Re: potassium and Tears 2

J. Moore (
Sat, 4 Nov 95 11:42:00 -0500

Ja> >So far it looks like potassium gives no preference to
Ja> >any kind of lifestyle whatsoever.
Ja> >--
Ja> So the fact that we actively excrete potassium in our tears does not
Ja> exclude the possibility of a marine or aquatic ancester. It can't be a
Ja> marker for a purely terrestrial lifestyle.

Ja> Phew! You definitely had me worried with that one for a bit, Jim!
Ja> James Borrett.

Looks like I have to repeat things you didn't understand the first
time 'round:

"The sodium chloride-excreting gland of marine birds and lizards
must have been an adaptation to this particular habitat which
involved the loss of the ability to secrete potassium and
bicarbonate" (Peaker and Linzell 1975:262).

The "salt" glands of these terrestrial birds and reptiles, *unlike*
those found in marine birds and reptiles, are used primarily to
excrete potassium. "In other words the secretion resembles that
of the ostrich and differs markedly in composition from that of
marine birds and reptiles" (Peaker and Linzell 1975:247-8).

Tears are *not* hypertonic in regard to sodium but *are strongly*
hypertonic in regard to potassium. In this regard, the human
lacrimal glands call to mind the "salt" glands of terrestrial
reptiles and birds and *not* the salt glands of marine birds and

So the actual observed and measured excretions in human tears
far more closely resemble the excretions from the "salt" glands of
terrestrial birds and reptiles than those of marine birds and
reptiles. Thus for human tears' excretory abilities to indicate a
terrestrial background requires no change. For their abilities to
indicate a marine background requires not just one but two massive
changes, starting and ending with a passive secretion of sodium.

This idea is certainly far less parsimonious than the idea that
the observed and measured active secretion of the lacrimal gland
indicates our actual environmental past habitat, which evidence
from tears suggests would be terrestrial rather than aquatic.


So you have the choice of either going with the idea of
a land-based lifestyle for hominids, or the *extremely
non-parsimonious* idea of an aquatic lifestyle.

Your brain -- your choice.

********************** References **************************************

1975 *Salt Glands in Birds and Reptiles* by M. Peaker and
J.L. Linzell (Dept. of Physiology, Agricultural Research Council,
Institute of Animal Physiology, Babraham, Cambridge). Monographs
of the Physiological Society No. 32.
Cambridge University Press: Cambridge, London, New York, Melbourne.

Jim Moore (

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