Re: At the Water's Edge

On Wed, 11 Jun 2008 16:58:22 -0700 (PDT), Ken Shackleton
<ken.shackleton@xxxxxxx> wrote:

<snip all except quote from Zimmer's book "At the Water's Edge"

<start quote>
Before [Keith] Thomson's work, our own urea was a bit of a puzzle: if
tetrapods evolved from freshwater lobe-fins, they must have evolved it
from scratch. But when Thomson considered all the evidence at his
disposal, the simplest explanation was that the earliest vertebrates--
the ancestors of sharks, lobe-fins, and ray-finned fish--had invented
urea to survive in the ocean. Sharks that stayed in the ocean simply
went on producing it, while lobe-fins, which evolved in briny coastal
lagoons, still depended on urea to avoid being overwhelmed by salt.
The coelacanth, later moving out to deeper waters, held on to the
cycle, while African lungfish shut it down for the most part as they
lived in freshwater, although they could still resort to it. By
contrast, ray-finned fish moved to freshwater and lost their ability
to produce urea; later, when some of their descendants came out to the
ocean, they couldn't recover it. They had to invent glands instead to
fight salt.
<end quote>

It looks to me that Zimmer is saying above that the earliest
vertebrates lived in fresh water and have to evolve urea as they moved
back out to sea. I am not sure what to make of this....for even the
freshwater tetrapods would have had marine ancestors making urea at
some point, so I do not understand the significance of urea production
in determining who evolved from what. He also stated that Thomson's
work was done in the 1960's.....a very long time ago and I am quite
sure that the knowledge has come along quite a bit since then.

What Zimmer said was wrong even in the 1960's. It has always been
known that urea synthesis goes way, way back evolutionarily and that
there is absolutely no problem understanding why we terrestrial
tetrapods all have it. In particular, many fresh water fish do
synthesize and excrete urea, just in rather small quantity relative to
ammonia. The main enzyme responsible for urea synthesis, arginase I,
is found in eubacteria and archaea and fungi and plants and animals. I
found genes for it in zebrafish (Danio), the common carp (Cyprinus),
rainbow trout (Onchorhynchus) by searching (BLAST) for proteins that
very closely resemble the human version. Just google "urea freshwater
fish" for 197,000 hits. One paper titled "Excretion of ammonia and
urea by phylogenetically diverse fish species in low salinities"
describes urea synthesis in channel catfish and goldfish, both species
with limited tolerance to salt (stenohaline) as well as rainbow trout,
striped bass, and brown trout, species with broader salt tolerance
(euryhaline).

Historically, there is a 1969 paper by V.A. Cvancara titled "Tissue
arginase and ureogenesis in fresh-water teleosts" in Comp. Biochem.
Physiol 30(3):489-496. Homer Smith, in "From Fish to Philosopher"
written in 1953, describes lungfish excreting urea. At the time, the
lungfish was considered a key indication of the switch between
freshwater fish and amphibians. In the 1959 revision, he includes in
the notes mention that up to the arrival of the reptiles, urea was the
universal excretory product of vertebrates although, as we now know,
the quantity can vary enormously with the type. Unfortunately, I got
very efficient just a few months ago and discarded all my old
textbooks dating back to the early 60's.

So it is absolutely not true now and was not true in the 1960's that
people believed that urea production was any sort of problem at all
for any type of fish or amphibian or amniote or whatever evolutionary
stage you would like to name.

Keith Thompson was, I believe, the person who said that the coelocanth
retention of urea was some sort of evolutionary problem. It is true
that one of the major reasons to find elevated levels of urea in the
blood of marine vertebrates is as an adaptation for water balance. But
it simply is not true that only marine fish can produce urea -- some
have just specialized to produce an awful lot, to retain it, and to
tolerate the high concentration. Fresh water fish can also produce it
and apparently always have even if its synthesis is suppressed in some
forms.

The urea story is a real pisser. Don't believe it!

.

Relevant Pages

  • Re: At the Waters Edge
    ... urea to survive in the ocean. ... lived in freshwater, although they could still resort to it. ... many fresh water fish do ... The main enzyme responsible for urea synthesis, arginase I, ...
    (talk.origins)
  • Re: At the Waters Edge
    ... urea to survive in the ocean. ... lived in freshwater, although they could still resort to it. ... many fresh water fish do ... The main enzyme responsible for urea synthesis, arginase I, ...
    (talk.origins)
  • Re: At the Waters Edge
    ... urea to survive in the ocean. ... still depended on urea to avoid being overwhelmed by salt. ... many fresh water fish do ...
    (talk.origins)
  • Re: At the Waters Edge
    ... urea to survive in the ocean. ... Sharks that stayed in the ocean simply ... lived in freshwater, although they could still resort to it. ... vertebrates lived in fresh water and have to evolve urea as they moved ...
    (talk.origins)