Well, inside evolution, the eye is difficult.
Here on Science 2.0 we have written lots of articles on the evolution of the eye and it's easy to lose patience with people who show up and haven't read all of them or are skeptical (no one likes people who are skeptical about the wrong things) but I have long believed that what evolution needs is a great book on eyes. We've been gifted with some terrific books on general evolution by Neil Shubin and Sean Carroll and Science 2.0 guest columnist Jerry Coyne but I couldn't remember seeing a book that dealt with the evolution of the eye.(1)
Because it's difficult to convey. Even a brilliant thinker like Charles Darwin could not fathom how an eye formed by natural selection, though he knew it must have. He did not have parsimony analysis and molecular biology but modern scientists do, they just need someone who can communicate it all.
Enter Dr. Ivan Schwab, M.D., ophthalmologist and professor at the University of California Davis and his new book Evolution's Witness: How Eyes Evolved from Oxford University Press. Schwab sets out to decipher the complex amalgam that goes into this organ, and assuage the concerns of those who are skeptical because soft tissue like an eye won't have a complete fossil record.
I used "War and Peace" in the title of this piece partly because I joke about possessing the same divine genius as Tolstoy far more than I should, but also because "War and Peace" can be many things to many people; if you want it to be just an adventure novel, it can certainly be that, and if you want to find the deeper significance, you can do that too. So it goes with "Evolution's Witness", though I am sure Schwab will object to any Tolstoy comparison. In my defense, I am not a journalist so I am unencumbered by any need to be measured or balanced; unless you are an expert in spider eyes, you will learn something about spider eyes, but if you just want to follow a narrative about how eyes evolved and get some interesting insights along the way, you can do that also. That's what any writer sets out to do in a book like this.
For a Science 2.0 audience, the science meat is often paramount but the book unravels the process of science for the less initiated as well. In science, there is no One Science To Rule Them All so people who insist nothing is valid until it is found in the fossil record are missing a key understanding of science. Paleontologists even caution people that a fossil itself is downright miraculous so fossils of soft tissue preserved at various stages is not a reasonable expectation. Instead we have biologists who discovered an opsid protein for collecting light and the discovery of how eyes have evolved went from there.
3.75 billion years is a lot of ground to cover in a short-form piece so to help me distill all this down, I got together with Dr. Schwab over a coffee.
The first question I had, since I know writing a book is a lot of work, is why this was the book he had to write. He has two textbooks to his credit and a 2006 Ig Nobel award in ornithology for figuring out how woodpeckers can bang their skulls off a tree and not suffer brain damage. He has nothing left to prove to anyone, he got an Ig Nobel, and he has already done his part for science. Why tackle this big subject in a controversial area?
"Well, I don't play golf. Biology is also my hobby and I go out into the field on weekends and I look around. That probably stems from growing up in West Virginia in a rural community where there weren't a lot of other things to do. So if I saw an eagle I would wonder how come that eagle had such good eyesight. Over time I learned that the human cornea is not perfectly clear. We always think because we are looking through it, it must be window glass clear. It turns out it isn't - neither is window glass - our corneas have some scattering of light and eagles have less than we do because their cornea is thinner. Each thing evolves to its own niche.
"Writing a book like this is for people who have that same curiosity but won't necessarily have biology or ophthalmology expertise, they understand science the way a Science 2.0 or a Scientific American reader does but they are not experts."
One misconception in evolution is the notion that evolution occurred in some sort of straight line. In reality, evolution, being unplanned, does things that are a little crazy, as any man who got a look at the biological workings of his reproductive system for the first time and started chuckling at how poorly designed it is can attest. The evolution of the eye is crazy in too many ways to count. It's craziness is the best affirmation of natural selection. You just couldn't make it up.
To explain that craziness takes a certain amount of narrative but it's always difficult to make proper metaphors and analogies for complex topics. In explaining how eyes have evolved so differently, how do you convey that a mantis shrimp has 16 visual pigments while a human only has 3 (well, maybe 3.5 with that nocturnal part)? We're only going to conceptualize it in terms of our own vision, as in theirs is just 'better' - we simply cannot comprehend it.
With 16 visual pigments, the mantis shrimp looks even more incredible to another mantis shrimp. It's okay to be jealous. Photo: Wikipedia.
We might liken it to how a cat can hear frequencies higher than we can or how a dog can smell better. Heck, we could liken it to how much different my advanced press copy in black and white looks compared to the published book. I am serious, it is stunning to see the final version if you look at the press copy first. The paper stock is terrific, the colors are vibrant. That is how those shrimp see all of the time, except throw in UV and black body radiation.
Only fickle Mother Nature could have a laugh by giving crustaceans who just need to outrun barracudas such ridiculously great eyes while we're stuck without trinocular vision in each eye to help us play Battlefield 3.(2)
"A biologist can predict what an ancestor should be like and a geologist can predict where and in what conditions a fossil of it might be found and a paleontologist can go and find it. How do you do that with an eye?" I asked.
"Parsimony. If we have two lineages that are not directly related but they have common features that would otherwise be hard to evolve, it is likely that they were a node that diverged and had those features, maybe a little more basal. Then you can get molecular biology to back that up, to show that the genes are similar."
"How many times do you estimate the eye has evolved?"
"Tough to say but probably 40 times. Any audience reading this will know that evolution occurs in gradual steps, stops and starts, rather than just being fully formed. Some people have asked 'What is the use of half an eye?' and it can be quite useful. There is a shrimp that has a fully formed eye in its larval stage closer to the surface of the water and as it grows it sinks and when it is fully formed it no longer has an eye, it has less than half of an eye, really just a strip of retina, but it lets the shrimp detect IR and black body radiation and the risks from vents on the ocean floor."
As a final question, I asked what period he would regard as most vital to the eyes we have today, something I didn't see addressed in the book. It's a philosophical question.
"That's a little like asking you to name your favorite child," he laughed, so I assured him his answer was completely on the record. No pressure.
He said he believed it would turn out to be the Ediacaran, which was a surprise, because the Cambrian after that gave rise to our camera style eye.
"The first eye is not known to be Ediacaran," he said, "I am not saying that it was, but I think we will eventually find that the Ediacaran did have the first eye, it may turn out to have a more evolutionarily stable, robust eye than we know about right now, and it may even be as old as 600 million years, well before the 543 of the Cambrian.
"A study came out a short while ago from work on Kangaroo Island showing mosquito eyes were around 540 million years ago - that was a fossil - so that was the distant Cambrian but I don't think it sprang up even then, I think it developed before that."
I was a little sad; I wanted him to say the Permian because that is what I would have thought and I wanted to feel clever after reading the book. When 96% of life goes extinct the survivors are determining how we got where we are. It would have made me feel a little smarter had he agreed but he surprised me with a better answer I hadn't considered, as does the book when it introduces questions and then answers them. His knowledge is comprehensive and you can tell he wrote the book with the same spirit that he used learning science in West Virginia, wandering through whatever valley and stream his questions took him. He tackles questions and time periods, then revisits them again because he knows a smart audience will have new questions based on new information, and sometimes that will mean coming to a dead end and going a different direction before finally arriving at the destination.
That's a process a lot like the evolution of the eye itself.
(1) Apparently I am not wrong in not remembering. Schwab listed examples from the 1940s and the 1970s, yet a lot of ground has been covered since then and no definitive books on the eye have been written since.
(2) Trinocular vision is probably the only thing that could make this game look better: