In The Blink Of An Eye (42 page)

Emphasis has been on the great importance of light as a stimulus to animal behaviour today. In fact all of the terrestrial animals (excluding domestic species) we are familiar with are wonderfully adapted to light not only in terms of their colour, but also their behaviour and sometimes shape. Colour is the logical animal adaptation to light, and the external colour of an animal living in an environment with light is usually an evolutionary response to that light. For instance, it is argued that in spiders the production of colour is chemically costly and is principally maintained by the action of sight-hunting predators. Shape, on the other hand, is largely governed by chemical processes, movement, reproduction, feeding mechanisms and other behaviours. But for some of these activities light may also be a major consideration. Here behaviour becomes important. A stonefish not only has to be coloured like a stone, but must also have a similar shape and behave similarly, spending long periods stationary. Also praying mantids possess the colours and shapes of the plant parts on which they live, whether they be green leaves or pink petals. Then there are the stick and leaf insects, which are related to the praying mantids but are the hunted rather than the hunters. Stick and leaf insects possess the light adaptation characters of colour and shape, but unlike stonefishes and praying mantids they must move to find food. And to complete their adaptation to vision, they walk with the quivering movement of leaves or petals in the wind.

Once again we are led to consider eyes, albeit those belonging to animals other than those in question. The above mentioned colour, shape and behavioural characteristics are not directly an adaptation to sunlight, but rather adaptations to the presence of animals with eyes. But in particular it is the eyes of either predators/enemies or prey. There is a potent relationship between eyes and predators, or between the visual appearance of animals and staying alive. Staying alive, according to The Laws of Life, can mean eating and/or avoiding predation.

We can now understand why camouflage is common among animals today. Many insects are green so as to be camouflaged against leaves. Although green is generally a difficult colour to achieve, pea aphids
are
green where their predators, ladybirds, abound. Ladybirds hunt mainly using vision, and so camouflage is a good strategy for their prey. But when ladybirds are scarce, the pea aphids stop producing the energy-expensive green pigments and turn a less costly red. Similarly, guppies change their visual appearances in response to predators with eyes. Populations of this fish, found in Trinidad and South America, vary markedly from each other and so have become classic animals for the study of evolution in action. A population can transform in terms of colour and anti-predator behaviour within a few years, or ten generations, of a change in predator pressure. Of course mating is another important behavioural and evolutionary consideration, leading to sexual selection. Sexual selection acts in unison with predator-driven evolution, or natural selection. When the threat of predation is relaxed, bright mating colours will evolve in guppies via sexual selection. But all of this evolution is driven by vision, whether the vision of other guppies or of their predators.

Mating leads to well-known exceptions to the rule of camouflage, particularly in birds, where vision is usually the primary sense. Consider the peacock, where Newton's analysis of colour applies only to the spectacular males with their imposing tail feathers, not to the dull-brown, short-tailed females. Yet both sexes of peacock share the same feeding strategy. A key element here, however, is the relatively modest threat of predation, and this is a luxury afforded to most birds. Flight in vertebrates has generally provided an evolutionary ‘time out' from the camouflage constraints imposed upon most animal species on
land and water. So many birds are free to display colours suitable for another important behavioural process - courtship. And as could be predicted from this philosophy, birds have evolved some of the most sophisticated, visually oriented courtship displays. They can stand somewhat clear of the cat-and-mouse world sculpted by the presence of predators with eyes.

Back on the ground or in the water, The Laws of Life are far stricter. There are no magical hiding places or extra dimensions into which animals coloured with maladapted hues could instantly vanish. But at the same time, light paves the way for increased adaptive radiation here. Cases discussed in this book have included those of the East African Rift lake cichlid fishes and Caribbean
Anolis
lizards. Adaptive radiation involves movement into different available niches. Light generally creates more available niches - shade and bright light, and different coloured backgrounds, for instance. Hence sunlit environments support a greater diversity of animal life than do cave environments.

Put together all of the considerations listed and we have a world where light shapes most ecosystems. Consider the marine environment. One can choose to live in different light regimes. There is the sea floor to burrow into, or crevices in rocks and corals. Similarly, sponges provide suitable hiding places, and the stinging tentacles of anemones or Portuguese men-of-war can be another safe option (if one is immune to their toxins). Then one can be brave and shun the protection afforded by external sources, but living out in the open potentially places one in the line of fire. So a survival strategy must be evolved to reduce the risk of predation. One may be camouflaged or transparent. Then there is the conspicuous option - don warning colours or protective armour. Or one can be fast and on the ball, capable of spotting and outrunning any predator. Alternatively one can concede defeat to predators, and choose an unusually successful breeding strategy at the expense of reducing individual chances of survival. At least this way one's species may survive (although this would not work if employed by
all
prey species, since ‘space' for this niche is limited). But either way, a good strategy to counter those predators with eyes is essential.

Although this is not strictly the language of an evolutionary biologist,
it does sum up the idea of selective pressures that act on evolution. And all of this comes about because predators exist with eyes. Without eyes, light would not be a major stimulus to animals.

At this point I feel like a university lecturer who has just finished teaching a foundation course - weary but relieved. Not a single educational stone has been left unturned in the bid to reveal the facts and figures needed to progress to a new stage in learning. There is a certain amount of relief because this is where things become interesting and exciting. We are now equipped to tackle evolution's grandest event of all. We can now go back 543 million years, to the beginning of the Cambrian.

Consider dividing geological time into two parts - pre-vision and post-vision. The boundary separating these parts stands at 543 million years ago. Considering vision as the most powerful stimulus on Earth, the way the world functions today is the same way it functioned ten million years ago, 100 million years ago and 537 million years ago, after the Cambrian explosion. Similarly, the world was without vision 544 million years ago just as it was 600 million years ago. In the interval of life's history of these two parts, a light switch was turned on. For the second half it remained on, although during the first half it was always off.

We know that vision places major restrictions on the external forms of animals today, but before the Cambrian it could not have played such a role because eyes did not exist. Consequently light did not exist as a major stimulus in the behavioural system of animals. By vision I mean the ability to produce visual images, which can be achieved only by animals with
eyes
. Light is used to determine the direction of sunlight in numerous forms of simple animals. Testament to this are the algae found in the snow at the Burgess quarries in Canada, with their red eyespots but lack of vision. But these have nothing to do with vision. Indeed, some plants even possess simple light perceptors that regulate the shift from vegetative growth to floral development. But this
form of light detection is not vision. Vision is the capacity to perceive and classify objects using light, or seeing.

The Precambrian was a time where only soft-bodied representatives of the multicelled animal phyla existed. On the following pages is a snapshot of life in a Precambrian environment as pictured by the most advanced form of light perceptors of the time.

Effectively light as a major stimulus is, or rather visual appearances are, removed from the Precambrian environment because the animals of that time did not possess eyes. Presumably Precambrian animals possessed chemical, sound and/or touch receptors. They may also have possessed simple light perceptors, like the algae in the Canadian snow, but nothing that could form an image. Light could be considered a very minor selection pressure in the Precambrian. It could not have had a direct effect on the evolution of multicelled animals (it could have had an indirect effect in that animals which fed on photosynthetic algae would have been restricted to sunlit zones).

Competition and predation would not have been major selective pressures in the Precambrian, but they were taking a foothold. The Ediacaran animals of the Precambrian were gradually developing brains. They were developing ways to pick up environmental cues, or news items, and process that information. They were also evolving the ability to chew, and were gradually developing a rudimentary form of rigidness in their limbs. Precambrian trace fossils or footprints suggest that legs could support bodies off the ground. But as in dark caves today, evolution in general would have been slow in the Precambrian, and may well have continued at a gradual pace had it not been for a single but monumental event. This was an event that, in terms of body parts, would have seemed like any other evolutionary innovation, of which there have been many. But this event was different - it changed the world forever on a scale not since witnessed. At the end of the Precambrian, while most phyla were evolving gradually, a serious transformation was taking place in the soft-bodied trilobites. A light sensitive patch was becoming more sophisticated. It was dividing into separate units. The nerves servicing each unit were becoming more numerous, and so too were the brain cells they serviced. These nerve and brain cells were either multiplying or being borrowed from the wiring and processing system of another sense. Then the outer covering of each unit began to swell and take on focusing properties. One day all this reached a crescendo - a compound eye had formed.

Figure 9.1
(overleaf) This is how all Precambrian animals would have pictured their neighbours using light as a stimulus.

Let there be images! A new interpretation of a sense had entered the animal world . . . but this was no ordinary sense. What was to become the most powerful sense of all was unleashed with the birth of one individual proto-trilobite (during its transition to a trilobite) - the first to entertain an eye. For the first time in the history of the Earth an animal had opened its eyes. And when it did, everything on the sea floor and in the water column was effectively lit up for the first time. Every worm crawling over every sponge, and every jellyfish floating through the water, was in an instant revealed as an image. The lights on Earth were switched on, and they put an end to the gradualness of evolution that had characterised the Precambrian.

Simply put, the visual appearance of animals suddenly became important with the introduction of eyes. But it took just a single pair of eyes - the first eyes - to introduce vision as a stimulus to the world around them, including all its inhabitants. Now if we add vision to the Precambrian scene depicted in
Figure 9.1
, the animal inhabitants appear as shown on pages 274-5.

The most powerful sense of all had been launched on Earth. Suddenly, and for the first time, an animal could detect everything in its environment. And it could detect it with pinpoint accuracy.