What use is half an eye? (Talk.Origins)
What use is half an eye? Source: Paley, Richard, 2000. The eye.
(Talk.Origins quotes in blue)
1. Half an eye is useful for vision. Many organisms have eyes that lack some features of human eyes.
Talk Origins is confusing not having all the features of the human eye with being incomplete. What is actually meant by half an eye, is an eye that is not complete enough to function as an eye. Consider what is needed for minimum sight.
- Light sensitive cells that can transmit signals to a brain. This is the only part that can serve a useful purpose—sensing light sources—but that is as good as it get until one has a functioning eye.
- Brain that can process the signals back into an image.
- Container to keep out extra light and contaminants.
- Lens to focus the image and adjust that focus as needed.
All of these features need to be in place and functioning or the eye is at best an overly complex light detector. Beyond that at least two more features are needed to keep the eye functioning.
- An adjustable aperture so the eye can handle varying light. Too much light for too long would harm the light sensitive cells, like overexposing film.
- A cleaning mechanism to keep the aperture clean. Otherwise dust and other debris would, over time, reduce and even totally block incoming light.
Once again these features need to be in place and functioning for an eye to last long enough to be a benefit to the organism.
2. Humans themselves have far from perfect vision:
A fully functional eye does not necessarily equate to perfect vision. Many of the features that Talk Origins lists have other tradeoffs that make combining them in one eye impractical if not impossible. For example, many animals that see better at night than humans do not see color as well as humans. The simple fact is that in most cases these eyes are simply different from normal human eyes. This has nothing to do with the claim.
The two human examples listed could indicate that normal human vision has actually degenerated as mankind has diversified.
Only one example requires a specific response.
- Humans have a blind spot caused by the wiring of their retinas; octopuses do not.
What Talk Origins does not tell you is that the wiring of the human retina protects the retina from UV radiation in air that would blind an octopus if it were in air. In the water an octopus’ retina is protected by the water.
Also Humans have binocular vision, so the left eye sees the tiny bit the right eye misses (0.25% of the visual field!), and vice versa. Thus, the blind spot is a non-issue.