In The Blink Of An Eye (45 page)

The capacity to discriminate temperature differences is slight in members of most phyla, but thermal sensitivity is acute in most chordates, one of the more highly derived phyla. Although within the chordates fishes are the best-known marine animals in terms of sensitivity to pressure changes, most swimming members of other phyla are also known to respond to pressure variations. This includes some of the least derived phyla - jellyfish and comb jellies - as well as bristle worms and arthropods. A reaction to gravity is also shared by most phyla. Comb jellies and some jellyfish are equipped with this sense, along with bristle worms, echinoderms (including sea cucumbers), lamp shells and arthropods.

Sound waves travel readily through water, with greater speed and less dampening than in air. But hearing as such is an adaptation of the chordates. Members of other phyla may detect sound waves, or at least vibrations of some description in the water, by means of less complex organs. The reflex of some bristle worms to underwater sounds is to start, and certain crabs are known to produce sounds. The means of sound detection, which is certainly very limited, is unknown in these animals, but large, specialised acoustic organs are absent. In the case of crabs, gravity detectors may be involved. Insects began their history as
deaf animals, so the noisy cicadas, crickets and grasshoppers had to evolve sound detectors in addition to sounds. This dual evolution is a lengthy process, requiring fine-tuning of both receptor and transmitter. Importantly, this sense has little direct effect on its neighbours from other animal phyla. And it was the effect of vision on
all
phyla, not just those with the detectors, that made eyes instrumental to the cause of the Cambrian explosion.

So at the phylum level there is a definite relationship between sensory efficiency and the branching point from the evolutionary tree, light detection excluded. Sponges, the least derived phylum, possess simple forms of mechanical and chemical receptors. The next phyla to branch from the tree, the cnidarians (including jellyfish) and comb jellies, again have simple forms of touch receptors and slightly more sensitive chemical receptors, but also reasonable pressure and gravity receptors. Flatworms, one of the next most derived phyla, possess further improved mechanical receptors. But the more highly derived phyla show a general improvement of most types of sensory receptors. This is to be expected. The trend is one of increasing sensory perception with increasing complexity of the body, and this includes brains and nervous systems, those attributes vital to sensory perception. Again, this suggests that the senses other than vision evolved gradually, beginning their history before the Cambrian. Eyes, it would seem, are the oddballs of sensory evolution.

Finally, there is the argument, touched on several times already, that the sense of light detection is different from other senses because of its stimulus. In most environments, sunlight is present, and any animal will leave its optical signature, or image, in that environment. This image is ripe for detection. So to adapt to vision, an animal
must
evolve a response in terms of adapting its visual appearance, whether it is warning shapes and colours, camouflage or hiding behind physical barriers.

Most common senses other than vision begin with a stimulus created by an animal. So if an animal does not create the stimulus, it can't be detected. And then chemical receptors and, to some extent, mechanical receptors, are often finely tuned to detect only a narrow range of the
potential stimuli. So animals can evolve to avoid only that specific range. It is not so simple to adapt to vision, however, since eyes usually detect most of the stimulus range, or spectrum, in their environment. I saw this principle in action, curiously enough, while writing this chapter, when I observed a jumping spider take on a ‘flesh fly' twice its size. The spider was positioned on a wall, against which it was well camouflaged. The fly landed just 10 centimetres from the spider but did not detect its presence. The fly has excellent chemical receptors, but not specifically for the smell of jumping spider. And because the spider was neutral to vision, the fly could not sense it. As the spider approached the fly, however, it was compelled to make movements. These movements translated to changes in its visual appearance and were detected by the fly, which flew off. Fortunately for the fly, the sun always shines, and the spider cannot help but leave a signature in the visible spectrum. Even evolution cannot provide a perfect solution to that problem.

In essence,
all
animals must adapt to light, but this is not the case for other stimuli. And to adapt to a radical advance in chemical perception, for instance, an animal must reduce the chemicals it exudes to a minimum. But this change would have little to do with hard external parts. In fact most changes of this nature would occur inside an animal, in its chemical processes. So a revolution in chemical reception could not have caused the Cambrian explosion - the evolution of external parts.

From another perspective, adaptations to vision do affect other senses. As the door is closed to visually oriented predators, it is opened to predators mainly employing other senses. Hard, protective shells are often ornaments to predators with eyes, and signal that an attack would be a waste of energy and might even harm the attacker. But blind predators are oblivious to this signal. The shelled animals have evolved best to counterattack by the greatest threat in the water - highly active predators with eyes. And in doing so they created a new niche - one for less active predators. Enter starfish, creatures that are blind but can prey on less mobile but even well-protected animals. Starfish rely on
smell and touch to locate their prey, which they then smother until an opening to the soft, edible parts is located. But this is only possible because animals can't be adapted to everything, and they are generally adapted to counter the greatest threat. Other threats, then, can enter the system through the back door. This back door, however, was once the front door.

It should be remembered that there was never really a race waiting to begin in the Precambrian, a race to attain eyes. That's not the way evolution works, and would represent a teleological view. Rather, something happened in the environment one day that changed the rules. Then selective pressures changed either in their direction or size. Evolution works by adaptive radiation, usually caused by a change of some description in the environment. In
The Theory of Evolution
, John Maynard Smith explained further that ‘when a reversal or change in the direction of evolution has occurred . . . it perhaps more often [reflects] a change in the methods of exploiting that environment'. Whichever way you look at it, the appearance of eyes was the biggest change in the environment of all, even for those blind animals. But although vision can be found in only six of the thirty-eight phyla today, over 95 per cent of
all
animal species, taking account of
all
phyla, have eyes. Eyes certainly proved a significant method of exploiting an environment.

In his 1992 review on ‘The Evolution of Eyes', Michael Land began with the statement ‘Since the Earth formed more than five billion years ago, sunlight has been the most potent selective force to control the evolution of living organisms.' This is true for life in general, particularly those forms that photosynthesise, but for
animals
, barring the inefficient sense of simple light perception, it is true for the past 543 million years only. Although the figure of ‘five billion years' does not apply to animals, Land's statement otherwise supports my inferences made in Chapters 3 to 5. But of greater importance to this book is the understanding of why ‘five billion' does not apply to animals. If one divides the history of the Earth into pre- and post-eyes, then considering
the power of vision - generally the most potent selective force for animals today - its day of birth must have been a monumental event in the history of life. Forgetting the Cambrian explosion for a moment, the evolution of vision, that opening of the first eyes, must have caused a remarkable change in the way life works, particularly with respect to external forms of animals. That this day coincided with the day animal life began to explode seems more than a coincidence.

In his conclusion to
Origin
, Darwin wrote:

Walking around the extensive garden at Down House, I noticed a similar diversity. But I should have seen more. According to a book about local fauna, there is much more to see in the countryside visible from Darwin's garden paths. Set against the white background of the pages of a book, the rabbits, several species of common birds, further species of even commoner beetles, frogs, snakes . . . many local animals would seem easy to spot. But against their natural backgrounds, they simply cannot be seen. They are adapted to the light in their environment - they maintain a low visual profile. Even though the birds could be heard, they could not be seen. One sees mainly plants - and plants generally abstain from adapting their colours to avoid the attention of animals.

If Darwin could have travelled back in time, donned Scuba gear and walked through the Late Precambrian seas, he would have seen animals from all phyla everywhere. He would have noticed worms and other soft-bodied forms, including those ancestors of the mammals, crawling and floating in front of his very eyes. Simply, in the Precambrian, animals were not adapted to vision, and there was no danger in being incidentally bold. That could not happen today.

The eye of the trilobite tells us that the sun shone on the old beach where he lived; for there is nothing in nature without a purpose, and when so complicated an organ was made to receive the light, there must have been light to enter it

JEAN LOUIS RODOLPHE AGASSIZ, ‘Geological Sketches' (1870)

 

 

So the evolution of vision via that very first eye in a trilobite triggered the Cambrian explosion. This is the answer to the problem - the Cambrian enigma - I set out to solve. In 2000, I presented this solution at a Royal Institution Lecture in London, where it sparked many questions. I could answer all of them . . . except one. The Light Switch theory also succeeds in posing a further question. As one door closes, it seems that another one is opened.

At the end of my Royal Institution lecture came the question, ‘What triggered the evolution of the eye?' I believe this does require an answer, that we should not assume an eye was always going to evolve as soon as the genetics and building materials in an animal became appropriate (a teleological view). Recently this question has attracted the attention of geologists and meteorologists, who have begun to search for an answer. Logic suggests the solution must lie in an event which led to an increase in light levels at the Earth's surface just prior to the Cambrian. This would suddenly enhance the selective pressures for an eye to evolve. But what was that fateful event, which indirectly changed the course of the history of life on Earth?

The first eye must have evolved in response to an increase in
sunlight
, a factor independent of evolution - bioluminescence (light
generated by animals) would not have evolved significantly until there was an eye to see it. And indeed the geologists have revealed an increase in sunlight levels at the Earth's surface precisely at the very end of the Precambrian. Due to its direct relationship with the Earth's magnetic field, an increase in luminosity is proportional to an increase in the elements carbon-14 and berylium-10 preserved in the rocks. And temperatures increased on Earth at that time too. So we have our answer, or at least part of it - eyes evolved when the dominant selection pressure for an eye stepped up a gear. But we still seek a factor that caused an increase in sunlight levels. Light passes from the sun, through the space of our solar system (the interplanetary medium), through the Earth's atmosphere and through the sea (remember, Cambrian life was exclusively marine). So for sunlight levels to increase at the Earth's surface, one of two events must have taken place: either the sun's light output increased, or the media between the sun and Earth's sea floor became increasingly transparent.

Through theories of stellar construction, it has been well established that the sun was between 25 and 30 per cent less luminous 4,600 million years ago than it is today. But the pattern of this increase in light output is unknown, although it is assumed to have been gradual. Because of the immense time period under consideration, a gradual increase, or even a stepwise increase, translates to a very minor boost in sunlight during the few million years prior to the Cambrian explosion. But it is still possible that sunlight levels rose to a critical level at the end of the Precambrian - critical in that light sparked new reactions within the Earth's atmosphere that led to increased transparency. And this brings us to the second possibility for a rise in Earth's measure of sunlight.

Certainly, the content of the Earth's atmosphere affects its transparency to light - different elements absorb sunlight to different degrees. And the atmospheric contents
have
changed throughout geological history. Some meteorologists suggest that a blanket fog (with various possible sources, including volcanic activity) cloaked the Earth's surface in the Precambrian, thus blocking out a high proportion of sunlight like a giant umbrella. So the lifting of this fog at the very end of the Precambrian would have greatly increased light levels at the
Earth's surface. Precisely how the fog lifted is another issue altogether. One suggestion is again linked to a slight but critical increase in radiation from the sun. The merest increase in solar output and the blanket fog becomes transparent water vapour. So, almost overnight in geological terms, the Earth has clear skies and a line of sight. This would seem the tidiest explanation for a sudden increase in sunlight at the end of the Precambrian. But there are other possibilities.