In The Blink Of An Eye (46 page)

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 Chapter 1, I described how the Earth passed through spells where it was covered, or nearly covered, in ice a kilometre thick. Certainly the retraction of this ice could have stirred up minerals in rocks on a grand scale. As those huge ice sheets traversed the land, they would have ripped open the surface layers of rocks and absorbed minerals, transporting them to the sea. Unfortunately, though, the timing is a little out. The
Cambrian explosion took place between 543 and 538 million years ago, and the last Snowball Earth event ended 575 million years ago at the latest. So there is a difference of at least thirty-two million years between these two events. This might be just too great - theoretically an eye can evolve within half a million years. So I still believe that the last Snowball Earth event should be coupled to the Precambrian ‘surge' in evolution rather than with the Cambrian explosion.

Research in this area of the geological history of media transparency is still in its infancy; hence my discussion of this subject has been brief. In the future it is to be hoped that all will become as clear as the Late Precambrian environment itself.

The Light Switch theory is a consequence of recent fossil finds and evolutionary analyses (although the philosophy of colour today weighs in heavily, too). There remains an imperfection in the geological record that is still to be reckoned with, but it no longer looms before us as it did in Darwin's days. Palaeontologists today are striving to fill the ever narrower gaps in the fossil record, searching all corners of the globe for new species that lived near the time of the Cambrian explosion.

Originally I was afraid that the Light Switch theory might appear far-fetched, particularly since most alternative theories had been heading in very different directions. Eyes the cause of the Cambrian explosion? How ridiculous! But it was the amalgamation of modern biology with Cambrian palaeontology that finally settled my nerves. Now, after considerable contemplation of the power of vision today, I am convinced that the evolution of that very first eye must have been a monumental event in the history of life on Earth. For this fact alone I am happy to share my ideas with a wider audience. Whether that introduction of the eye really did coincide with the beginning of the Cambrian explosion should be answered with greater precision as new fossil finds are unearthed from near that Early Cambrian border. But at this stage in our knowledge, this relationship appears remarkably close.

My final reassurance that the Light Switch theory is both a judicious
and logical one came from the editor of a newspaper. James Woodford, a journalist with the Australian newspaper the
Sydney Morning Herald
, wrote a comprehensive article on my theory. This made the front-page headlines, and gave the newspaper's editor cause for concern. The night before publication, and just before the article and the paper went to press, James received a question from his boss. The question was, ‘Are you sure this has not been said before?' That was extremely comforting. It meant that this was an obvious answer. In fact it was so obvious that it had little scientific merit - anyone could have come up with it. True. Now
I
think that this is the obvious answer.

Recently I went swimming off the coast of Sydney. Here I encountered a group of cuttlefish similar to those that had initially woken me up to biodiversity, as described in the first chapter. Again the cuttlefish surrounded me in an arc and displayed spectacular colour changes. Again they looked at me with their large sophisticated eyes, and flashed their sophisticated colour display, as if confirming the importance of light in nature. Yes, I thought, vision has really entered the behavioural system of animals. Then I noticed a crab on the sea floor. I zoomed in on its eyes, and reflected:
the origin of those arthropod eyes had a lot to answer for
. . .