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.
The ‘Light Switch’ theory
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.
The difference between the previous two pictures, or light perception in the Precambrian and Cambrian, is comparable to that experienced when we close then open our eyes. With our eyes closed we can determine the direction of sunlight but we cannot, for example, find and identify a friend. So, using light, some Precambrian animals could have known which way was up in mid-water, but they could not have found a friend or foe. Nevertheless, in their favour, a potential predator could not have found th
em either. So there were no strong selective pressures for Precambrian animals to become adapted to light, even though light was to become the most powerful stimulus of all. In fact it became the most powerful stimulus of all almost overnight (in geological terms), with the evolution of the first eye at the beginning of the Cambrian.
With our eyes open, suddenly we see the world very differently. We can see food from some distance, although we can only smell it if it produces a smell, hear it if it produces a sound and touch it if we are very close. So in the Precambrian, not releasing certain chemicals or producing sounds was enough to avoid a potential predator, unless it was bumped in to. But in the Cambrian life was lit up. The light switch was turned on, for the first and only time - and it has been on ever since. With our eyes open we see the size, shape and colour of animals, but we also see their behaviour - we can judge how fast they can move and whether we can catch them. All of these animal attributes suddenly mattered at the beginning of the Cambrian, when the first active predators with eyes were introduced on Earth. At that very point all animals had to become adapted to light, or vision. Near the end of the Precambrian, selective pressures had been acting on proto-trilobites to evolve an eye. But they had not been acting on the other animals to gradually be adapted to vision, in readiness for this eye. An animal will always be releasing an image into its sunlit environment, and the race to produce adapted images began. All those adaptations to vision that exist today were quickly conceived. The worm-like forms had to display armoured parts, warning colours, camouflage shapes and colours, or signs of the ability to swim so as to outmanoeuvre a pursuing enemy. Or, on the other hand, they could opt out of the visual environment and evolve bodies capable of burying themselves into rock crevices or other substrates. But after the initial chaos, further adaptations would become gradual - evolution would have settled down to its habitual pace.
That first eyed individual literally saw a whole new suite of niches open up. It observed areas of the sea floor in light and shade, which had previously been combined. But importantly it could easily identify the other animals sharing its environment. It could determine how far away they were, where they were heading, and how fast they were moving. At this point, nonetheless, there were to be few immediate consequences apart from a competitive edge this eyed individual had over other members of its species - it could find food and a mate more easily. This advantage would translate into retention in the species of those new genes that code for an eye. And soon all individuals of that proto-trilobite species would possess an eye, possibly making them a new species. But selective pressures for all multicelled life on Earth would have changed the moment that first eye opened, and the consequences of these would soon be realised. The next selective pressures were for active predation and its countermeasures.
Figure 9.2 Soft-bodied multicelled animals living at the end of the Precambrian. This is how the most sophisticated light receptors of the time - eyes - would have pictured the Very Late Precambrian or Early Cambrian world, around 543 million years ago.
The first eyed proto-trilobites must have been frustrated individuals. They had a taste for meat and were feeding on whatever scraps they came across on the sea floor, probably detecting the chemicals wafting from decaying ‘food’. But now they could literally see a far greater potential. They saw their soft-bodied neighbours, from all animal phyla, as chunks of protein, or potential meals. But they had neither the mobility nor the jaws to capture and kill all of them. They needed to swim to capture those floating forms, and they needed stabbing mouthparts or limbs to perform their acts of murder. In other words, they needed hard parts. But considering the potential for proto-trilobites to take over the world, the selective pressures for hard parts were massive. And hard parts and active predation would follow, very quickly. Soon, proto-trilobites would become trilobites.
In seas across the globe trilobites with eyes and predatory limbs appeared at the beginning of the Cambrian. Active predation was born. Now there was a menace in the sea like nothing seen before. These trilobites set the scene for what was to follow, from T. rex carnivorising the Cretaceous, to lions in the Serengeti today. Another big factor in being a highly active predator was the ability of the trilobite to move up into open water - to swim. Today the bristle worms with the best eyes, the alciopids, are also the best swimmers of all the bristle worms - eyes are most useful if one is also highly mobile. The Precambrian predators in open water were those jellyfish which sensed the world mainly by touch. Animals cannot be adapted to touch, so this form of predation provided no selective pressures for the evolution of prey.
It really was the appearance of the trilobites that shook the world. Arrow worms were early Cambrian predators, but they were not known to be numerous and are rather tiny. In fact they hunted only small, planktonic prey and so could not have played a role in the Cambrian enigma. And then, with some exceptions, no defences were evolved during the Cambrian explosion against the non-visual predators, such as the priapulid worms. The Cambrian explosion is really all about defences to visually oriented predation.
So when that first eye appeared, the potential for proto-trilobites to rule the world was recognised in the selective pressures acting on other animals. Selective pressures are invisible forces. No one is ever aware of them. One cannot ‘urge on’ evolution, even if one thinks one knows better. So as selective pressures for active predatory lifestyles mounted on the proto-trilobites, so did selective pressures for countermeasures build up on the other multicelled animals. And these pressures were massive too. Evolution is a balance, and the balance will not continue to tilt one way. With the exception of extinction, it continuously levels.
That first eye effectively created new niches for everyone, even though only the proto-trilobites could actually see them. Today, fishes do not know that they are silver to avoid predators. They evolved silver colouration to fill an available niche, one where large animals could live in mid-water if they were not visible to predators. And selective pressures targeted that available niche. So all those new potential niches at the beginning of the Cambrian, those areas of light and shade, were there for the exploitation of all. The rules were simple, but new.
Soon the free for all for trilobites was over. There was a new selection pressure acting on them - to avoid becoming prey. As they jetted through the water, and sprinted or skimmed over the sea floor, they came into contact with other trilobites. These other trilobites would have appeared as tasty morsels themselves. It became dog eat dog, or, rather, trilobite eat trilobite. The emphasis of trilobite evolution was shifting from eat to avoid being eaten. Some small Cambrian trilobite fossils have been found inside empty worm tubes. They were probably keeping out of sight of the larger trilobite hunters. But another evolutionary response was that the hard exoskeleton of the trilobites, that had granted their ability to swim, became endowed with armaments. And now, for the first time on Earth, armaments were ornaments. Let us return to the trilobite’s soft-bodied food that was drying up, and in particular the reason why it was doing so. It was not simply that the trilobites were overconsuming.
The soft-bodied forms exposed on the sea floor started to become scarce because they were evolving. Before now they had been exposed to inactive predation only. This was a fairly inefficient process, in which maybe one in ten individuals would meet a sticky end. This may have been the sticky end of a predatory priapulid worm, or the sticky end of an anemone’s tentacle, but a species can live with odds of one in ten. The remaining 90 per cent of individuals would have been safe - safe to carry the species into the next season. Stay out of the way of a priapulid or anemone and you are safe. A trilobite, however, will come looking for you. Things changed at the Cambrian border.
The most obvious requirement for adaptation to this new world of light would seem to be the possession of hard parts. This was precisely where evolution’s emphasis was placed. Hard parts evolved for armour just as they had evolved in proto-trilobites to provide strong jaws. In mo
st cases of ground-dwelling animals, their armour was directed towards attacks from above. This provides further evidence that active predators were swimmers - as suggested in Chapter 8, trilobites were probably the fishes of Cambrian seas. And then eyes themselves took off in the arthropod phylum not only to enhance a predatory lifestyle in their owners, but also to prevent them being eaten.
On close inspection of the fossil record, it becomes clear that it was the arthropod phylum that diversified most, or evolved the greatest range of hard parts, in the Cambrian. They were the active predators of the Cambrian, and eyes go a considerable way towards helping an animal become an active predator. The other thirty-three phyla that were to take on hard parts formed smaller armies. With the exception of molluscs and lamp shells, these other phyla were represented by relatively few species - species that saw their phyla through the Cambrian transitional period. This was achieved via adaptations to active predators with eyes, including the abilities to swim, hide in rock crevices, burrow efficiently or be protected by armour. Many of these adaptations required hard parts. Camouflage was probably another major adaptation but we have no evidence either for or against this - the Cambrian explosion was probably an event involving hard parts/shapes and colour. The other thirty-three phyla did not, however, evolve eyes in the Cambrian (with the possible exception of Insolicorypha, a Cambrian bristle worm comparable to the swimming, eyed alciopids today). Maybe this could explain their reduced diversification in comparison with the eyed arthropods of the Cambrian. Eyes did evolve in five other phyla, becoming common only in the chordates and molluscs, but they evolved after the Cambrian - these five phyla remained eyeless during the Cambrian. For instance, according to the fossil record and evolutionary analyses, the group of eyed animals to which squid and cuttlefish belong did not evolve within the mollusc phylum until well after the Cambrian.
In The Blink Of An Eye Page 30
