Many believe that animals use their eyes in the same way as people, but there is evidence that shows this is not the case.
Dan-Eric Nilsson, a Swedish scientist, studies the box jelly fish’s eyes. Although we only have two eyes, the box jelly fish has 24 dark brown eyes which are in four clusters, known as rhopalia. Nilsson has created a model which resembles a golf ball with tumours appearing as sprouts. They are anchored to the jelly fish by a stalk, which is flexible.
In each rhopalium there are 4 eyes for detecting light, but the other 2 have lenses that focus on light, allowing them to see images, but the resolution is lower than in human eyes. So what information does the jelly fish need? It is a simple animal made of gelatin with stinging tentacles trailing behind it.
Nilsson and his group of researchers showed that some of the eyes are used to pick out obstacles. Further research showed that the upper eyes look upwards, and continue to do this even if the jelly fish is upside down. This eye can detect cark patches, which tells the jelly fish that there are mangroves above, so the small crustaceans that it feeds on are in the area. If it is in open water, it can only see bright light, so there will be no food available. Its eyes assist the jelly fish to find food and watch out for obstacles.
The animal kingdom has a myriad of eyes. Some can see only in monochrome, black and white, others are able to see the entire rainbow of colors, and others see light that we are not able to see at all. Some animals are not able to work out the direction light is coming from, but others can see prey from a great distance. Fairy wasps have eyes almost the size of an amoeba, but others, such as the giant squid have eyes almost the size of dinner plates.
Although the eyes of a squid work in a similar way to ours, the fly has a compound eye. The incoming light is divided between many separate units, and each of these units has their own lens and photo receptors. The eyes of humans, flies and squid are to be found in pairs on the head. Scallops, on the other hand, have eyes on the mantles. The eyes of a sea star are to be found on the tips of their arms, and the sea urchin seems to behave as one big eye. There are different forms of eyes – some with bi-focal lenses, some with mirrors, and others that can see in all directions at the same time.
The diversity of eyes can be bewildering. Although all eyes are able to detect light and light behaves in a manner we can predict, it has many different uses. It can tell the time of the day, how deep the water is, and if it is shady or sunny. For example, the box jelly fish uses it to find safe feeding areas. We use it to see around us, to understand the expressions on other faces, and to read. The types of tasks that eyes perform seem to be unlimited. In order to see how eyes evolved, we need to look at their structure, and more importantly, learn how animals are using their eyes.
It was around 540 million years ago that our predecessors appeared on earth, during the Cambrian explosion. There are fossils of many of these creatures and some are in a state which has allowed scientists to fully examine their eyes and to study how they saw the world.
Although we are able to examine these eyes, we are not able to find information about how animals with no sense of sight first started to ‘see’. Even Charles Darwin found the development of the eye puzzling, he said suggested that natural selection has certainly played a role in the development of the many different gradations of the eye.
The range of types of eyes goes from the more primitive patches that are sensitive to light, for example, the earthworm, to the superior sharp eyes of a bird such as an eagle. Nilsson has shown that evolution from the more primitive type of vision to a sharper, clearer vision could happen quite quickly.
In an experiment Nilsson began with a small patch of pigmented cells which were sensitive to light. It became a little thicker and increased its curvature at each generation. A basic lens formed, which also improved. He found that with even the smallest improvement in each generation, it took 364,000 years for the organ to function as a ‘camera’.
This doesn’t mean that all simple eyes move along the same path. Eyes today have evolved to meet the users’ needs. A sea star doesn’t see color, intricate detail, or objects moving quickly, because it doesn’t need these qualities. The sea star needs to be able to locate coral reefs so it can go back home. It has no need of other qualities.
Nilsson notes that eyes didn’t change from simple and basic to perfect; they evolved to perform tasks more efficiently. Nilsson developed a model of eye evolution, based on the tasks of the eyes, not the physical structure. There are four stages:
- The initial stage is being able to see the ambient light so the animal can tell day from night, and the depth in water. To do this an animal only needs a photoreceptor. A small relative of the jelly fish called a hydra does this. Other researchers have shown the hydra uses these receptors to control the stinging cells, and allows it to ‘see’ with greater ease in dim light.
- The second stage will allow animals to see where the light is coming from. In order to do this a shield is developed which blocks out some light. It is similar to a one-pixel view. It is not vision as we know it, but allows an animal to move either toward or away from the light.
- The third stage allows these photoreceptors with shields to gather into groups, allowing each to point in a different direction. This allows the view to become more integrated. Although blurred and grainy, images can be seen. This is the moment when the photoreceptors become more like eyes as we know them.
- In stage four lenses are formed and vision becomes clearer and more detailed. Nilsson suggests that it may have been the development of this stage that started the Cambrian explosion as prey and their predators had additional capacities. Animals evolved in many ways during this time, and so did eyes.
The basic structure of eyes that we see today was present during the Cambrian era, but there is a lot of variety. The male version of the mayfly has a large compound eye which allows it to watch the sky for the form of a female. The four-eyed fish is able to look up to the surface of the water, and also keep watch below for any danger. Human eyes are fast, but birds of prey have superior resolution.
Nilsson’s research has shed light on an ongoing debate about the evolution of eyes. Some scientists believed there were many origins; others suggested there was only one step in their evolution. This latter idea was based on the discovery of a gene, called Pax6, which controls the development of eyes.
They are all right. The eyes of many organisms have evolved from Nilsson’s first stage. All eyes begin with the same building block, a protein called an opsin. Opsins hold a chromophore which is able to take in the energy of a photon. The chromophore and the opsin set off the start of vision.
Although there are a myriad of different opsins, they are related. Some years ago a researcher in Hawaii found that all opsins come from a single predecessor. From here they developed into a huge variety.
Opsins didn’t come from nothing. It seems they developed from proteins that behaved more like clocks. These original proteins contained melatonin. Melatonin is a hormone that manages our body clocks. Light destroys it, so when it is not there an animal can sense that daylight is coming. It needs to be continually made as it will only work once.
Chromophores linked to opsins change shape when light enters, and they are able to revert to the original shape. These are so efficient that now we have variations on that theme.
Lenses are different. Lenses are almost all made from proteins, called crystallins. All different animal groups have developed their own crystallins. In nature the strangest lenses don’t even have crystallins. A group of marine molluscs called chitons appear as ovals decorated with armour-like plates. Each plate has a myriad of eyes from Nilsson’s stage 3, and each has a lens. These lenses are created from a mineral, aragonite, which forms from calcium and carbonate.
Evolution has ‘played’ with vision, changing materials for new functions and creating complex structures from simple ones. Despite this there are imperfections. For example, if a fly was to see the world as we do, its eye would need to increase in size to almost a metre wide. Insects are very successful with the compound eye, but could do even better if they had developed an eye more like ours.
A colleague of Nilsson, Eric Warrant points out that an insect’s temporal vision is much faster than ours. The eye of a dragonfly is able to see almost 360 degrees, but our eyes cannot. The elephant hawk moth is able to discern color with only the light from the stars. Warrant suggests there is no one eye that does everything better.
Our eye has some problems. The design of our eyes leaves us with a blind spot due to the structure. Despite this blind spot there are long cells, known as Muller glia, that allow our brains to fill in this blind spot. The structure of our eyes can also lead to blindness. If the design was different this wouldn’t happen.
Eyes have become as complex as is needed, and if the need decreased, the eyes evolve accordingly. Whereas most birds and reptiles see colors with 4 types of photoreceptors, each with an opsin for a different color, mammals lost two of these cones. Because mammals evolved from nocturnal creatures, they didn’t have the same need for color vision.
It remains the same for most mammals today. Dogs have 2 cones, one each for blue and red. Some primate redeveloped a cone sensitive to red, probably to enable them to see food. When marine mammals went to the sea, they lost the cone for blue, and some whales lost the red one as well. They need to see in the deep ocean, which is dark, and they don’t need to see in color.
If there remains no reason to see at all animals can completely lose their eyes. One example is the Mexican tetra. During the Pleistocene era some of these fish moved into deep caves. Being a pitch black environment, their eyes were not much use, and today they have skin over the area where their eyes once were. This happens because of the energy required to create and maintain eyes. They don’t want to waste energy on something that is no longer needed.
Another scientist, Tom Cronin, is studying the mantis shrimp. The mantis shrimp was 2 compound eyes that appear like muffins that have been set up on a stalk. This shrimp has eyes made up of three different regions, able to focus on a narrow strip of the environment, and allow the shrimp to perceive depth without help from the others. They are able to see ultraviolet light, as well as polarised light. They also have 12 color receptors, while we only have 3.
For a long time scientists thought the mantis shrimp must have had the most superior ability to discriminate color, but in 2013 a scientist from the University of Queensland showed they were definitely not good at color discrimination. The next question then is why they have all these receptors. One theory suggests that although the mantis shrimp cannot discriminate, it can recognise color, enabling it to strike its prey quickly. Still, may feel there is more to this.
Nilsson says we need to know how the animals use them to help us understand why they are a certain way. We need to see the world through their eyes to fully understand.