The holes in the tubes of Claudina provide the first definitive evidence of predation on Earth. And it seems that what can be best described as ‘inactive predation’ was common in the Precambrian. Although, based on the lack of armour worn in the Precambrian, this type of predation obviously did not present a strong selection pressure for counter-predatory measures. It did not provide the stimulus for hard, protective parts.
In particular, there was one interesting soft-bodied animal that roamed the Precambrian sea floor. In 1984, petroleum companies were exploring parts of southern Morocco and eastern Siberia. They drilled vertically into the ground and removed cores - long, thin cylinders of rock that revealed the layers of sediment built up over 600 million years, while these areas were underwater. As expected, rocks that formed just before the Cambrian showed signs of stromatolites. But there were further, unusual layers just above the stromatolites. At the time they were termed ‘thrombolites’ and were assumed to be the result of grazing by soft-bodied arthropods, including ‘proto-trilobites’. Indeed, the first signs of trilobites, the first hard parts of any type, were found some tens of metres above the lowest thrombolites. Along with trace fossils of soft-bodied arthropods, this is an important clue in piecing together a picture of the ancestors of arthropods. But most enticing is the term ‘proto-trilobite’. Did trilobites as such exist without their armour before the Cambrian explosion? In 1991 this question was answered. Incredibly, a new expedition to the original Ediacaran site, the Ediacaran Hills in South Australia, yielded a soft-bodied trilobite.
Figure 8.7 A soft-bodied ‘trilobite’ from the Precambrian (about 565 million years old). Shaded regions in the head could be the precursor to compound eyes.
First, raking trace fossils were found from an animal predicted to be 4 centimetres long with twelve pairs of long, thin legs. Then came the real breakthrough. Several specimens of the bodies themselves were discovered - the originators of the trails. The bodies would have been soft compared with those of Cambrian trilobites. From above they were round, but showed clearly a semicircular head with a well-marked border, a thorax with thirteen large segments and eight smaller segments, and a tiny, oval tail. Some specimens were distorted, indicating a degree of elasticity in the skin like that of the Cambrian Naraoids. But the general body architecture matched that of Cambrian trilobites - except for an elastic skin in place of the hard exoskeleton. Also of interest in the proto-trilobites were curved, shallow ridges on the head, in the region that eyes were housed in Cambrian trilobites. But eyes themselves, like grasping limbs and spiny mouthparts, were absent in the Precambrian forms.
The proto-trilobites of the Precambrian were grazers, feeding on algal mats and probably dead animal matter lying on the sea floor. It seems the voracious predators that emerged with the Cambrian had rather peaceful beginnings. If anything, the proto-trilobites would have been prey themselves - the tables may really have turned at the Cambrian border. In general the Precambrian was rather an experimental stage for predation, occupied mainly by peace-loving vegetarians that were willing enough to accept any occasional animal matter they stumbled upon. For they were developing a taste for meat.
Was this shift in emphasis towards predation a gradual phenomenon? It seems not. Carnivores really made the headlines 543 million years ago. Suddenly, predation was not only a major option within the food web; it encompassed a new form. If the Precambrian predators were considered passive, the second wave of predators that swept through the early Cambrian seas were undeniably active.
The end of this chapter will duplicate the end of the previous chapter, where we learnt that the first animal with eyes was a trilobite - the first trilobite. The first true trilobite was also a predator. Fallotaspis, Neocobboldia and Shizhudiscus, all trilobites with eyes, were also icons of the beginning of the Cambrian, around the time the Cambrian explosion began. Their limb shapes indicate that these trilobites were predators; their spiny shields affirm that they were also prey. They probably attacked each other - the archetypal attacks on Earth, since their bodies were armoured in only rudimentary form. Their skins had become less soft than those of the Precambrian proto-trilobites, but they were still not fully hardened, as were exoskeletons of trilobites that appeared a few million years later. They were, however, highly active animals. They could swim rapidly, they could manoeuvre in mid-water . . . and they were predators with spiny, robust limbs. They were bad news for Precambrian-style, soft-bodied forms everywhere. Life was about to be stirred up.
So the beginning of the Cambrian was also the beginning of active predation. This is a simple concept that warrants little additional discussion. But there is one detail we should consider further - we must distinguish in our minds the difference between the Cambrian explosion and the cause of the Cambrian explosion. The signs of predation we are using to denote the beginning of active predation are the spiny tools and swimming limbs of trilobites - or hard parts. But the acquisition of hard parts was the Cambrian explosion. This chapter will close with a question: ‘Did a few species of predatory trilobites evolve from proto-trilobites and kick-start a chain reaction?’ That chain reaction, of course, was the acquisition of hard parts and other external characteristics in all animal phyla - the Cambrian explosion. Did all hell break loose simply on the appearance of a few armoured forms, or did something else happen that sparked the evolution of hard parts simultaneously everywhere? It is finally time to put two and two together.
9
The Solution
From a perception of only three senses or three elements, none could deduce a fourth or fifth
WILLIAM BLAKE, There is No Natural Religion (1788)
The sun emits a continuous array of electromagnetic waves - radiation ranging from cosmic and gamma rays, with wavelengths smaller than an atom, to radio waves with wavelengths of over a thousand metres. Visible light waves lie within this spectrum, at the peak in the sun’s energy emission. They include only a narrow range of wavelengths. When light waves fall upon an object, they can be deflected and relay news of that object into the environment. If the deflected waves meet our eyes, they can be focused on to a retina, and we can interpret the news. One item of news that helps us to ‘see’ is the direction from which these waves last came. This we can determine simply. With two eyes, we can also judge the distance of the object deflecting the waves. But a third trick of the eye is to convert light waves varying slightly in wavelength into different colours. So for an animal without eyes there is no such thing as colour in its environment.
This is difficult to comprehend. But just think: all those wonderful colours we see around us, wherever we are, do not actually exist. In the environment there is no colour, only objects that happen to deflect different types of electromagnetic radiation. Roses are not beaming out reds, nor do leaves generate greens. Perhaps the one chance we have of dealing with this truth lies with ultraviolet.
To birds and insects there is even more happening in the environment, even more colour. Their palette also contains ultraviolet - they are communicating with private wavelengths, oblivious to us. But birds and insects could not comprehend that some other animals cannot detect ultraviolet light. So in turn we should remember that not all animals see images nor understand what we mean by colour. That’s not to say that light and colour are not a big part of the lives of all animals. The word ‘colour’ can be found in the dictionaries of all animals living where light exists. Although not all are conscious of the fact, light is a major selection pressure acting on everyone . . . or at least it is today.
Plants are governed by very different rules to those of animals, yet even many plant colours are adaptations to animal vision. Leaves generally have to be green because their component chlorophyll deflects the wavelengths that we interpret as green (those wavelengths not used for photosynthesis) - this is incidental colour. But many plants produce flowers that display a vast array of colours to attract pollinating insects, and also colourful fruits to attract seed-dispersing mammals and birds. In
fact, animals with eyes may even provide the main selection pressure in the evolution of some plant groups. For instance, the flowers of the Ophrys orchids have evolved to mimic females of different species of Campsoscolia wasps in terms of colour and shape. This mimicry is so effective that the male Campsoscolia wasps are deceived and attempt to mate with the flowers, but succeed only in transporting pollen.
In his book The Universe of Light, published in 1933, Sir William Bragg introduced the concept that ‘Light brings us the news of the universe. ’ Light is not the only messenger, or stimuli, on Earth - there are other conveyors of the news of the universe, most notably sound and chemicals - so it needs to be put into perspective. It may be useful to make analogies between the natural stimuli and the different forms of media that supply our political news (excluding the Internet).
We can receive our daily political news from television, radio and newspapers. The producers of the news in these three different formats operate very differently. In terms of history, newspapers were the first to appear. Reporters roved around newsworthy scenes, and brought home their stories on paper. Their job became easier with the introduction of the telegram machine and telephone. In fact their job changed a little following these innovations - reporters ‘evolved’ in response to their changing environment.
The introduction of radio saw further changes to the reporters’ technique, but previous technology could still be used, albeit in a new way - telephone messages could be broadcast directly. But now all the print could be read out into a microphone, and the news producers’ job had changed, or ‘evolved’, once more. Small improvements in technology translated to equivocal developments in the news service. As technology advanced, the news producers responded to adapt to their new environment. If they did not adapt, they would have been overtaken by rival companies. They would have been forced to target an unenviable minor audience, or elbowed into a remote and limited niche. ‘Micro-evolution’ was taking place in the world of news broadcasting.
Then came a momentous change - television was invented. The news producers’ job had to evolve once more . . . but this time dramatically. New equipment was needed, along with new people with the skills to operate it. Old-style reporters were replaced with non-camera-shy reporters, who also met new demands on visual appearances. New buildings and vehicles were required. Basically the whole news scene changed - a different type of worker was required at every position. There had been a ‘macro-evolutionary’ event in the conveyance of news that turned the trade upside-down. The gradual changes that had been taking place in the other types of media now would seem trivial in comparison.
The introduction of television happened almost overnight. Some significant changes happened subsequently, such as the conversion from black and white to colour, and the introduction of satellites, but eyes were already focused on the television at news time, and that event was the really big one (again, the Internet excepted).
The introduction of television to the field of news broadcasting would have had even greater impact if everyone on Earth had possessed a television set. The effect would have been similar to everyone suddenly evolving eyes overnight. That’s an interesting thought, and a concept that also applies to this book, particularly when considered with the previous comment that light is a stimulus for all animals on Earth . . . at least today.
The link between the power of light and the behaviour and evolution of all animals today is the eye. Eyes make light a stimulus for everyone - even individuals without eyes. Today the importance of eyes to animals living in sunlit environments is often considerable, as is evident from the size of most animals’ eyes. Dragonflies have big heads, with eyes occupying three-quarters of the area, and some seed-shrimps have eyes which monopolise a third of their body volume. And a large proportion of the brain of eyed animals is always devoted to vision.
When the first eye was traced, it emerged that it belonged to the first trilobite, or ‘last’ proto-trilobite, and appeared at the very beginning of the Cambrian explosion. There is a link here which suggests that eyes may have been the ‘television’ of evolution, and it is one that cannot be ignored.
Should we consider predation too?
The last chapter introduced a new variable to the equation - feeding. It also threw a real spanner in the works of the neat theory that was forming. It shed further light on that first trilobite to evolve at the beginning of the Cambrian. This was the first animal with eyes, but it was also both predator and prey. We learnt that predation was evolving gradually during the Precambrian. But the first trilobite was the first highly active predator. This is different. It means that another factor - active predation - can also be associated with the beginning of the Cambrian explosion.
So where do we stand now? Are two possible causes of the Cambrian explosion developing in this book? First we should examine these possibilities further, and perhaps try to integrate the evidence.
Consider the military expression ‘search and destroy’ used in Chapter 8. The word ‘search’ precedes ‘destroy’, and that is exactly the order of action in the process of active predation. Before destroying, one must search, identify and capture. Active predators would be useless without eyes or a comparable detector for another sense. At the beginning of the Cambrian, animals began madly chasing and eating each other. A prerequisite for this behaviour is an appropriate search capability - the attributes of speed, agility and grasping hooks would be redundant without a knowledge of where the prey is. And indeed, at the beginning of the Cambrian predators first set their sights on prey. Setting sights, as in getting a victim within the telescopic sights of a rifle, is an appropriate term because the early Cambrian killers did place their victims within sights - their eyes. It seems that Chapters 7 and 8 are beginning to overlap. Now the long spines extending from the bodies of many early Cambrian trilobites can be interpreted.
Armaments are ornaments
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.
In The Blink Of An Eye Page 29
