to
the nature of our bodies (blood going in one side of the heart, coming out the
other, etc.).  Of course that doesn't explain why it favors one side or the 
other, just that it has to be one side.

>really asking is, that as the fetus cells are busily dividing along, how do
>ones on the left know not just that it's time to become heart cells, but
>that they're not on the right? 

This is simple.  Once the pattern has developed, it is easily reproducable
with the body's biochemical mechanisms.

>Wild guesses (in descending likelihood):
 ^^^^^^^^^^^^
Exactly!  They are all pretty much wrong.
>
>1.   I know there's left and right-handed proteins and humans are all made
>     of only one kind (which?).  So maybe this low-level asymmetry works its
The body uses L (left-handed) forms of amino acids, the D (right-handed) form
of sugars (notably glucose), and B-DNA (a right-handed twisting).  I very 
much doubt that this is the source of assymetry.
 
Guesses 2,3, and 4 are just wild!

Some biologist may just tear this opinion to pieces.  This is the theory of
a chemist with some biochemical knowledge.

-- 
				Rich Engler "The Wanderer"
				"Did I err?" -Groo

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