The larvae of marine ragworm Platynereis dumerilii have the simplest eyes that exist. They resemble the first eyes that developed in animal evolution and allow the larvae to navigate guided by light. (Credit: Copyright EMBL)
The larvae of marine creatures, the first form that emerges from the egg, are tiny and swim with the plankton in the ocean. These larvae have the simplest eyes that exist. No more than two cells: a photoreceptor cell and a pigment cell. These minimal eyes are called eyespots.
Eyespots cannot form images. But can sense the direction of light. This ability is critical in helping these tiny animals to swim with changing light conditions towards good feeding conditions and away from dangerous zones. These movements drive the biggest transport of biomass on earth.
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.
Understanding how simple eyes work "unravels the first steps of eye evolution." In fact, an earlier study of the eyes of a marine worm suggests how eyes could have evolved through natural selection. So we have lots to learn even from our most humble marine creatures.
Simple Eyes Of Only Two Cells Guide Marine Zooplankton To The Light
ScienceDaily 19 Nov 08;
Researchers unravel how the very first eyes in evolution might have worked and how they guide the swimming of marine plankton towards light.View to a krill: Secrets of plankton eyes
Larvae of marine invertebrates – worms, sponges, jellyfish - have the simplest eyes that exist. They consist of no more than two cells: a photoreceptor cell and a pigment cell. These minimal eyes, called eyespots, resemble the 'proto-eyes' suggested by Charles Darwin as the first eyes to appear in animal evolution. They cannot form images but allow the animal to sense the direction of light. This ability is crucial for phototaxis – the swimming towards light exhibited by many zooplankton larvae. Myriads of planktonic animals travel guided by light every day. Their movements drive the biggest transport of biomass on earth.
"For a long time nobody knew how the animals do phototaxis with their simple eyes and nervous system," explains Detlev Arendt, whose team carried out the research at EMBL. "We assume that the first eyes in the animal kingdom evolved for exactly this purpose. Understanding phototaxis thus unravels the first steps of eye evolution."
Studying the larvae of the marine ragworm Platynereis dumerilii, the scientists found that a nerve connects the photoreceptor cell of the eyespot and the cells that bring about the swimming motion of the larvae. The photoreceptor detects light and converts it into an electrical signal that travels down its neural projection, which makes a connection with a band of cells endowed with cilia. These cilia - thin, hair-like projections - beat to displace water and bring about movement.
Shining light selectively on one eyespot changes the beating of the adjacent cilia. The resulting local changes in water flow are sufficient to alter the direction of swimming, computer simulations of larval swimming show.
The second eyespot cell, the pigment cell, confers the directional sensitivity to light. It absorbs light and casts a shadow over the photoreceptor. The shape of this shadow varies according to the position of the light source and is communicated to the cilia through the signal of the photoreceptor.
"Platynereis can be considered a living fossil," says Gáspár Jékely, former member of Arendt's lab who now heads a group at the MPI for Developmental Biology, "it still lives in the same environment as its ancestors millions of years ago and has preserved many ancestral features. Studying the eyespots of its larva is probably the closest we can get to figuring out what eyes looked like when they first evolved."
It is likely that the close coupling of light sensor to cilia marks an important, early landmark in the evolution of animal eyes. Many contemporary marine invertebrates still employ the strategy for phototaxis.
Yahoo News 19 Nov 08;
PARIS (AFP) – Biologists on Wednesday explained how the larvae of marine zooplankton can see with just two cells, using what is believed to be the world's simplest vision system.Darwin's Greatest Challenge Tackled: The Mystery Of Eye Evolution
Zooplankton are tiny creatures such as copepods and krill that drift in the ocean's water columns, swimming up from the depths towards the light in order to graze on marine plants called phytoplankton near the surface.
This movement, called phototaxis, is the biggest biomass displacement in the world.
In a study published by the British-based journal Nature, European scientists looked at the larvae of the marine ragworm Platyneris dumerilii to try to explain how plankton are able to do the phototaxis trick.
The larva has just two eye cells, consisting of a pigment cell and a light-sensitive cell, say the investigators.
The cells are unable to form images but enable the plankton to sense the difference between light and dark and send appropriate signals to its swimming mechanism, say the investigators.
First, the pigment cell absorbs light and casts a shadow over the photoreceptor cell. The shape of the shadow varies according to the position of the light source.
The photoreceptor cell then converts this light signal into electricity, sending it in a signal along a nerve that connects to a band of cells endowed with thin hairs, called cilia, that beat to displace water.
The basic but effective system could explain how the very first eyes in evolution may have worked, say the team from the European Molecular Biology Laboratory (EMBL) and the Max Planck Institute.
"For a long time, nobody knew how the animals do phototaxis with their simple eyes and nervous system," said EMBL's Detlev Arendt.
"We assume that the first eyes in the animal kingdom evolved for exactly this purpose. Understanding phototaxis thus unravels the first steps of eye evolution."
ScienceDaily 1 Nov 04;
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."
The "living fossil," Platynereis dumerilii. (Credit: Maj Britt Hansen, Photolab, EMBL heidelberg)
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."