In The Blink Of An Eye

by Andrew Parker

  That relatives of Canadia and Wiwaxia today also have diffraction gratings is a nice test of the Cambrian finds. The spines and hairs of many living bristle worms, particularly those most closely related to Canadia and Wiwaxia, are highly iridescent. They have similar diffraction gratings and they produce colours comparable with those of the reconstructed surfaces of their Cambrian relatives. This makes the colour reconstructions of Canadia and Wiwaxia seem quite reasonable, and removes them from the realms of science fiction.

  The Burgess colours quickly made the news. New scenes of life in the Cambrian were computer-generated by a number of magazine artists, but these scenes were different from those we had become used to. These were in colour, and now the colours were accurate. The Cambrian was seen as never before.

  Full-colour models of Burgess creatures were also constructed in natural history museums. That ultra-impressive walk-through Cambrian reef at the Royal Tyrrell Museum also features an iridescent Wiwaxia, a couple of feet long of course. The addition of colour really does help to bring ancient animals to life, and now Wiwaxia is almost alive.

  This Burgess project had certainly revealed some interesting results, but what did they mean? A standard physics textbook, Born and Wolf’s Principles of Optics, affirms that diffraction gratings were conceived in 1819, when Joseph von Fraunhofer wound fine copper wire around a metal screw. Others credit the diffraction grating to the US astronomer David Rittenhouse, after his experiments of 1785. Now the date for the first diffraction grating has been pushed back a little further - some 515 million years. But on the serious side, some intriguing biological questions surfaced following the find of the Cambrian gratings. Why were these Burgess animals reflecting colour in the Cambrian? Was there a wide-ranging consequence to all of this? It was at this point that studies of animal colour and the Cambrian explosion first began to cross paths. It was not any old fossil that had been reconstructed accurately in colour, but one that existed relatively close to evolution’s grand event.

  These questions changed the course of my research and lie at the origin of this book. The book itself holds the answers. Although the finding of Cambrian colours adds nothing directly to the Cambrian enigma, it does provide a cryptic clue. And this was the first clue that I uncovered, which ultimately led to the writing of this book.

  Up to this point the chapters in the book have contained the thoughts that go through one’s mind, in the order they happen, while contemplating the questions that followed the Cambrian colour discovery. But there are further thoughts to be introduced, involving subjects that make up the final pieces of the Cambrian jigsaw puzzle. These will be covered in the next two chapters; the first of these subjects may indeed seem overdue.

  So much discussion of colour warrants consideration of its counterpart. There is a reason for the variety and sophistication of the colour we see today; ‘see’ is the operative word. One particular organ exists that conceives both the observer and the observed - the eye.

  7

  The Making of a Sense

  To suppose that the eye, with all its inimitable contrivances . . . could have been formed by natural selection, seems, I freely confess, absurd in the highest degree

  CHARLES DARWIN, On the Origin of Species (first edition, 1859)

  The preceding chapters have explained and emphasised the importance of light as a powerful stimulus to animal behaviour in the past and present, and revealed it as a driving force of evolution and a promoter of great biodiversity. This chapter is devoted to the eye and the reason for this influence of light on animals and their evolution - the sense of vision.

  Eyes are the detectors that convert the light waves travelling through the atmosphere into visual images. These light waves enter the Earth’s atmosphere from the sun, and bounce and reflect off objects that exist all around us. They are the same light waves that change when they strike an animal to relay information about its identity and whereabouts within the environment. Eyes pick up all this information. Eyes and only eyes conceive the sense known as vision. Electromagnetic radiation of different wavelengths exists in the environment; colour exists only in the mind.

  In Chapter 4 I questioned whether the Precambrian environment was similar to that found in caves today. By the end of this chapter we will be able to link light, eyes and vision, and understand that such a question is not well founded. We have established that the Earth is said to be 4,600 million years old, as is the sun. So sunlight would, to some degree, have struck the Earth’s surface well into the Precambrian - but it would not have entered caves. Not now, not then. Moving from here to the next question I will pose, we will approach the final solution to the Cambrian enigma. Two further clues remain to be found in Chapters 7 and 8, and these will provide the final pieces of the Cambrian puzzle. For the moment, however, we can look for a more immediate clue in the question: ‘When did eyes invent vision?’

  Before attempting to answer this specific question, a tour of the wide range of eyes found today is necessary if only to interpret fossil eyes. Darwin referred to the eye as an ‘organ of extreme perfection and complication’. The word eye implies an organ capable of producing visual images in order to distinguish objects using light. Extreme perfection and complication are obligatory characters of the more efficient eyes, and so the reference in Chapter 4 to the eye being a very expensive piece of equipment is really quite valid. But the eye itself is only Act One in the complete performance of seeing. Act Two involves transmitting visual information, in the manner of electrical cables, from the eye to the brain. In Act Three an image is formed in the brain. Vision employs the eye and brain of the beholder.

  The central aim of this chapter is to trace the introduction of the eye to Earth. Since only the eye is preserved in fossils, and not information relating to Acts Two and Three of the visual performance, this chapter will centre on the architecture of the eye itself - the main hardware. We will assume that an eye with good optical apparatus is linked to a brain where a good image is formed, and a poorly designed eye to a brain producing poor images. In other words, the complexity in the hardware is mirrored in the software. Only the box jellyfish can throw a spanner in the works of this theory, but the box jellyfish is destined to emerge as an oddball anyway.

  Vision - the formation of an image or picture from light waves - is the most sophisticated form of detecting light, but it is not the only one. The less sophisticated, or elementary, forms are relevant to Precambrian life, and so to the theme of this book. The elementary form of detecting light will be called ‘light perception’, and the receptors that perform this task ‘light perceivers’. The question of interest in the first part of this chapter is ‘To see or not to see?’ Throughout the remainder of this book, it is vital that these two possibilities and their associated organs are kept very separate.

  Not to see

  Light perception in bacteria, animals and plants ultimately involves organic molecules that undergo a simple reaction when hit by a package of light called a photon. Light perception takes place in many single-celled animals, such as amoebae and Euglena, where the fluid within the cell is sensitive to light. These animals use light to orientate themselves - to distinguish up from down.

  In multicelled animals, independent light-sensitive cells or organs of various complexities perform the task of light perception. The most elementary forms of light-perceptive organs are called ocelli. These are small cups containing a light-sensitive surface backed by dark pigment. Sometimes they are capped by a rudimentary lens. The simplest multicelled animals with these structures are the jellyfish.

  The marginal sense organs of jellyfish in some cases include ocelli, in addition to gravity, touch, chemical, pressure and temperature receptors. Indeed, ocelli are generally the most poorly developed sense receptors in jellyfish, with lenses lacking from most groups. The pigmented patches of most jellyfish are not known to detect light, and may have evolved rather as a light barrier - to absorb light and so shield the underlying sensory
cells that detect other stimuli. But in some jellyfish, where a lens covers the cup-shaped light-sensitive surface, the ability to respond to light on or light off has been established.

  Similar cup-shaped ocelli occur in members of many other animal phyla such as flatworms, ribbon worms, bristle worms, arrow worms, molluscs and sea squirts. An advantage of a cup-shaped light perceiver over a flat one lies in its curved surface. A beam of sunlight illuminates a curved surface, such as a hemisphere, in one region only. A flat surface, on the other hand, would be completely lit. So a curved surface can perceive the direction of the light source. Some maggots - the larvae of flies - possess flat light perceivers but still manage to find a light source by swinging their heads from side to side. This mechanism, not surprisingly, is uncommon.

  The elementary light detectors discussed so far cannot be called eyes because they don’t form images. Eyes are born when the light detection cells get serious and form a ‘retina’, a thin plate of nerve cells lining the inside of the eye. The retina will detect with accuracy whatever is projected on to it, so it is important that an image is first focused sharply on to the retina by some additional apparatus. A camera loaded with highly sensitive film would be useless without a lens. When all these conditions are satisfied, we have an eye - we have reached the stage of being able ‘to see’. And the size of the step taken to get here cannot be overemphasised.

  Figure 7.1 Marginal sense organs of the jellyfish Paraphyllina intermedia and Aurelia aurita, showing different levels of complexity (particularly in their light detectors).

  Based on the number of entrances for light, eyes can be divided into two types - simple and compound.

  To see

  ‘Simple’ eyes

  Simple eyes are so called because light is received through a single entrance - the simplest design solution for an eye . . . in theory. Molluscs may exhibit a wide variety of light perceivers, or ‘eyespots’, but they also boast the broadest range of eyes. And these are all simple eyes. But despite their inept title, simple eyes do produce visual images, and ironically their hardware is often quite intricate. There are three forms of simple eyes known in animals, and all can be found in molluscs.

  Nautilus, the subject of a palaeontological mystery discussed in Chapter 2, has a simple eye that is unique because an image is produced on its retina without the aid of a lens. For more than 2,000 years the Chinese have known that an inverted image is produced on the inside wall of a dark chamber if light enters only through a small hole in the opposite wall. Leonardo da Vinci revived this principle with his ‘camera obscura’. But the Chinese had, unknowingly, revived it too - the principle was practised by nautilus long before.

  The image-forming structure in the ‘pinhole eye’ of nautilus is a small pupil, or ‘pinhole’, formed in its dark iris. Light is not focused, but is received only through the pinhole, providing at least some degree of control. To gain accurate directional information, the retinal mosaic is remarkably fine so that light coming from a single point will illuminate several receptor cells. But serious disadvantages are inherent with this type of eye, which accounts for its rarity. A bright image requires a large pupil, whereas a sharp image requires a small pupil. The nautilus’ solution - a large range of pupil sizes or pinholes - unfortunately results in blurred images.

  In his book Optiks, published in 1704, Sir Isaac Newton revealed his plans for a telescope without a lens but with a curved, concave mirror instead. This mirror would focus light towards a focal point, in the same way that a modern satellite dish focuses radiation towards its receiver. At the focal point was positioned a small, flat mirror, angled to redirect the focused light out through a gap in the side of the telescope - the eyepiece. This ‘Newtonian’ telescope works well - it is popular today.

  Curved mirrors can also be successful substitutes for lenses in eyes. The scallop has many eyes just inside the edge of its shell. These eyes appear silver, like tiny mirrors - and indeed they do contain mirrors. Within each eye, a hemispherical concave mirror similar to the reflector in a car headlight lies behind the image-forming retina. Light passes almost unfocused through the transparent retina before it is reflected back, this time focused by the mirror. The light is focused precisely at the position of the retina. And now the retina absorbs the light rays, and an image is grabbed. The mirror is achieved by the same mechanism found in the skin of the Mexican cave fish - stacks of thin layers of various thicknesses. The mirror eye is an improvement on the pinhole eye because it can focus light. But with light first passing through the retina unfocused there is potential for this to be detected, and so the performance of the eye is limited. For this reason, the mirror eye is confined mainly to the scallop and a few related clams.

  The third type of simple eye in molluscs is found in a snail. The snail has an eye separated from the skin and containing a large spherical lens. This eye is known as the camera-type. It works in the same way as a camera in that a single lens focuses light on to a film, or retina, with an adjustable iris included to alter the quantity of light passing through its ‘pupil’. The general design is quite simple, but it is ideal for seeing images, and the variety of camera-type eyes to be found in other animal phyla testifies to its success.

  The most efficient eyes of bristle worms belong to a group known as alciopids. One member of this group lives on the surface of the sea and possesses camera-type eyes complete with a paired retina with refined layering, two distinct layers of ‘humoral’ material which fills the eyeball, and a well-developed spherical lens and ‘cornea’ - the outer covering of the eye. The retina contains about 10,000 light detection cells, and is positioned at the focal plane of the lens - the position where the lens focuses an image.

  Figure 7.2 The three types of simple eye - pinhole, mirror and camera-type - and their effect on light rays. Light receptors (retinas) are shaded. The mirror eye has an underlying mirror (dashed region) and the camera-type eye has a lens, both of which focus light to form clear images.

  The camera-type eye is the standard hardware for vision in vertebrates, both on land and underwater. Humans are one beneficiary, but in addition to bristle worms and molluscs it has also emerged in spiders and crustaceans within the arthropod phylum, velvet worms within a phylum all of their own, and in box jellyfish within the cnidarian phylum. The precise design of the camera-type eye is determined by how the lens is formed - it can be formed either inside the eye, or outside, where it is actually part of the skin or exoskeleton, and is technically the cornea.

  Focusing is all about bending light rays from different parts of the environment towards a common point. There are two factors which affect the bending of light rays - the differences in materials either side of a boundary, and the angle of that boundary relative to a light ray (think of a prism). Adaptations to vision on land are different from those underwater because, as we learnt in Chapter 3, light behaves differently in air compared to water - there is a material difference. Light does behave similarly in water and in the cornea, so it barely recognises a boundary as it enters the eyes of aquatic species. In this case, the lens within the eye must be responsible for most of the focusing. But light recognises a considerable difference between the cornea and air, and it is bent as it crosses their boundary at an angle. So the cornea of land animals acts as a powerful lens in its own right.

  James Clerk Maxwell tackled the subject of underwater focusing in the nineteenth century while contemplating his breakfast herring. Following a spontaneous dissection, Maxwell noticed his herring had a spherical lens. This is typical in fish - it bends light rays more than a thinner, oval lens because its surfaces are more steeply curved and present steeper tangents to light. But there is a problem with a spherical lens - spherical aberration. This is the reason why we do not use spherical lenses in cameras, and instead choose a series of ‘oval’ lenses. Spherical aberration occurs when light striking the periphery of a lens is focused at a different plane to light striking the central axis of the lens - the perip
heries bend light too much. So to focus both sets of rays simultaneously, the retina must be in two places at the same time. This is impossible. But fishes can see by focusing very sharp images - the question is ‘How?’ There is only one solution here, and Maxwell worked it out.

  If the curves, or rather tangents, are made less steep by flattening the lens, the focal point moves too far from the lens - a huge eyeball would be needed to house the retina. So if the angles can’t be changed there is only one other option for solving spherical aberration - change the materials. And indeed Maxwell suggested that the material of the fish lens is not uniform but is graded from the centre outwards.

  Today we know from precise measurements that the periphery of the fish lens has optical properties similar to those of water and causes light to bend only slightly. This compensates for the comparatively glancing angle with which light strikes the edge of the lens, which alone causes considerable bending of the light path. Near the central axis, this angle is not glancing but nearer to 90°. So to keep central light rays synchronised with those from the periphery of the lens, the lens material in the centre is optically very different from water and causes light to bend more, but also to slow down more. This effect on the speed of light is important since the path through the centre of the lens is now the shortest. To sum up the effect of this lens, all the light rays striking the eye at one instant will be focused on to the same point on the retina at the same time. Clever! Now a sharp image is formed . . . and formed equally well in all directions.