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Darwin's Greatest Challenge Tackled: The Mystery Of Eye Evolution

The "living fossil," Platynereis dumerilii.
(Credit: Maj Britt Hansen, Photolab, EMBL heidelberg)
ScienceDaily (Nov. 1, 2004) — October 28, 2004 -- When Darwin's skeptics attack 
his theory of
evolution, they often focus on the eye. Darwin himself confessed that
it was "absurd" to propose that the human eye evolved through
spontaneous mutation and natural selection. Scientists at the European
Molecular Biology Laboratory (EMBL) have now tackled Darwin's major
challenge in an evolutionary study published this week in the journal
Science. They have elucidated the evolutionary origin of the human eye.
Researchers in the laboratories of Detlev Arendt and Jochen
Wittbrodt have discovered that the light-sensitive cells of our eyes,
the rods and cones, are of unexpected evolutionary origin – they come
from an ancient population of light-sensitive cells that were initially
located in the brain.
"It is not surprising that cells of human eyes come from the brain.
We still have light-sensitive cells in our brains today which detect
light and influence our daily rhythms of activity," explains Wittbrodt.
"Quite possibly, the human eye has originated from light-sensitive
cells in the brain. Only later in evolution would such brain cells have
relocated into an eye and gained the potential to confer vision."
The scientists discovered that two types of light-sensitive cells
existed in our early animal ancestors: rhabdomeric and ciliary. In most
animals, rhabdomeric cells became part of the eyes, and ciliary cells
remained embedded in the brain. But the evolution of the human eye is
peculiar – it is the ciliary cells that were recruited for vision which
eventually gave rise to the rods and cones of the retina.
So how did EMBL researchers finally trace the evolution of the eye?
By studying a "living fossil," Platynereis dumerilii, a marine worm
that still resembles early ancestors that lived up to 600 million years
ago. Arendt had seen pictures of this worm's brain taken by researcher
Adriaan Dorresteijn (University of Mainz, Germany). "When I saw these
pictures, I noticed that the shape of the cells in the worm's brain
resembled the rods and cones in the human eye. I was immediately
intrigued by the idea that both of these light-sensitive cells may have
the same evolutionary origin."
To test this hypothesis, Arendt and Wittbrodt used a new tool for
today's evolutionary biologists – "molecular fingerprints". Such a
fingerprint is a unique combination of molecules that is found in a
specific cell. He explains that if cells between species have matching
molecular fingerprints, then the cells are very likely to share a
common ancestor cell.
Scientist Kristin Tessmar-Raible provided the crucial evidence to
support Arendt's hypothesis. With the help of EMBL researcher Heidi
Snyman, she determined the molecular fingerprint of the cells in the
worm's brain. She found an opsin, a light-sensitive molecule, in the
worm that strikingly resembled the opsin in the vertebrate rods and
cones. "When I saw this vertebrate-type molecule active in the cells of
the Playtnereis brain – it was clear that these cells and the
vertebrate rods and cones shared a molecular fingerprint. This was
concrete evidence of common evolutionary origin. We had finally solved
one of the big mysteries in human eye evolution."
Adapted from materials provided by European Molecular Biology Laboratory.
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European Molecular Biology Laboratory (2004, November 1). Darwin's Greatest 
Challenge Tackled: The Mystery Of Eye Evolution. ScienceDaily. Retrieved 
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