Re: Breathing space

Stephen Jones (sejones@ibm.net)
Tue, 31 Mar 98 21:50:04 +0800

Reflectorites

I was overseas on holiday from September-December 1997, so I am
catching up on my journal reading. I recently found this little gem
in New Scientist of 11 October 1997. I apologise if it had already
been posted.

Obviously lungs had to exist before animals could live on land, and
so lungs had to be developed in water, before they were needed on
land. But that was not enough. It seems that lungs out of water
need a special coating called the surfactant system, which stops the
lung tissue sticking together in air.

So right on cue, 350 million years ago, just before it was needed on
land, an ocean-dwelling fish happened to have a special mutation
which gave it a surfactant system that it didn't need, so that down
the track its descendants could crawl onto the land.

But of course that special mutation had to have happened to the same
line of fish that also developed legs underwater, before they were
needed on land. No doubt it will also be found that special
mutations had to appear on cue to provide special land-dwelling eyes,
ears, nostrils and eggs, in readiness for the `invasion' of the land
later down the track by tetrapods.

Not bad going for a `blind watchmaker' process that can't plan ahead!
;-)

Steve

------------------------------------------------------
Breathing space

Do lungs work because of a mutation 350 million years ago?

AIR breathing developed independently many times in the history of
evolution. But a subtle biological mechanism that allows lungs to
function seems to have developed only once, and is still used by all
air-breathing vertebrates, say researchers in Australia.

Lungs need to expand and contract, but if the surface tension of
liquid in the lungs is too high, the tissue sticks together and breathing
is impossible. So in air-breathing vertebrates, the surface tension is
lowered by a coating of proteins and lipids, called the surfactant
system, on cells lining the lungs.

Christopher Daniels and student Lucy Sulivan of the University of
Adelaide compared the surfactant systems of 18 vertebrates,
including fish, lizards, chickens and humans, and found that all make
use of a key protein called surfactant protein A. They conclude in this
month's Journal of Molecular Evolution that the surfactant system
developed once in an ocean-dwelling ancestor about 350 million
years ago, and has been used again and again as vertebrates
developed lungs and crawled out of the water.

"It's similar to the insulin system and haemoglobin-it's one of the
things you have to have for the organism to do well," says Daniels.
"Since it evolved, it hasn't changed."

Daniels thinks the surfactant system may have developed first in the
gut, as a way to regulate surface tension between the organs there.
Even today, there are similar surfactant proteins in the guts of rats, he
says. Because the lungs and the gut are closely related in the
developing embryo, Daniels believes a genetic mutation may have
shifted the surfactant to the lungs and made air breathing possible.

---
Nonstick: surfactant stops lung cells clumping together

---

Allan Smits of Quinnipiac College in Hamden, Connecticut, who also studies pulmonary surfactants, says Daniels's research on the surfactant system is convincing. "Clearly air breathing couldn't have developed without it," he says. "It is definitely necessary for air breathing."

Kurt Kleiner

(Kleiner K., "Breathing space," New Scientist, Vol 156, No 2103, 11 October 1997, p21) ------------------------------------------------------

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