Scientists Propose Incorrect Origin Of Backwards Eye
Researchers claim to discover why the retina of most vertebrates is built backwards with the light receptors behind a mesh of intervening cells and neurons. They say this is because there are a subset of these cells that focus the light onto the receptors. Is this the real reason though?
Researchers at the Israel Institute of Technology, Technion will give a talk on this topic at the American Physical Society on March 5th in San Antonio Texas.
The premise of this talk is the apparently counterintuitive design of vertebrate eyes. The basic layout just doesn’t make sense. You’d think an Intelligent Designer would make a straightforward unblocked path for light thru the eye directly onto the light-sensing cells, the rods and cones in the retina. In fact, this is something Darwin noticed and to this day this poor design is offered as solid evidence that our eye is a result of a contingent quirky evolutionary design rather than a perfect end-product of a divine designer.
What are these intervening cells then? Well, the first leg of the trip through the eye makes sense; you have the cornea, the lens and the gel-like vitreous and aqueous humours. Things go a little wacky though at the far end where the retina is. You’d think the photoreceptors would be there waiting, but the photons have to wade through a tangle of cells first. This includes blood vessels, ganglion cells, amacrine cells, bipolar cells, etc. This is a little like your cellphone camera having its various components laying around in front of its digital light sensor.
Why this backwards design? Well, these Technion researchers think they know. Using special techniques employed for astrophysics and astronomy, they created a way to image the retina in detail never attained before. They focused (so to speak) on a special type of metabolic cell called Muller glia cells. Previous research suggested that they may have some interaction with the photoreceptors in terms of delivering focused light to them. Using these new techniques, the researchers showed that that was indeed the case. The cells acted like lenses or fiberoptic cables that brought and focused light to the light-sensitive cells mitigating some of the distortion and loss of clarity engendered by all the stuff that gets in the way.
Erez Ribak, a professor at the Technion said:
“For the first time, we’ve explained why the retina is built backwards, with the neurons in front of the photoreceptors, rather than behind them”
This was an interesting discovery and I love how they used astrophysics techniques to improve the biological imaging, but I’m afraid I cannot agree with the context in which they’re framing their conclusion. They have not discovered why the retina is built backwards. We already have a real good idea why it was. The retina of vertebrates is inverted probably because of one or both of the two leading ideas why:
- Our eyes are actually extensions of the layered cortex of the brain which made a major impact on its current structure. Cephalopod eyes, like the squid, did not evolve their eyes in this way and do not have such an inverted structure.
- It is also likely that the structure of our retina is due to sensible genetic compromises made in the very early evolution of the vertebrate eye. Perhaps when it was just a couple evolutionary steps beyond a simple light sensitive cavity. This makes even more sense if sharp vision was just not important to these ancient ancestors. Once certain evolutionary pathways are taken, however, it could be almost impossible to make fundamental design changes once they’ve been implemented. Some of those early design decisions were probably helpful, even if it put the receptors in a weird place. If that means that our visual acuity was not optimized or took a minor hit in the process, so be it. It was a good trade-off.
Once these major design elements of the eye were laid down, it was of course still possible to make minor tweaks to improve our vision to overcome some of the degradation caused by the photoreceptor placement. Focusing the light with the Muller Glia cells was likely one of those tweaks. Just because these glia cells take advantage of the layout already in place doesn’t mean that the layout itself is due to the Muller cells.
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