So what evolutionary purpose did the membrance covering the (octopus) eye serve prior to the existence of the eye?
It was most likely the octopus "skin" and regulated water - salt balance.
Single celled bacteria swim toward light? Because they're "unlikely to encounter food" deep under water?
Yes, one of the oldest groups of bacteria, the Cyanobacteria, are well known for swimming toward light. These bacteria are of such a simple construction that they don't even have a nucleus in their cell.
The chemicals sensitive to light are slightly repelled by the chemicals that make up the flagella. This means those chemicals concentrate on the "front" or the side away from the flagella. If they are hit by photons they have a reaction that causes the ATP in the cell to release and it gets the flagella turning. This way the flagella turns when the cell is pointed toward light and doesn't turn when it points away.
Of course Cyanobacteria , in spite of the 3 billion years they have been swimming, do not understand that there is more food (especially phytoplankton) near the ocean surface than at the depth. It is just that the ones who swam toward the light found more food and multiplied while the ones that swam toward the dark, or didn't swim at all, found less and multiplied less.
The eye of the cynanobacteria is certainly primitive, but as in all evolution it is just a finite series of steps over a long time until more complex eyes are favored.
(other bacteria have simple sensors for sound, smell, and temperature)
TP |
