Convergent Evolution of the Eye and my Epigenetic Quest for a Future Super Hero Race

Watch out, I am currently training my self to see like a Mantis Shrimp so my children can one day become super heroes. Don't laugh, it could happen!

We humans think we have it all when it comes to vision. We do have pretty good vision but in many ways our vision is very poor compared to other species.

I'm not just talking about having "Eagle Eyes" either, there are whole dimensions of vision which our evolution has left unexplored. Yet, as I will explain, we may have the nascent ability as a species to evolve at least one of these improvements in a way which is completely novel and also a great example of how convergent evolution can occur through the accidental function of a structure designed for another purpose.

The simplest improvement in human sight would be to broaden the spectrum we can see into the UV and IR spectrum. Even better, how about being able to discern the phase shift of light (whether light waves are in sync or not)? We humans are completely unable to do that yet other organisms can. Or how about being able to see the polarizaton of light, the direction in which the waves are undulating, and discern the difference between linear polarization and circular polarization? That would be pretty amazing.

Turns out that the stomatopods and especially the Mantis Shrimp (pictured above) have all of this and more. They also see a greater range of color than we do, have more types of photoreceptors, have three distinct eye regions for different purposes, and pipe all this data to the brain in parallel rather than trying to jam it all in in one nerve pathway like we do. Wow! (Check out an excellent overview on Pharyngula and a paper on Mantis Shrimp polarized vision here)

Of course humans don't share much evolutionary lineage with the Mantis Shrimp and don't have compound eyes. So how, you ask, could we ever evolve to see this way? The answer lies in a little understood ability to see the polarization of light called Haidinger's Brush...
Some people are able to see a cross of yellow and blue which orients itself in their vision according to the direction of polarization. It is best seen on a plain color background (the sky 90 degrees from the sun on a cloudless day is best apparently) and most people need to train themselves to see it as it appears as a visual "ghost". Some people are even able to train themselves to see circularly polarized light using this method although it is difficult. Moreover, this ability varies across the population, with some able to see it better than others and some unable to see it at all.

Wikipedia has an excellent entry with a great description of how to view the phenomenon on your LCD screen. You too can feel the rush of excitement that comes when you finally see it! (And the incredulous stares and comments from significant others that come as you tilt your head back and forth and squint at your laptop screen for the better part of an hour)

The effect is thought to be caused by the circular orientation of the macular cells of the retina. This circular pattern helps to align a small portion of the molecules in the retina in this circular fashion, one of which is the protein Lutien. Lutein absorbs light depending on its orientation to the light source, which, when paired with an oriented retinal structure means that in certain orientations the light reaching the receptor is absorbed within the frequency of Lutein and thus shifts the color perceived slightly, creating a visual ghost in alignment with the polarization. (Think of the way that light reflects from engine turned metal in a pattern that changes with the direction of the light)

Now, when you think of this trait in terms of evolutionary biology you realize immediately that what evolution needs is a variable trait to act on. It also needs selective pressure. For the Mantis Shrimp it is the need to see translucent prey (translucent things like a window are invisible because the color and intensity of light is not affected by passing through them, however, they often alter the polarization of the light going through them) which are only visible with phase shift vision or polarized vision.

So, all we humans need is a driving need to see translucent things in order to survive and we might also develop this ability to a greater extent. One could easily envision the retina developing a more rigid circular orientation and/or specifically oriented Lutein binding proteins in the retina in order to magnify this effect. All we need is selective pressure (an invading race of nearly invisible aliens bent on human destruction perhaps?).

How this effect could be translated into actual polarized vision in the brain is hard to guess but it is probably possible given enough evolutionary time. I imagine once Lutien was bound in an organized fashion within the macular cells it would be only a small step towards specialized Lutein binding cells arrayed in a pattern through the retina, which would be a small step towards these cells becoming specialized as polarization receptors. An viola!, humans with polarized vision! (Well, humans plus whatever other evolutionary changes occured over the mabye a new species...or a super hero species!)

The really interesting thing is that this is an example of nascent convergent evolution. As humans we share none of the structural or developmental resources that allow the mantis shrimp to see polarized light, yet we have an accidental structural anomaly which allows us to visualize it. Just as there are many ways to skin a cat there are many ways to evolve complex visual abilities that go beyond simple color vision.

Seeing organisms in this light it is quite easy to think of the ways in which evolution could act on those little anomalous traits that we otherwise take no notice of, or fail completely to notice. I challenge a creationist to explain why they think there is no evidence for evolution given the facts of living organisms and the fossil record, and the ease with which we can see ways in which simple traits could be improved upon with the help of natural selection.

Until then, I'm off on a Lamarkian quest to speed the process along by training myself to see Haidinger's Brush. If I'm lucky my ability will be methylated into my DNA and epigenetically transmitted to my future progeny, thus bringing them one step closer to Super Hero status!