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Re: Breathing

A while ago, I wrote:

>the condition prior to primitive lungs in vertebrates was gills.<

I've been corrected on that point by a couple people, on and off-list,
although I remain curious about the evolutionary and physiological factors
that apparently *precluded* (?) in terrestrial vertebrates the true
counterflow respiratory systems (aka gills) seen in more primitive
vertebrates.  A couple of posts I received are relayed below. Thanks to
those who responded and increased my still woefully inadequate understanding
of this issue:

>>The oldest known jawed vertebrates had *both* structures! <<

The condition of being a "jawed" vertebrate is not one I made, but the point
is taken.  I still hold to my conditional acceptance of the "previous
anatomy" argument, however, since, if the vertebrate lung is not derived
from the gill, at some point in vertebrate evolution an innovation had to
occur to produce the terrestrial vertebrate lung.

And, from another:

>>the vertebrate lung has a different embryological and evolutionary origin
than the fish gill, and started off as a blind-ended outpocketing of the
anterior portion of the gut.  It has been such a structure ever since, with
only one opening to the outside and with air coming in and out of the same
opening.  Subsequent modifications have been primarily to the pouch and not
the connecting tube; the original paired single chambers have been
subdivided and subdivided again, primarily as an attempt to increase the
diffusional surface area. <

I'll open myself up to correction again and ask why an (arguably) close to
optimal gas exchange sytem (gills), which evolved in a fluid medium that was
approximately 50% oxygen (H2O), needed to be abandoned for a *less*
efficient exchange system (the lung) in a medium (the atmosphere) that
contains *less* than half (21% ?) the oxygen concentration? Terrestoriality
apparently had a huge cost, energetically speaking.  Perhaps endothermy was
unavoidable, and perhaps the continued taxonomic dominance among vertebrates
of the archosaurian lineage that includes birds can in part be ascribed in
some degree to the fact that they are the only lineage to *reinvent* an
approximation of the counter-current (birds, I am told, have only a
cross-current) respiratory system.