Evolution of Eyes

While eyes are common in larger animal species, about a third of all animal phyla lack eyes altogether; sea urchins do not bother with them, nor do many worms. Another third have eyes that look rudimentary to us; spots and patches and pits that can sense whether it's night or day or whether a shadow is passing overhead, but that do not form any kind of image. The final third have true image-forming eyes that can capture a picture of what's going on around them and pass that on to some kind of brain or nerve net. The phyla that have true eyes are a diverse subset of the multicellular animals, including jellyfish and sea anemones, molluscs, annelid worms, onychophora (velvet worms), arthropods, and us chordates, which is a strange distribution. It's as if eyes popped up in scattered lineages interspersed with groups that lack them. For a long time, one of the hypotheses to explain all these eyes was that they evolved independently, multiple times within the animal kingdom.

"Speaking as a physicist, I cannot believe that there has been enough time for an organ as complicated as the eye to have evolved from nothing. Do you really think there has been enough time?" (An argument that stems) from the Argument from Personal Incredulity. ... I have usually fallen back on the sheer magnitude of geological time.It now appears that the shattering enormity of geological time is a steam hammer to crack a peanut. A recent study by a pair of Swedish scientists, Dan Nilson and Susanne Pelger, suggests that a ludicrously small fraction of that time would have been plenty.When one says "the" eye, by the way, one implicitly means the vertebrate eye, but serviceable image-forming eyes have evolved between 40 and 60 times, independently from scratch, in many different invertebrate groups. Among these 40-plus independent evolutions, at least nine distinct design principles have been discovered, including pinhole eyes, two kinds of camera-lens eyes, curved-reflector ("satellite dish") eyes, and several kinds of compound eyes. Nilsson and Pelger have concentrated on camera eyes with lenses, such as are well developed in vertebrates and octopuses.

The first was: is there a smooth gradient of change, from flat skin to full camera eye, such that every intermediate is an improvement?...Second, how long would the necessary quantity of evolutionary change take?

But even with these conservative assumptions, the time taken to evolve a fish eye from fiat skin was minuscule: fewer than 400,000 generations. For the kinds of small animals we are talking about, we can assume one generation per year, so it seems that it would take less than half a million years to evolve a good camera eye.

The eyes of the organism is very similar to the Ophiocoma Wendtii's eyes, something that exists today.

More of, from what I understand from the pamphlet "That was pretty complex for back then, seeing as organisms still have such a structure."

It turns out the dorsal side of the brittle star is covered with microscopic lenses embedded in its skeleton, making the entire back of the creature into a compound eye.

Although they were not the first animals with eyes, trilobites developed one of the first sophisticated visual systems in the animal kingdom. The majority of trilobites bore a pair of compound eyes (made up of many lensed units). They typically occupied the outer edges of the fixigena (fixed cheeks) on either side of the glabella, adjacent to the facial sutures. At least one suborder of trilobites, the Agnostina, are thought to be primarily eyeless. None have ever been found with eyes. In contrast, a few secondarily eyeless species (in which a clear evolutionary trend toward reduced eye size with eventual disappearence of eyes altogether) have developed within several groups, even those known for large, well-developed eyes (e.g., Phacopina).

Yeah, I saw that before making this thread. I'm curious about the transition between "c" and "d" on that.

But aren't they both just a cluster of lenses that group together to form an eye? The placing is different, as they are different species, but to me they sound like the same general idea.

MBG in Message 6 writes: Among these 40-plus independent evolutions, at least nine distinct design principles have been discovered, including pinhole eyes, two kinds of camera-lens eyes, curved-reflector ("satellite dish") eyes, and several kinds of compound eyes.

However, there probably are extant animals with eyes like d, does anyone know how their eye fluid progresses over their lives?

The transparent cells over the pinhole eye's aperture split into two layers, with liquid in between. The liquid originally served as a circulatory fluid for oxygen, nutrients, wastes, and immune functions, allowing greater total thickness and higher mechanical protection. In addition, multiple interfaces between solids and liquids increase optical power, allowing wider viewing angles and greater imaging resolution. Again, the division of layers may have originated with the shedding of skin; intracellular fluid may infill naturally depending on layer depth.

I think that it would be helpful if Lyston were to post a bit more from this pamphlet, the whole thing if it's at all practical.

"But if we look at the individual elements of the trilobite eye, we find that the lens systems were very different from what we now have. Riccardo Levi-Setti (a Field Museum research associate in geology and professor of physics at the University of Chicago) has recently done some spectacular work on the optics of these lens systems. Figure 7 shows sketches of a common type of trilobite lens. Each lens is a doublet (that is, made up of two lenses. The lower lens is shaded in these sketches and the upper one is blank. The shape of the boundary between the two lenses is unlike any now in use either by humans or animals. But the shape is nearly identical to designs published independently by Descartes and Huygens in the seventeenth century. The Descartes and Huygens designs had the purpose of avoiding spherical aberration and were what is known as aplanatic lenses. The only significant difference between them and the trilobite lens is that the Descartes and Huygens lenses were not doublets - that is, they did not have the lower lens. But, as Levi-Setti has shown, for these designs to work underwater where the trilobite lived, the lower lens was necessary. Thus, the trilobites 450 million years ago used an optimal design which would require a well trained and imaginative optical engineer to develop today-or one who was familiar with the seventeenth century optical literature." (Raup D.M., "Conflicts Between Darwin and Paleontology," Field Museum of Natural History Bulletin, Field Museum of Natural History: Chicago IL, January 1979, Vol. 50, No. 1, pp.22-29, p.24).

I would like you to elaborate on these transitional forms, and be specific please. I am currently researching the topic of transitional forms and transitional eyes would be greatly appreciated.

It's a natural process that takes place every time an egg is fertilized. Nothing remotely new or dramatic.