In a Marine Worms Eyes-Biology

In a Marine Worms Eyes, the Theory of Evolution

By CARL ZIMMER

Charles Darwin considered the evolution of the human eye
one of the toughest problems his theory had to explain. In “On the Origin of
Species,” he wrote that the idea that natural selection could produce such an
intricate organ “seems, I freely confess, absurd in the highest possible
degree.”

But Darwin dispelled that seeming absurdity by laying
out a series of steps by which the evolution could take place. Making this
sequence all the more plausible was the fact that some of the transitional
forms Darwin described actually existed in living invertebrates.

Now, a team of American and European researchers
report that they have discovered an eye that could represent the first step in
this evolution. They have found, in effect, a swimming eyeball.

“This is in no way the ancestor of the human eye, but
it’s the first time we have had a model of it,” said Yale Passamaneck, a
postdoctoral researcher at the University of Hawaii. He and his colleagues report the discovery in
the online journal EvoDevo.

The researchers made the discovery while studying a
species of brachiopods, or lamp shells, which live in shells but are marine
worms unrelated to mollusks like clams and oysters. Lamp shells have existed
for over half a billion years, but their biology has long remained a mystery —
including the simple question of whether they can see.

Four-day-old lamp shell larvae, for example, have
puzzling dark spots on either side of the front end of their bodies. Recently,
Carsten Lüter, a biologist at the Berlin Museum of Natural History, and his
colleagues dissected the eyespots of some lamp shell larvae. They discovered
that each spot was actually a pair of neurons, one for capturing light and one
containing pigment. The neurons connected to a brainlike clump of neurons
inside the larva.

Their anatomy suggested the spots were simple eyes. So
Dr. Lüter and his colleagues contacted Dr. Passamaneck and his colleagues at
the University of Hawaii, who are experts on the genes for animal photoreceptors.
The Hawaii researchers discovered that, indeed, photoreceptor genes were active in the dark spots.

But to be thorough, Dr. Passamaneck checked to see if
the photoreceptor genes were active at other stages. “I thought, ‘I’m just
going to eliminate that possibility,’ ” he said.

Just the opposite happened. Dr. Passamaneck discovered
that the genes were active much earlier, just 36 hours after fertilization,
when the lamp shell embryo was merely a cup-shaped mass of a few hundred cells.

Dr. Passamaneck was baffled. “There are no neurons at
that stage,” he said. Nevertheless, it was clear that the outer surface of the
cup was covered with photoreceptors.

To see if the embryos were doing something with the
light, Dr. Passamaneck and his colleagues put a light on one side of a dish of
embryos. The lamp shell embryo is covered with tiny beating hairs, which it
uses to swim in a spiral pattern. Dr. Passamaneck found that after 20 minutes,
twice as many embryos would end up on the illuminated side of the dish as on
the dark side.

Dr. Passamaneck and his colleagues hypothesize that
the cells can detect the direction of light because it is blocked in some
directions by the embryo’s yolk. It can then use this information to change the
rhythm of its hair.

It is possible, Dr. Passamaneck said, that in the
course of evolution, our own eyes started out as swimming eyeballs. Only later
did the job of catching light get relegated to only some cells, which could
send signals to their neighbors. And only much later did these specialist cells
relay signals to brains.

Todd Oakley of the University of California, Santa Barbara, an expert on the evolution of vision, called the
results “tantalizing.” But he cautioned that just because the photoreceptor
gene was active in the early embryo, that did not necessarily mean that the
lamp shells were able to see. “Other possible photoreceptive mechanisms should
also be ruled out,” Dr. Oakley said. “Correlation does not mean causation.”

Dr. Passamaneck is making plans for these sorts of
studies. For now, though, he remains a bit stunned at what he has stumbled
across.

“It’s like Yogi Berra said,” he said. “You can observe a lot by
watching.”

http://www.nytimes.com/2011/03/01/science/01eyeball.html

Why are the lamp shell organisms important to understanding how the eye evolved?  What do you think about this discovery?