How Eyes Evolved - A Worm's Eye View

Vanadis cf. formosa

Photo: Michael Bok

Marvelous eyes, but you be the judge of this sea critter’s beauty. Vanadis is a byname of the Norse goddess of love, Freya.

Photo: Michael Bok

Mediterranean marine worm has developed eyes "as big as millstones"; – University of Copenhagen

Creationists love to cite the eye as an example of irreducible complexity' which could not have evolved by Darwinian step-wise evolution because anything less than a whole eye can't function as an eye.

They even misquote Darwin who, so they claim, admitted the evolution of the eye could not be explained, as though the entire unifying theory of biology rests on the opinion of one man who wrote his books about 160 years ago. But in their usual intellectually and morally bankrupt way, what they fail to do is to give the whole quote in the context in which Darwin used it to show that his theory of evolution was fully capable of explaining how something as complex as an eye could have evolved. It was typical of his style that he would set out a problem for biology, then show how his theory solved that problem. (see the full quote later).

Tell me how many distinct types of eyes there are in nature, with examples, please. Eyes in nature have evolved into a fascinating array of forms, each suited to the specific needs and environments of the organisms possessing them. Here are some examples of distinct types of eyes found in nature:

1. Simple Eyes (Ocelli):
   
   • Arthropods:
     Many insects, such as bees and butterflies, have simple eyes known as ocelli. These eyes are typically used to detect changes in light intensity and aid in navigation.
   • Mollusks:
     Some mollusks, like certain species of snails, also possess simple eyes that detect light.
2. Compound Eyes:
   
   • Arthropods:
     Insects like flies, beetles, and dragonflies have compound eyes. These eyes consist of many individual visual units called ommatidia, each with its own lens and photoreceptor cells. Compound eyes provide a wide field of view and are excellent for detecting motion.
3. Camera-Type Eyes:
   
   • Vertebrates:
     Most vertebrates, including humans, have camera-type eyes. These eyes consist of a single lens that focuses light onto a retina containing photoreceptor cells (rods and cones). Camera-type eyes provide high-resolution images and are capable of forming detailed visual perceptions.
4. Pit Eyes:
   
   • Some Fishes:
     Certain species of fish, such as deep-sea fish like the barreleye fish, have eyes that are adapted to low-light environments. These eyes are characterized by tubular structures that point upwards, allowing the fish to detect prey silhouetted against the dim light above.
5. Reflective Eyes:
   
   • Nocturnal Animals:
     Some nocturnal animals, such as cats and certain types of frogs, possess eyes that appear to glow in the dark. This effect is due to a layer of cells called the tapetum lucidum, which reflects light back through the retina, enhancing low-light vision.
6. Eyespots:
   
   • Various Organisms:
     Many simple organisms, such as certain types of algae and single-celled organisms like Euglena, possess eyespots. These are light-sensitive organelles that can detect the direction of light but do not form images. They help these organisms move towards or away from light sources.
7. Multiple Eyes:
   
   • Some Arachnids:
     Certain spiders, like jumping spiders, have multiple pairs of eyes arranged in different configurations on their heads. These eyes serve various functions, such as detecting prey, assessing environmental conditions, and navigating.

These are just a few examples of the diverse range of eyes found in nature. Evolution has led to the development of eyes adapted to an astonishing variety of habitats, lifestyles, and visual needs across the animal kingdom.

And it's demonstrably false to claim that anything less than a full eye can't serve any useful purpose because examples of simple eyes, from light-sensitive eye-spots to varying degrees of complexity exist in nature. It is also a matter for debate whether eyes have evolved multiple times or whether they all have the same common origin. If they have evolved multiple times this is additional evidence for the possibility of their evolution.

This article, in part, goes some way to inform that debate.

Two biologists, Michael J. Bok and Anders Garm, working at the University of Copenhagen's Marine Biology Department, together with Armando Macali of Department of Ecological and Biological Sciences, Tuscia University, Italy have discovered that a bristle worm, Vanadis cf. formosa, which is found around the Mediterranean island of Ponza in the Bay of Naples has a massive pair of eyes which weigh as much as twenty times as much as the worm's head. Scaled up to human size, this would mean each of our eyes would weigh about 50Kg.

These massive eyes give the worms an unprecedented visual acuity for an invertebrate, approaching that of the best mammalian eyes, yet the worm is wholly nocturnal!

So, the mystery is why have they evolved such massive eyes if they don't use them in daylight? There must be some evolutionary advantage for the benefit to outweigh the cost of growing such large eyes.

The scientists have published their findings, open access in the journal Current Biology and explain it in a Copenhagen University news item:

MARINE BIOLOGY Scientists are amazed at the discovery of a bristle worm with such sharp-seeing eyes that they can measure up to those of mammals and octopuses. The researchers from University of Copenhagen and Lund University suspect that these marine worms may have a secretive language, which uses UV light only seen by their own species. The advanced vision of such a primitive creature helps to finally settle an epic debate about the evolution of eyes.

The Vanadis bristle worm has eyes as big as millstones – relatively speaking. Indeed, if our eyes were proportionally as big as the ones of this Mediterranean marine worm, we would need a big sturdy wheelbarrow and brawny arms to lug around the extra 100kg.

As a set, the worm's eyes weigh about twenty times as much as the rest of the animal’s head and seem grotesquely out of place on this tiny and transparent marine critter. As if two giant, shiny red balloons have been strapped to its body.

Facts: The Worm
The Vanadis worm belongs to a family of large-eyed bristle worms, or polychaeta, found in many parts of the world.

Its eyesight rivals that of rodents such as mice and rats. Vanadis' eyes weigh about 20 times more than the rest of its head

The worms can see UV light and focus on relatively small objects, tracking them as they move.

It is nocturnal. The researchers believe that these bristle worms use their eyes to communicate for mating and hunting prey.

Vanadis bristle worms, also known as polychaetes, can be found around the Italian island of Ponza, just west of Naples. Like some of the island's summertime partiers, the worms are nocturnal and out of sight when the sun is high in the sky. So what does this polychaete do with its walloping peepers after dark? And what are they good for?

Neuro- and marine biologist Anders Garm from the University of Copenhagen’s Department of Biology couldn’t ignore the question. Setting other plans aside, the researcher felt compelled to dive in and try to find out. He was hooked as soon as his colleague Michael Bok at Lund University showed him a recording of the bristle worm.

"Together, we set out to unravel the mystery of why a nearly invisible, transparent worm that feeds in the dead of night has evolved to acquire enormous eyes. As such, the first aim was to answer whether large eyes endow the worm with good vision," says Michael Bok who together with Anders Garm, authors a new research article that does just that.

It turns out that the Vanadis’ eyesight is excellent and advanced. Research has demonstrated that this worm can use its eyes to see small objects and track their movements.

It's really interesting because an ability like this is typically reserved for us vertebrates, along with arthropods (insects, spiders, etc.) and cephalopods (octopus, squid). This is the first time that such an advanced and detailed view has been demonstrated beyond these groups. In fact, our research has shown that the worm has outstanding vision. Its eyesight is on a par with that of mice or rats, despite being a relatively simple organism with a miniscule brain.

Anders Garm, co-corresponding author.
Marine Biological Section
Department of Biology
University of Copenhagen, Copenhagen, Denmark

This is what makes the worm's eyes and extraordinary vision unique in the animal kingdom. And it was this combination of factors about the Vanadis bristle worm that really caught Anders Garm's attention. The researcher’s work focuses on understanding how otherwise simple nervous systems can have very complex functions – which was definitely the case here.

UV light and a secret language
For now, the researchers are trying to find out what caused the worm to develop such good eyesight. The worms are transparent, except for their eyes, which need to register light to function. So they can't be inherently transparent. That means that they come with evolutionary trade-offs. As becoming visible must have come at a cost to the Vanadis, something about the evolutionarily benefits of its eyes must outweigh the consequences.

Precisely what the worms gain remains unclear, particularly because they are nocturnal animals that tuck away during the day, when eyes usually work best.

Facts: Bioluminescence
Bioluminescence is when organisms are luminescent, i.e., capable of producing light using their own power. This can be done chemically within the body, as with glow-worms.

Should the researchers succeed in documenting it, the Vanadis worm could become the first animal proven to use UV bioluminescence, meaning that they create ultraviolet light naturally, for communication, among other things.

No one has ever seen the worm during the day, so we don't know where it hides. So, we cannot rule out that its eyes are used during the day as well. What we do know is that its most important activities, like finding food and mating, occur at night. So, it is likely that this is when its eyes are important.

Anders Garm

Part of the explanation may be due to the fact that these worms see different wavelengths of light than we humans do. Their vision is geared to ultraviolet light, invisible to the human eye. And according to Garm, this may indicate that the purpose of its eyes is to see bioluminescent signals in the otherwise pitch-black nighttime sea.

We have a theory that the worms themselves are bioluminescent and communicate with each other via light. If you use normal blue or green light as bioluminescence, you also risk attracting predators. But if instead, the worm uses UV light, it will remain invisible to animals other than those of its own species. Therefore, our hypothesis is that they’ve developed sharp UV vision so as to have a secret language related to mating.

It may also be that they are on the lookout look for UV bioluminescent prey. But regardless, it makes things truly exciting as UV bioluminescence has yet to be witnessed in any other animal. So, we hope to be able to present this as the first example.

Anders Garm

Exciting for robotics research and evolutionary history
As a result of the discovery, Anders Garm and his research colleagues have also started working with robotics researchers from the Maersk Mc-Kinney Møller Institute at the University of Southern Denmark (SDU) who find technological inspiration in biology. Together, they share a common goal of investigating whether it is possible to understand the mechanism behind these eyes well enough so as to translate it into technology.

Together with the robotics researchers, we are working to understand how animals with brains as simple as these can process all of the information that such large eyes are likely able to collect. This suggests that there are super smart ways to process information in their nervous system. And if we can detect these mechanisms mathematically, they could be integrated into computer chips and used to control robots.

Anders Garm

According to Garm, Vanadis' eyes are also interesting with regards to evolutionary theory because they could help settle one of the heaviest academic debates surrounding the theory: Whether eyes have only evolved once – and evolved into every form that we know of today, or whether they have arisen several times, independently of one another, in evolutionary history.

Vanadis' eyes are built simply, but equipped with advanced functionality. At the same time, they have evolved in a relatively short evolutionarily time span of just a few million years. This means that they must have developed independently of, for example, human eyes, and that the development of vision, even with a high level of function, is possible in a relatively short time.

[How Creationists Lie To Us]

Extra Info: The eye and evolution
In general, eyes come in complex sizes, which is the case with the human eye, for example. Evolutionary skeptics have often pointed to the eye and said 'see for yourself, this must have been created by God'.

The eyes of the Vanadis worm have a surprisingly simple natural "design" that has evolved in a relatively short time span compared to typical evolutionary timelines – i.e., a few million years. Despite their simplicity, they are advanced.

The emergence of eyes has been the subject of many debates since Darwin presented his theory of evolution in Origin of Species, both among those who are religious and skeptics outside science, as well as among eye biology and vision researchers.

One of these debates has been about whether eyes have only evolved once – and into every form that we know today, or whether they have arisen several times, independently of one another, in evolutionary history. Research in recent years has provided a number of pieces of evidence to support the latter, and the eyes of the Vanadis worm are another powerful piece of evidence in that direction.

This means that they must have developed independently of, for example, human eyes and that the development of vision, even with a high level of function, is possible in a relatively short time. Because, this worm is so young on an evolutionary scale.

Michael J. Bok, co-corresponding author.
Lund Vision Group
Department of Biology
Lund University, Lund, Sweden.

Darwin and the eye
In Charles Darwin’s major work, On the Origin of Species, he wrote about the incredible nature of the eye in relation to his theory of evolution by natural selection. He is often quoted by evolutionary skeptics as saying:

To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I confess, absurd in the highest degree...

Charles Darwin

But this quote forgets to add the end of the passage:

Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.

Charles Darwin (Origin of Species, chap. 6)

Having vindicated what Darwin said, and exposed creationists claims as bogus and designed to mislead, the scientists have published their findings, open access in Current Biology:

Summary

High-resolution object vision — the ability to separate, classify, and interact with specific objects in the environment against the visual background — has only been conclusively shown to have evolved in three of the thirty-five animal phyla: chordates, arthropods, and mollusks (cephalopods)¹. However, alciopid polychaetes (Phyllodocidae, Alciopini), which possess a pair of bulbous camera-type eyes, have also been hypothesized to achieve high acuity. In this study, we examined three species of night-active pelagic alciopids from the Mediterranean Sea. Our optical, morphological, and electrophysiological investigations show that their eyes have high spatial acuity and temporal resolution, supporting the notion that they are capable of active, high-resolution object vision. These results encourage interesting hypotheses about the visual ecology of these enigmatic polychaetes.

Main text

Though hypothesized to be capable of high-resolution vision, there are sparse data on the eyes of alciopids (Figure 1A,B), partly due to inconsistency in obtaining high numbers of healthy individuals for experimentation. Previous studies have characterized the morphology of their eyes² and the fine structure of their retina, revealing a dense array of microvillar photoreceptors³. Electrophysiology subsequently suggested multiple photoreceptor spectral types, maximally sensitive to ultraviolet and green wavelengths, respectively⁴. These previous studies, however, lacked information about the resolving power of the optics and the temporal resolution of the retina, both of which are crucial for high-resolution object vision¹. In order to examine large numbers of healthy alciopids we identified a location near Ponza Island, Italy where three species (Torrea candida, Vanadis cf. formosa, and Naiades cantrainii) could be collected by hand in shallow water.

Figure 1 Optics, morphology and physiology of alciopid eyes.

Morphological and optical analysis of the eyes and lenses of the three species of alciopids revealed that the spherical lenses ranged between ∼150 and ∼550 μm in diameter (Table S1). Lens diameters and eye sizes varied within species, with N. cantrainii having smaller eyes on average (Figure S1A,B). We used low frequency square-wave gratings imaged through isolated lenses to measure their focal length and found that it correlated closely with lens diameter in all three species (Figure 1C–E and Table S1). The F-number (focal length/aperture or lens diameter) ranged between 1.2 and 1.45, with T. candida generally having longer focal lengths (Figure 1E inset and Table S1). We saw no signs of spherical aberration, suggesting a graded refractive index in these lenses. The lenses also appeared to have little or no chromatic aberration at human-visible wavelengths.

By imaging square-wave black and white gratings of varying frequencies through the lenses (Figure 1C,D), we found that the potential imaging quality threshold approached 4–5 cycles per degree in all species (Figure 1F). Micro-CT images of T. candida and V. cf. formosa eyes showed a well-ordered receptor mosaic throughout the retina, with receptor densities equating to a sampling frequency between 0.5 and 1.5 cycles per degree (Figure 1G and S1). The examined eye from T. candida had slightly lower receptor densities, but both species showed similar relative density patterns in vertical and horizontal transects through the retina, with higher densities in ventral and anterior regions. These results show that alciopid retinas are notable for achieving high anatomical acuities over a large field of view (>150°) when compared with other small invertebrates5. Some spiders, for instance, achieve higher acuities, but only for very limited viewing angles, and even human acuity rapidly drops to 10% of the maximum 20° from the center of the fovea⁶.

Electroretinograms recorded from the eyes of all species in response to a sinusoidal light intensity stimulus of varying frequency showed flicker fusion thresholds (fft) of approximately 10 Hz, with T. candida, extending to nearly 15 Hz (Figure 1H). This is notably high among polychaete lateral cerebral eyes⁷, though the unrelated radiolar eyes of fanworms achieve temporal resolutions up to 35 Hz8. Here it should also be noted that alciopids are night active, stressing that their eye evolution has selected for relatively high temporal resolution. A fft of 10–15 Hz is putatively sufficient temporal resolution to detect and track other planktonic animals. However, this will depend on the hunting strategy for alciopids and movement patterns of their prey under natural conditions, both of which remain to be shown.

Our results show that the eyes of alciopids possess the anatomical, morphological, and physiological properties requisite for high resolution tasks and object vision. Previously only vertebrates, arthropods and cephalopods have been documented to possess object vision, with most polychaetes only having directional photoreception or low-resolution ‘ancient’ vision⁷.

It remains to be revealed why exactly alciopids have evolved such spectacular eyes. Their wide field of view without an obvious fovea would suggest that their vision is primarily concerned with spotting small or distant objects (potential prey, threats, or conspecifics) that could occur at any relative direction in the water column, while the alciopid itself engages in undulating swimming movements. The sparse behavioral observations available indicate that alciopids are active swimmers in the water column and we collected these at night, where we observed them to be attracted to the white light of our dive torches. Though we observed fluctuations in their numbers on subsequent nights, we did not observe any spawning behavior, besides a small number of T. candida that were apparently guarding floating egg masses. The smaller eyes of N. cantrainii, relative to body size, are also notable in that this species was observed to be less active, often adopting a curled-up posture and floating. These smaller eyes could affect sensitivity to point sources and the less active lifestyle putatively allows them longer integration times, which is supported by them having the lowest temporal resolution (Figure 1H).

The higher receptor densities in the ventral retina may suggest that alciopids are concerned with spotting objects silhouetted against the downwelling light. The previous observation that they have ultraviolet sensitivity⁴ would suggest that they are also active at shallower depths during the day where intensities of ambient ultraviolet light are high enough to support visual tasks. There is some evidence that alciopids feed on salps and ctenophores⁹, whose otherwise transparent tissue is absorptive in the ultraviolet¹⁰, and ultraviolet-vision could allow for easier detection of their silhouettes against ultraviolet backlighting. Additionally, as night active animals, alciopids’ vision may detect bioluminescent cues. Further studies will be required to fully explain the visual ecology of the alciopids and why they, apparently alone amongst the annelids, have evolved the capacity for object vision.

It's unusual that serious biologists these days bother to address the dishonest claims and misrepresentations of creationists, but these scientists obviously felt their field of expertise was being misrepresented by creationists, while their discovery had vindicated what Darwin actually said and supported the idea that eyes have evolved on multiple occasions. So, claims that they couldn't have evolved are demonstrably bogus - which of course is why creationists try to support their bogus claim by fooling their target dupes into thinking Darwin had somehow, perhaps carelessly or inadvertently, admitted they were right, or even that Darwin was an intelligent design creationist at heart.

It's an old creationists maxim that if you're going to misrepresent a scientist, wait until they're good and dead and can't refute your lies.

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