The human eye was once considered so complicated and well-designed that it posed a problem for evolutionary biology — and an opportunity for creationists. How could something so useful evolve from random genetic mutations? Creationists took advantage of the conundrum of eye evolution, posing it as an example of something that, they claimed, must have been made by a higher power — or at least steered by one, which is the crux of the pseudoscientific idea of intelligent design. To this day, an online search for "eye evolution" yields multiple results from religious groups trying to muddy the scientific waters.
Yet to Mother Nature, the evolution of the eye isn't really a difficult feat at all, it turns out. Indeed, there are about 10 different types of eye design in the animal kingdom, and eyes have evolved independently across animal species approximately 50 times or more. Furthermore, genetic evidence reveals that eyes can evolve relatively quickly — in as short as just a few hundred thousand years. That may sound like a long time, but on geological timescales, it's a blink of an … well, you know.
Even though vision is complex, the genes that make it possible are abundant and it doesn't take much to tweak them in a way that benefits an animal.
In other words, the eye isn't really a problem for evolution; in fact, nature can't stop evolving eyes. And those eyes are incredibly varied: just look at some of the varied and bizarre eyes on animals like sea urchins, vultures or dragonflies. But scientists have only recently begun to figure out how eye evolution is able to happen so fast, and how and why it happens so often in natural history.
"Vision is remarkable because it's a process that reciprocally shapes what the world looks like," explains Adriana Briscoe, an evolutionary biology professor at University of California, Irvine who specializes in butterfly vision. In other words, we not only use our eyes to generate reality, but to influence it.
To get even more fundamental, eyes are capable of capturing and interpreting wavelengths of light. But for a long period in early life, some 3.7 billion years ago, most creatures were completely blind. Then, around 538 million years ago came the Cambrian Explosion, one of the most momentous periods in the history of life on Earth, in which life began to rapidly diversify. This is when most of the major animal groups started to appear in the fossil record.
"Before the Cambrian Explosion, we don't really see much going on or at least we can't really detect eyes very well in the available fossils," Briscoe explains. "And what's really interesting about this is that during the Cambrian Explosion, we see both invertebrate compound eyes and vertebrate camera style eyes suddenly appear in the fossil record. And they do so alongside organisms that have simple eyespots or more primitive-looking compound eyes."
The first eyes in history were extremely simple light-sensitive cells known as photoreceptor cells, which are a type of neuron. When a light particle called a photon hits them, they change shape, sending nervous impulses to the brain. (It's worth noting that it's not really possible to have vision without a brain. Sorry, plants and fungi, but you can't technically "see" like animals.)
"Because these genes are used over and over in development in in the same organism, it's relatively straightforward to make a few changes in gene regulation that that are going to produce eyes. So a lot of the mystery associated with eye evolution goes away with that knowledge."
These primitive photoreceptors can detect light, but that's about it. They're too rudimentary for forming images. For example, if a species of blind worm at the bottom of the ocean could develop the ability to detect shadows or movement, that could be enough for the creature to flee or detect food. This advantage would allow the worms to pass on their genes, allowing for natural selection to continue favoring genes that produce photoreceptor cells and benefit its survival.
Sometimes these photoreceptors form clusters called eye spots, like in some flat worms or jellyfish. But this sensitivity to light can get too dialed up, a process known as "bleaching." In response, an organism might develop pigment cells that prevents photoreceptor cells from becoming overwhelmed. It also starts to give more dimension to what one is seeing.
"If you put the pigment cells in one part of the tissue compared to another, you can begin to get a sense of directionality to light at that point," Briscoe says. "What we think starts to happen on the pathway to develop a camera-type eye is that simple layer of photoreceptor cells starts to invaginate. Sort of forming a little cup, a little dimple. And together with the pigment cells, it enhances directionality."
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This directionality becomes even more precise if the eyes can move around. The eyes in some insects, like ants, can't change position. But everyone is familiar with the googly-eyed chameleon, which can move its peepers independently of one another. Human eyes can wobble around in their sockets, which is helpful so we don't have to turn our heads to focus on something. They essentially function as cameras — or more precisely, cameras were invented using the same principles as certain eyes.
Essentially, a camera is a chamber in which light is funneled through a pinhole. Whether it's a camera on your phone, an old school film camera or the globes in your skull, all cameras work on the same principles. The tiny hole allows the light to be focused, bringing out definition and allowing for more focused imagery. Our eyes have lenses called corneas and instead of a sliver of film or an image sensor, we have a retina, a layer of tissue in the back of our eyes that absorbs light and sends signals about it to the brain.
These types of eyes evolved in humans, but also in cephalopods, which include squids and octopuses. It's a little weird that we have eyes so similar to underwater molluscs which have more in common with slugs than us, but it's true. This is an example of convergent evolution, in which something functional emerges entirely separately. The quintessential example being the development of wings in insects, birds and bats, which all came via different forks in the tree of life.
Some animals evolve eyes and then later shed them. Creatures that live in total darkness their entire lives, such as blind salamanders that dwell in caves or fish that live in total darkness at the bottom of the ocean, have no need for vision. Indus River dolphins have eyes so small that experts believe they are functionally blind, barely able to sense more than the intensity or direction of light. But they spend most of their time in extremely muddy water, so seeing isn't really a possibility, let alone a priority.
Why would evolution work "backwards" like this? First, discard the idea that evolution flows in a single direction. Humans are not the "apex" of evolution, though we're certainly quite good at natural selection. Everything from our eyes to our complex intelligence are simply the result of a long cascade of genetic traits that allowed us to continue breeding, but there is no true hierarchy to evolution. In some instances, discarding vision makes total sense.
"Photoreceptors are incredibly energetically costly to maintain. Perhaps they're the most expensive of all tissues," Briscoe says. "Because energy is scarce, that scarcity drives tradeoffs in how organisms allocate their resources. If an animal ends up in a cave or in the deep sea, where vision is no longer needed, natural selection will reallocate that energy and that tissue to an alternative purpose. And so taste, smell, touch, for example, might become more significant for that animal."
Even though vision is complex, the genes that make it possible are abundant and it doesn't take much to tweak them in a way that benefits an animal. In 1997, for example, scientists were able to generate extra eyes on fruit flies' antennae, wings and legs by manipulating a single gene.
"There are these circuits of interacting genes that get turned on to specify an eye or a leg or an antenna. But a lot of those genes get deployed in other tissue types and other developmental stages," Briscoe explains. "Once you have a circuit evolve, because these genes are used over and over in development in in the same organism, it's relatively straightforward to make a few changes in gene regulation that that are going to produce eyes. So a lot of the mystery associated with eye evolution goes away with that knowledge."
There's still a lot to learn about how eyes evolved and why it's happened so many times. For example, a study published this month in the Proceedings of the National Academy of Sciences revealed that bacteria played a crucial role in the development of the vertebrate eye by transferring a gene across domains. So not only does vision arise by modifying existing genes but also by acquiring and integrating new genes from different sources.
Eyes are beautiful, intriguing and extremely useful organs which have inspired endless metaphors and artwork. From a scientific perspective, their complexity is just as compelling, but by studying these fascinating organs, we can make their complexity a little less impenetrable.
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