There’s a new kid on the BloggerParty block, BigBadJohnny. Well, maybe he’s not a kid; if his avatar tells no lies then I’m a spring chicken by comparison. We can swap stories about when elephants had fur. Anyway, it appears that Johnny is a wondering sort, and one of the things he’s wondering about, he says, is where eyes came from. So here I am, sitting in front of the washing machine wondering how I’m ever going to work what I actually do for a living into the incoherent mishmash that is Felloffatruck Publications, and here comes an opportunity, wonderfully gift-wrapped. Thanks, John, the check’s in the mail. :)

We use eyes to see with. Obviously. Admin, here’s a new contest idea for you. Crushing Banality of the Week. I respectfully submit to you the first (ah, official) Candidate. Eyes are in fact wonderful machines to see with, and they’re not exactly simple, what with lenses and shutters and high-resolution sensors and high-speed data processors and all that. For a quick review, go here: for this kind of thing, the BBC wins once again.

But what is “seeing�? We think of it as taking a picture of the world around us so that we know where we are and what to do about it. Takes a good deal of thinking to deal with all those pictures, though. And this begets the “intelligent design� question. If you have to think before you can see, and you have to see in order to have something to think about … Brain cramp! Even Darwin’s brain had spasms. How can you evolve two interdependent sophisticated machines simultaneously? Is there a divinity in the house?

Now this is where I come in. I study algae, protozoa, and fungi. Basically, one-celled creatures. No brains, no thinking. And therefore, no seeing. Supposedly. (We’ll be coming back to this.) But this doesn’t mean they just sit there and take whatever Nature throws at them. They sense much of what goes on. Light in particular. You take a bowlful of swimming algal cells and put it on a windowsill, and in an hour you discover that all the cells are on the side of the bowl next to the window (or perhaps, if the light’s really bright, on the side away from it). The fancy Greek word for this is phototaxis. No, that doesn’t mean Yellow Cabs with cameras. Very funny. Photo, light; taxis (i as in is, not ees), movement. Movement towards (positive) or away from (negative) light.

How does this work? Each algal cell has a bright red spot in it. The eyespot. Catching on yet? Hang around, it gets better. The eyespot is actually a bunch of drops of fat. They serve as a shade for sensors that are in the outer membrane of the cell. When the sensors see light, they undergo chemical reactions that create an electric current in the membrane. When the sensors see less light (under the shade of the eyespot), they produce less current. The cell will turn towards, or away from, the light in response to the amount of current being produced by the sensors. For a sketch of what this sensing structure looks like, go here.

Now up in Syracuse University is a really smart dude named Ken Foster. He and his colleagues asked the simple-sounding question “what is the light sensor in algal cells?� This isn’t a simple question to answer, if it were dummies like me would have been all over it. Ken & Co. figured it out, a decade and more before you could cheat by logging into the web page of a genome project and doing a search, and discovered that the sensor is a protein, one belonging to a family of proteins called “rhodopsins�. The very same proteins that are found in the retinas of your eyes and mine.

Now this was, and is, big news. Especially since subsequent research has found rhodopsins in just about everything, even bacteria, and that they’re involved in light-sensing processes everywhere they’re found. Making an eye just got lots simpler. You put a bunch of eyespots together, just like on the chip in a digital camera, integrate the data from them, and you’ve got an image. Which is exactly what the rod cells in your retina are doing in conjunction with your brain. (Cone cells are more complicated. That’s another blog.) Don’t even need a lens, initially, though one helps. There’s lots of ways to invent a lens. Like a fly’s eye, for example.

Oh, and just in case you’re a brain chauvinist when it comes to seeing, let me introduce you to Warnowia. Warnowia is a one-celled ocean-going protozoon that belongs to a group called the dinoflagellates. You might have heard of dinoflagellates in connection with �red tides�. Warnowia and a few of its relatives have a structure called an ocellus - the word is Latin for “eye�. And it’s a damned good eye, with a large lens and large retina-like domain. (I say “retina-like� because a “retina� is made up of lots of cells; this thing, I remind you, is a small part of one cell.) If you check out the link, you’ll see in the pictures a dark line that represents the retina, with a clear globe attached to it that is the lens.

What, you (and the 17 people who heard of this before you did) ask, does a one-celled blob need with an eye? Well, this particular kind of cell is a hunter. It spies a prey and then, literally, reaches out and grabs it. And it doesn’t grab just anything. To pick a candidate out of all the plankton floating about, and then target it, it needs an image … How does it work? Well, we know that dinoflagellates have rhodopsins, but we don’t know much more about how dinoflagellate light-sensing systems work. Can’t you just probe the genome? Everything’s got genomes sequenced by now, don’t they? Well, with the dinoflagellates, there’s a problem. The bigger the genome, the harder it is to sequence and interpret. Somebody not long ago worked out how much DNA there is to sequence in a typical dinoflagellate genome. The amount is, like, 100 times greater than the DNA in a human cell.

And you thought you were so smart.

*   *   *   *

For the whole (and rather heavy) scoop on eye evolution as scientists currently understand it, have a look at Gehring WJ. 2004. Historical perspective on the development and evolution of eyes and photoreceptors. International Journal of Developmental Biology 48: 707-717. The link will take you to Google’s html translation of the source .pdf document. But you’ll need to download the .pdf to see the pictures – including some of the Warnowia ocellus.

  - O Ceallaigh

Copyright © 2006 Felloffatruck Publications, featuring The New Millennium Devil's Dictionary. All wrongs deplored.

All opinions are mine as a private citizen.