The eye is considered to have developed at rather quick pace during the â€˜Cambrian explosionâ€™, 540 million years ago.Charles Drawin always wondered how natural selection could have led to the development of an organ as complex as the eye and with a common origin, in so many different kinds of animals. A group of Scientists led by Detlev Arendt, in EMBL,Germany have found how the primitive sea creature, zooplankton, respond to light that marks an early stage in the development of the eye. Larvae of marine invertebrates â€“ worms, sponges, jellyfish – have the simplest eyes that exist consisting of only two cells: a photoreceptor cell and a pigment cell. These are called “eye spots” resembling the proto eyes suggested by Charles Darwin. According to Charles Darwin Proto eyes are the first eyes to appear in animal evolution.
These eye spots cannot form any image but only sense direction of light and this ability is crucial for phototaxis â€“ the swimming towards light exhibited by many zooplankton larvae. Detlev is of opinion that the first eyes in the animal kingdom evolved for exactly this purpose and hence understanding phototaxis will 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.
Caption: 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.
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 GÃ¡spÃ¡r JÃ©kely, 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.
Mechanism of phototaxis in marine zooplankton.
JÃ©kely G, Colombelli J, Hausen H, Guy K, Stelzer E, NÃ©dÃ©lec F, Arendt D.
Nature. 2008 Nov 20;456(7220):395-9.