In The Blink Of An Eye (30 page)

When a surface has the physical properties - the size and shape - of a diffraction grating, it
will
cause iridescence in the presence of sunlight. And sunlight would have existed in the environment of the Burgess animals - at least the blue, green and yellow part of sunlight. I applied some simple optical equations to the reconstructions of
Wiwaxia
,
Canadia
and
Marrella
and calculated the directions in which they would have reflected different colours. Because the parts with diffraction gratings were positioned in a variety of orientations, from any direction
Wiwaxia
, for instance, would have shimmered with all the colours remaining in sunlight. And those colours would have appeared relatively bright like the spectrum of a compact disc. They would have been visible even under the dim light conditions of deeper waters or during dawn and dusk - when pigments become invisible. Interestingly, I photographed the model of
Wiwaxia
's spine gratings under ultraviolet light only. Here I used the methods employed previously on the Atlas moth, as described in Chapter 3. Humans are blind to ultraviolet light, so I could see nothing through a camera with an ultraviolet-only filter. But when the ultraviolet-sensitive film was developed, very bright patterns emerged where human-visible colours were absent. The camera could ‘see' ultraviolet, and I was looking at the camera's view. So if
Wiwaxia
had lived where the ultraviolet part of sunlight existed,
such as in shallow depths, it would have shone brightly in ultraviolet along with the human rainbow. Unfortunately, we will probably never know the complete spectrum that illuminated the Burgess animals.

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.

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.

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.

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.