In The Blink Of An Eye (37 page)

When adaptations to vision include shape and behaviour, in addition to colour, it is clear that vision is a major tactic used in the struggle for both conspicuousness and illusiveness. Genuine strength or ability is actually a rare attribute in animals; rarely does an animal dominate an ecosystem without considerable employment of warnings or illusions. The lioness is the main predator in the Serengeti, but she cannot outrun her prey over short
and
long distances, so she must rely on camouflage colours and stalking behaviour to take up a competitive position in her race for food. An exception here can be found in many birds, and the reason for this exception will offer another clue towards solving the Cambrian enigma. Birds will be considered in the following chapter.

There are tactics animals can use other than vision to achieve either conspicuousness or illusiveness; other senses do exist, as also described earlier. Once again, adaptation to light is generally the main tactic to employ within The Laws of Life because of the factor separating light from all other stimuli - occurrence. Light exists, like it or not. Add Chapter 7 to the mixture and we have ‘Vision exists, like it or not'. Over 95 per cent of all multicelled animals today have eyes, so if one of
them is to avoid being eaten, it must be adapted to the light in its environment. We are beginning to take our knowledge of light and vision into the subject of predation.

Chapter 7 centred on the optics of eyes, the equipment that forms an image on a retina. The reason for this was the link between the living and extinct - the optical origins of today's eyes can be traced in the fossil record, right back to the very first eyes in the Cambrian. But there is something else we can learn from the type of image formed in the past, or the view of the world through fossil eyes, that is relevant to this chapter. Just as we did in Chapter 7, we must first look for evidence in the present day.

We have learnt that there are alternative ways of producing an image today - different types of eyes do exist. But that is not the end of the variation. There are also different ways in which eyes can be arranged in a head, and these provide different views of the world.

Among the vertebrates within the chordate phylum only camera-type eyes exist. In humans they lie next to each other in the front plane of the head - they face forward. But more than that, they always focus on the same object. So why bother with two eyes, when one would appear to do the job of seeing on its own? Has evolution been excessive in our case?

When eyes are positioned on the sides of the head, like those of rabbits, the wide field of view encapsulates almost the entire horizon. At first this would seem like the ideal form of vision, but to gain such a panoramic outlook, each eye sees a different picture - each approaching 180° of the horizon - and never the same object. With one eye, however, the view will be two-dimensional, and so distances are difficult to estimate.

When two eyes are positioned on the front of the head, distances and the direction in which one is travelling
can
be estimated. So it follows that eyes in this arrangement can perceive the three-dimensionality of an object. Differences in the positions of images create impressions of
depth, as can be demonstrated using stereograms. Each eye sees the same object but from a different angle. Stereograms probably work because the optic nerves serving slightly different regions of the two retinas converge on the same ‘binocular' cell in the brain. The view of an object from two different angles is superimposed and averaged - and its depth is perceived. So animals with two eyes facing forward are said to have stereoscopic vision - they can perceive images in 3D.

Figure 8.1
One of the original stereograms of 1838. Blur the picture to produce a fused image in the centre. The inner ring will appear nearer than the outer ring.

The stereogram, albeit merely a demonstrative game, would not work for a rabbit, or for ourselves if we closed one eye. So we should again consider whether it is better to have two eyes at the front of our head, facing forwards, or on the sides of the head, giving a panoramic view. The answer would appear to depend on the purpose of vision. Would you like to observe events happening all around you in two dimensions? Or would you prefer to view objects in front of you in three dimensions and with information on distances? Now we can return to The Laws of Life, and consider whether you are a predator or prey.

For a prey species, staying alive first means keeping off the dinner plate and
then
eating becomes important. So it is ideal for the prey species to be surrounded by open space, where the possibility of a
sudden ambush is minimised. Minimised, that is, if a 360° view of the terrain is possible - blind spots on the horizon are dangerous. We often find rabbits grazing in the middle of open fields rather than at its edges near hedgerows. And we always find them with their eyes positioned for a panoramic view: eyes positioned at the sides of the head are good for spotting predators.

For a predator, in contrast, staying alive usually means eating first and worrying about
their
predators and competitors after that. Eating lively animals involves hunting. Estimating distances is a critical part of hunting - the lioness cannot begin her charge when the prey is within its safety zone, where its head start is insurmountable for the lioness. Equally, a fox cannot catch a rabbit if the rabbit is given the distance in which to reach full speed. So where vision is the major sense employed by predators, two eyes at the front of the head are needed - an accurate assessment of distances is the difference between a meal and hunger. And that is just what is found in the lioness and the fox.

This trend can often be found within other animal phyla with eyes. But in mid-water, things become more complicated. There is not only the horizon to worry about, there is also above and below. In mid-water, danger can approach from
all
directions. The great bearers of marine compound eyes, the crustaceans, have evolved a solution to this problem - many crustaceans have eyes positioned at the ends of moveable stalks. They can move their precision eyes to cover a wide area of their surroundings. Because of this, stalked eyes generally do not provide clues as to predator or prey, although many crustaceans, like insects on land, are often both. Today they lie somewhere in the middle of the food web where avoiding predation is finely balanced with the need to eat. Other types of compound eyes, however, are more obliging to the Cambrian detective.

Later in this chapter, I will attempt to relate the feeding information provided by eyes to the inhabitants of the Cambrian. Eye stalks in this respect are like gloves to the fingerprint detective - they mask potentially useful information. But compound eyes that are fixed in position do offer some clues, and such eyes are found commonly in the fossil record.

In the air, dragonflies are expert hunters. They have three pairs of
grasping limbs positioned near to their blade-like mouthparts, and large wings to provide speed and manoeuvrability. But first the helpless prey must be found, identified as prey, and then tracked. This is achieved using vision - huge eyes are fused to the head. These eyes lock the prey in their sights, their ‘sights' being just parts of the eyes and not all the facets. This is food for palaeontological thought.

The compound eyes of dragonflies contain several hundred or even thousand facets, not all of which are equal. There are one or two regions of the eye that contain larger facets and these are known as the acute zones, the ‘sights'. Larger facets provide higher magnification and better resolution - they see with greater sensitivity. One acute zone is positioned at the top of the eye, and this is used to scan through the air and identify prey insects against the sky. When a prey insect has been spotted, the dragonfly moves into its horizontal plane and tracks it with a forward facing acute zone - the prey is now locked into a line of fire. But the relevant point here is that the size and positions of the facets within the eye provide information on feeding - predation in this case. The eyes of prey can be quite different.

For animals that require vision only to avoid being eaten, having two eyes is just one solution. Rather than evolving a pair of good image-forming eyes capable of scanning the entire environment, they may evolve numerous, less efficient eyes distributed over a large area of the body. At the sacrifice of images, numerous eyes are ideal for detecting movement - as an object passes over them, its moving shadow is detected. When the environmental light changes, as when a fish passes through the ocean, a response is triggered. Numerous compound eyes are indeed found in nature. They occur in ark clams (molluscs) and fan worms (bristle worms) where they are employed to detect predators.

The real advantage of this multiple eye system probably lies in the word ‘evolve' used at the beginning of the previous paragraph. Evolution involves changes, changes from one structure to another, for instance. Here we are back to Darwin's original doubts caused by the eye - from what could our highly complex and specialised eyes have evolved? We now know that skin and ears can share nerves, and that part of the animal brain may have converted from touch to vision at some stage. Dan-Eric Nilsson suggests that the light detector cells in the
compound eyes of ark clams and fan worms evolved from chemical detector cells that were inhibited by light. Originally, these chemical detectors were distributed over a large area of the body and, consequently, so too are the eyes today. In other words, it was most accommodating in these cases to evolve eyes all over the body.

Ark clams and fan worms are preyed upon by fishes. They have soft parts used for feeding, which can be enclosed within hard parts in the form of a shell and tube respectively. So these animals would benefit from a burglar alarm, an early warning system to detect a predator's approach. And that is the function of their eyes. When the movement detected in the water equates to that of a fish, the ark clam closes its shell tight and the fan worm withdraws into its tube. The armoured doors are closed. And their many compound eyes were the cheapest evolutionary option capable of performing this function from the building materials or starting points available.

Clearly, signs are appearing that the architecture and position of eyes can reveal not only how an animal sees but also its position in the food web - whether it is a predator or prey. Chapter 7 used fossil eye architecture to trace vision in the past. Now I will re-examine the fossil evidence, where appropriate, and use it in an attempt to trace the history of predation.

The Cambrian arthropod
Cambropachycope
had a single compound eye. Other than the weird
Opabinia
, the failed five-eyed experiment, all other Cambrian eyes producing good images and with the potential for image analysis were paired. When cross-sectioned, each of
Opabinia
's five eyes revealed the general architecture of a compound eye. But
Opabinia
had a flexible tube-like mouthpart extending from its head and terminating in a grasping jaw. The arrangement of the eyes at the front, side and top of the head are not so easy to interpret because of that mouthpart - it could extend in front of, to the sides or above the head. So which direction ‘faces forwards' for
Opabinia
? Because there are several ‘forward' directions for the mouth, we cannot say whether
Opabinia
's eyes served to view the entire environment or to centre on just one direction. Before tackling the remainder of the Burgess fossils, first we must reassess
Cambropachycope
.

Cambropachycope
was an ancestor of the crustaceans. Although
only a few millimetres in size, it is known in great detail from a fossil site in Sweden thanks to very favourable preservation conditions. As mentioned in Chapter 7, the bulbous front end of
Cambropachycope
was an eye - a single, large compound eye. An examination of the cornea of this eye revealed that it completely covered the slightly flattened front surface of the animal. Facets were evident on the surface as it curved away towards the sides, but generally the sides were bare. Importantly, the facets on the curved edges were small compared to those of the central part of the eye. It seems that the centre of the eye saw with the greatest precision.
Cambropachycope
's eye could scan a 120° sector of the environment - that sector in front of it. And just like in dragonflies of today, the central region of the eye could achieve finer resolution. In conclusion, this was the eye of a predator.
Cambropachycope
would have terrorised the tiny inhabitants of the Cambrian around 510 million years ago.

Unfortunately the eyes of the Burgess Shale animals do not reveal enough information on their optics to allow us to draw conclusions on feeding from just a single eye. We cannot resolve details of their individual facets. To add to this, most nontrilobite eyes in the Burgess Shale are stalked, so their manoeuvrability makes directional predictions difficult. But some are obliging to the palaeontologist.