Creationists often refer to the words of Darwin when he pondered the evolution of the complex eye. He said that to suppose that the complex eye evolve by natural selection seems "absurd in the highest possible degree". What they always ignore is that Darwin went right on in the very next sentence to propose how the eye could have evolved and that numerous gradations must exist between the simplest light detecting organs and the complex vertebrate eye.
In the 150 years since Darwin published his book not only have those gradations been discovered in the natural world but the very genes that build the eye have been discovered by science. They next few threads in this series will describe scientific discoveries that reveal how vision evolved.
"What use is half an eye?" is a challenge often heard in creationist debates. The answer is that half an eye is extremely useful. Ophthalmologists divide into two tribes - one that deals with the front of the eye and the other that is concerned with ailments of the retina. The earliest eyes that evolved at the beginning of the Cambrian period would only have been of interest to the latter.
Stripped of all the optical apparatus such as the lens and iris, the eye is fundamentally a patch of light-sensitive cells. This sort of simple eye is still to be found in many modern species. The shrimp Rimicarus exoculata that makes its living around hydrothermal vents beneath the Atlantic Ocean is one such example. Absolutely no light penetrates to the depths inhabited by these creatures and not surprisingly they appeared to be eyeless their name suggests. Thanks to the work of Cindy Van Dover of the Marine Laboratory at Duke University the species' moniker turned out to be a misnomer.
Van Dover became curious about two flaps of tissue on the shrimp's back and sent off a sample for examination to a specialist in invertebrate eyes. Results showed that the sample possessed the pigment rhodopsin - the same pigment that is responsible for light detection in our own eyes. But what would the point be of light detection in an environment where no light from the surface can penetrate? Van Dover persuaded geologist John Delaney to try an experiment by switching off the lights on the deep sea submersible vehicle Alvin and observing the vents with a digital camera. Sure enough they detected a sharply defined halo around the vent.
There was a problem however. The shrimp's rhodopsin was tuned to detect green light but the light vents glowed red. Were the naked retinas of Rimicarus exoculata a degenerate organ like the blind eyes of cave fish? The answer came in two stages. Firstly the larva of the shrimps were discovered living at much shallower depths of the ocean and they were found to have perfectly normal eyes on their heads. As they mature their eyes are completely reabsorbed and the naked retinas of the adult shrimp develop from scratch on their backs. This proves that the adult eyes are not the end result of generations of loss of function but something more deliberate.
The optics of regular eyes are a balance between the ability to resolve images and sensitivity. At lower light levels our pupils dilate letting in more light. The ultimate way to maximise sensitivity is to sacrifice resolution by completely removing the front of the eye so that every available photon is captured by the retina. The naked retina of Rimicarus exoculata turn out to be 7 million times more sensitive than the fully formed eyes of their own larvae.
But what about the mismatch between the red light emitted by the hydrothermal vents and the green sensitive rhodopsin of Rimicarus exoculata? Van Dover concluded that if the shrimp had evolved an ability to detect green light then green light must exist in that environment. She turned to NASA who were experts at light detection in the inky darkness of space. They had developed a device named ALISS (Ambient Light Imaging and Spectral System) which they agreed to fit to Alvin. Sure enough ALISS detected green light being emitted from the vents at a frequency corresponding to the rhodopsin found in the retina of Rimicarus exoculata.
In the strange world of hydrothermal vents creatures are poised between lethal extremes of temperature. The ability to detect the faintest glow of light and know if it is coming from above or below, ahead or behind is a matter of life and death.
There is a lot more to be said about the evolutionary journey from the naked retina to our own complex eyes, the origin of light-detecting proteins as well as the genes that trigger the development of eyes in the embryo. Future threads in this series will describe some of these details.
Information about the evolution of colour vision can be found in number 26 of this series here...
#31 Ten Questions For Creationists ...
The basic facts about reality covered so far pose an impossible challenge to creationism.
#32 Sexual Selection
How female mating preferences led to some of the most remarkable features of living things.
#33 A Tale About Tails
Human embryology reveals our primate history.
#34 Hiccups and Tadpoles
How hiccups are a relic of our amphibian ancestors.
#35 Nature Red in Tooth and Claw
Nature's ability to inflict pain and suffering in the battle for survival.
#36 Mass Extinctions
96% of life was wiped out in The Great Dying 250 million years ago.
The plumbing of the vas deferens gives evidence of our fish ancestry
#38 The Origin of Complex Cells
How a merger of simple cells made complexity possible
#39 Homologous Structures
Evidence for common ancestry in our shared anatomy.