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.
Eyes bring new opportunities
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.
Near-final thoughts
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:It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us.
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.
10
End of Story?
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. Be
cause 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.
So far I have considered changes in transparency within the Earth’s atmosphere. But are there extraterrestrial possibilities? Could there have been an event that reduced sunlight absorption between the sun and the Earth? There may have been, and its origins could exist deep within our galaxy.
Earth lies within a solar system that lies within a galaxy. The stars in our galaxy are clustered to form the shape of a ‘plate’ with a bulbous centre. But this galactic plate is not even - outside the central zone there are four ‘arms’ that spiral (logarithmically) out towards the edges. Although it has always existed near the edges of the plate, our star - the sun - has not always occupied the same position within the galaxy. It has moved around through time, passing in and out of the spiral arms. It streams through the arms at a speed of 68 kilometres per second, and spends tens of millions of years within each arm during crossover. And to a lesser extent it also moves up and down within an arm - the plate that is our galaxy does have some thickness.
As our solar system moves into a spiral arm, it encounters large, concentrated complexes of molecular gases and dust, but also a greater density of stars - it moves closer to other stars. Sometimes stars explode, causing ‘supernovae’, and at some stages in its history the Earth has been relatively close to supernovae. Supernovae probably represent the most violent events in our solar neighbourhood during geological history. And, of relevance to our discussion, they cause changes in the interplanetary medium of our solar system.
Supernovae cause the absorption of visible light by the formation of nitrogen dioxide. So in turn they reduce the light levels at the Earth’s surface. Additionally, while passing through a spiral arm, our solar system could also traverse a dense ‘Oort cloud’ that would raise the sun’s brightness but also make the Earth’s atmosphere more opaque.
Figure 10.1 Face-on view of our galaxy. Counterclockwise from the Sun (cross at top) are the Sagittarius-Carina arm, Scutum-Crux arm, Norma arm and Perseus arm. Triangles mark the times of the major post-Cambrian extinctions (modified from a paper by Erik Leitch and Gautam Vasisht). Some researchers believe the movement of our solar system into the spiral arms had an effect on these extinctions (such as a consequential encounter with giant meteors). The effect of unwinding is indicated by the dot-dashed lines defining the centroids of the arms for an unwinding of 1°, 4° and 8° for the first three arms, respectively.
Again, the net effect would be a reduction in light levels at the Earth’s surface. So as our solar system departed from a supernova or an Oort cloud, the Earth would have become a brighter place. Maybe this increase in sunlight could have been the enhanced selection pressure for eye evolution. This situation is comparable to, or even the same as, the ‘blanket fog’ scenario discussed earlier.
Supernovae can also cause ozone depletion in the Earth’s atmosphere via enhanced ionising radiation and cosmic rays. This, as we well know from the hole in the ozone layer today, increases the portion of some ultraviolet wavelengths reaching the Earth’s surface. But these ultraviolet wavelengths are not the same as those employed in vision - they are shorter, and are a concern for their damage to animal tissues rather than visual ammunition. And in terms of directly increasing the sunlight reaching Earth’s surface, a supernova emits only a flash of light - nothing long-lasting enough to be a selection pressure for evolution. So its effect on evolution could be only via changes in the interplanetary medium or within the Earth’s atmosphere. But maybe this was enough to give evolution a nudge in a particular direction. The next stage of research to be conducted in this area involves timing; did the Cambrian explosion coincide with the Earth’s passage through the spiral arm of the galaxy? That remains to be discovered.
Finally, we should consider changes in sea transparency. In terms of quality of light, or colours, today the sea acts as a narrow filter. Only a restricted range of wavelengths - mainly in the blue region - pierce seawater well, and the rest are absorbed or scattered. But change the mineral content of the sea and this filter may move within the spectrum or even widen. Could there have been an event at the Earth’s surface that released minerals previously locked in rocks? Today the lakes in the Canadian Rockies are a stunning emerald green. Glaciers have stirred up the rocks in their paths and so changed the mineral content of the waters encountered over time, and consequently shifted the light wavelengths reaching the lake floors. So the waters at the edges of the oceans, the hosts of the Cambrian explosion, could potentially have changed in mineral content and light transparency too. Maybe, at the end of the Precambrian, the light in shallow seas suddenly included ultraviolet light - the ultraviolet wavelengths employed in vision today. That would be interesting because it could have complimented the very first eye.
We are beginning to learn more about those private ultraviolet wavelengths used by some animals excluding ourselves. We cannot see ultraviolet light because our lens absorbs it. Earlier in this book I described how we became familiar with nature’s ultraviolet patterns - they were captured on camera film. Although an ordinary glass camera lens absorbs ultraviolet light, a quartz lens is extremely transparent, particularly to those ultraviolet wavelengths used for vision by arthropods and some other animals today. Quartz also formed the lenses of trilobite eyes. So that first eye could potentially see ultraviolet light, providing it possessed ultraviolet sensitive cells in its retina. And that was likely, since retinal cells for blue light also detect some ultraviolet in animals, including ourselves (people with artificial lenses can indeed see in the ultraviolet). Because blue light would have been optimal in the Cambrian seas, trilobites would certainly have possessed blue-sensitive retinal cells.
Although the sea is not particularly transparent to ultraviolet light today, there are some shrimps and other animals which have the ability to see these wavelengths. In fact this finding is becoming increasingly common. An increase in ultraviolet transparency in seas at the end of the Precambrian could have been due, again, to a change in mineral content, but also to a reduction in ‘particles’ that scatter light. These particles scatter shorter wavelengths of light, representing blues and ultraviolet, much more than longer wavelengths, representing the red end of the spectrum. So without these particles, the waters below the very surface of the sea would have contained more ultraviolet wavelengths available for vision.
Similarly, atmospheric events could have caused an increase in usable (for vision) ultraviolet reaching the sea. The ‘particles’ that scatter sunlight in the Earth’s atmosphere cause the sky to appear blue - and ultraviolet. Meanwhile the remaining wavelengths pass directly through the
scattering layers, and we see them during a sunset where they appear orange and red. So variations in the density of these scattering particles can shift the emphasis of the Earth’s spectrum from red and orange to blue and ultraviolet. But because we don’t know precisely which colours the first eye saw, we must end our search for the wavelengths that changed to provide an enhanced selection pressure for vision.
Figure 10.2 From left to right: a butterfly wing photographed in black and white through a crystal lens under white plus ultraviolet light; through a crystal lens under ultraviolet light only; and through a glass lens under ultraviolet light only. To the human eye each wing appears black with two blue stripes. These images reveal that the lower stripe also reflects ultraviolet light, which transmits through the crystal lens but is absorbed by the glass lens.
Now we are left to consider only the general quantity, or brightness, of sunlight as a selection pressure for eye evolution. But again, a mineral change in the water is the most likely explanation for increased light transmission in general (an alternative could be the clearing of dense algal blooms). So we require still an event that could have led to this. Maybe it is time to re-open the evolutionary file for Snowball Earth.
In The Blink Of An Eye Page 32
