From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Thu Jul 11 2002 - 09:00:34 MDT
Eliezer wrote:
> Perhaps some earlier single-celled organisms had a preferred vertical
> orientation, ... [snip] Still, I'm not sure such a physical vertical
> orientation can be regarded as the legitimate ancestor of our nervous
> system's assumption of a preferred vertical orientation in visual,
> spatial, and motor processing. If we're talking about "our" evolved
> preference for having things be right-side-up, I think it should be traced
> back to the earliest nervous systems, but not traced back billions of years.
Actually Eliezer, there is a lot more preference for environmental
cues than one might expect. Its looking like there are at least
dozens, if not hundreds of genes whose regulation is are dependent
on the daily "clock" which are quite organ specific. There are also
cyanobacteria that manufacture light absorption pigments that are specific
to water depth and they actively work to maintain that themselves at the
optimal depth. Furthermore there are clearly cells that have a "polarity",
e.g. intestinal cells that absorb nutrients on one side and export them on
the other side.
So it would not surprise me at all if there are sensors and response
mechanisms within biological systems with respect to whether one is
right-side-up or right-side-down. There clearly are gravity/stress sensors
(look at what happens to persons exposed to long term weightlessness).
Whether they are orientation specific remains to be determined.
I'd suspect these responses would be strongest in species most exposed
to conditions that might turn them upside down in potentially life
threatening situations (birds/bats?) and less developed in species
that might be able to fly inverted (dragonflys/other insects??).
While there may be some primates that have to function inverted
on a regular basis (e.g. hanging upside down to pick fruit), I
don't think the selection effect on humans would function to
preserve that. Most likely human discomfort with the upside
down position is due to it simply being unfamiliar to most
people. In any case, since suspended individuals are in genetic
stasis the issue of whether the are heads up or heads down would
not be a useful point of debate (anyone who wants to debate it
doesn't understand how gene regulation works or what the term
"frozen" means).
Robert
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