In The Blink Of An Eye (43 page)

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 them 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 most 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.

So it seems the evolution of hard parts everywhere, and ultimately the evolution of body forms of multicelled animals, was driven by active predators. This process
was
the Cambrian explosion. But it was triggered by the evolution of the eye. We are looking for that trigger rather than a detailed explanation of the event itself. The McMenamins' updated concept of food webs developing in the Cambrian is actually a description of the Cambrian explosion itself - but the event, not the trigger. The Cambrian explosion saw the writing of The Laws of Life as it exists today. The introduction of the first eye effectively tore up the previous Laws and gave rise to chaos among animals, creating a scenario without laws. It would have put evolution into top gear, perhaps moving it up from its lowest; fresh rules were required now. All animals needed to evolve to be adapted to vision before they were eaten, or before they were outwitted by their prey. The Early Cambrian thus became a race for adaptation to vision. This scramble for the newly available niches, this chaos during the writing of today's Laws of Life,
was
the Cambrian explosion. So finally we can be sure we have our answer.
The Cambrian explosion was triggered by the sudden evolution of vision.