The group of McAlpine flies, like the notched seed-shrimps, has evolved with light as a major stimulus. We may be so bold as to say that light has driven the evolution of this group. And this second example of the influence of light on evolution is not the last. Back in the sea, light has been known to insert its influence on the evolution of a group of crabs.
From sound to light
The snapping, or pistol, shrimp has one small and one large crab-like claw. The large claw is the pistol, which fires an underwater bullet of sound so loud that it can be detected by passing submarines. In fact it can even interrupt their sonar. Sound can have its drawbacks because it is an omnidirectional signal - it is, as the word suggests, sent out in every direction. So not only does one reach a target organism, but also every other organism in the vicinity.
Another crustacean making sounds in the sea is the oval (swimming) crab. Although spending most of its time on the sea floor, it is equipped with swimming paddles on its rear legs to propel it through the water whenever required. Individuals aggregate on the sea floor to form species groups, which are highly aggressive towards each other.
There are many species of oval crabs and the ancestral type, known from fossils, made sounds in prehistoric times. This ancestor possessed a file and pick that scraped together to make trademark music audible in ancient seas. This was probably the oval crab’s means of attracting its own species for aggregation. And it was successful because those sounds can still be heard today, made by descendants of the ancestral species. In fact about half of the oval crab species living today employ a similar instrument. The oval crabs, nonetheless, have greatly increased their diversity by succumbing to a selection pressure other than that for sound production - sunlight.
The ancestral oval crab and the living musical species all have strong shells. Their shells are strong because they are composed of a stack of thin layers. In fact a cross section of their shells appears like a multilayer reflector, except that the layers are too thick to cause a reflection. Still, a stack of layers is stronger, tougher and more resistant to cracking than a continuous slab of the same material. Think of pieces of wood used in DIY that are composed of thin layers glued together, for instance; they are both strong and effective.
Although one group of oval crabs continued with their music-making, another group gradually lost the ability to make sounds, while progressively acquiring the ability to reflect light. Throughout the evolution of this colourful group, the picks and files gradually diminished until they vanished completely. But early on in the evolution of this group, a change took place in their shells - the composite layers became thinner and, to maintain the overall thickness of the shell wall, more numerous. While retaining their strength characteristics, the layers had formed into multilayer reflectors. Shells began to appear iridescent.
The first oval crab species to evolve iridescence retained some ability to make sounds, which was probably useful because only a small area of its shell had become iridescent. It was a shy flasher. But then the floodgates opened. The next species to evolve contributed a greater spectrum to the oceans - it was more extensively clothed in iridescence. And so on until the most spectacular marine animal of all had arrived on Earth - the majestic iridescent crab. This is a large crab, with a shell the size of a grapefruit, that gleams with brilliant iridescence from every part of its body - shell, legs and claws. Imagine a crab made of the most spectacular opal. There would have to be great advantages to having such a bright attire, because the disadvantages are obvious, particularly the way the crab continually advertises its presence to predatory fish. Where seed-shrimps succeeded in concealing their iridescence when it was not required, oval crabs failed. But the iridescent advertising of the majestic iridescent crab is not as pointed as would at first appear to be the case because it has a trick up its sleeve - in its natural environment it can make itself invisible. Here lies an advantage of an iridescent signal - it is directional. Compare the explosion of a pistol to the flash of a torch in a bright environment. Unlike the explosion, the torchlight can only be detected when one looks directly at it. Either way, there really must be advantages in appearing brightly coloured, because the oval crabs that evolved iridescence also devolved their sound production. Time does tell. But these advantages could be confined to certain areas of the globe - the areas where the colourful oval crabs live. Maybe predatory fish have ‘bigger ears’ in these areas, so it is best to keep quiet.
Again, the relevant conclusion to be drawn for the purposes of this chapter is that light has played a major role in the evolution of an animal group. Sunlight could be considered the driving force for the evolution of the iridescent half of the oval crab tree. And the momentum of evolution in the direction of the sunlight selection pressure never slackened.
The list continues
I have dealt only with structural colours in this chapter because these can be represented by mathematical equations and granted efficiency values rather easily. But changes in pigments are also known to occur throughout evolution. Nudibranchs, or sea slugs - marine snails that have lost their shells (through evolution, of course) - demonstrate just how spectacular a pigment can be. Some of the most memorable underwater photographs seen in the coffee-table books on marine life are of sea slugs. But the taxonomy of sea slugs is problematic. Once their pigments have broken down in preservative, and their colours have faded completely, many of them look extremely similar. It is their colours that separate them into species without the aid of dissections or genetic analyses. Their unmistakable colours provide warnings to predators. Different species have different predators, and their colours have evolved to suit. As predator vision changes or evolves, so do the sea slug colours. Hence light is a major selection pressure to the evolution of sea slugs.
There are many other examples of evolution driven by light. Light is not only a governing factor of animal behaviour at any one point in time, such as today, but is equally important in the evolution from today’s ecosystem to that of the geological tomorrow. Light not only exposes an animal as conspicuous or camouflaged today, but can also drive the evolution of animals in the future. As inferred in Chapter 3, if an animal is not adapted to the light in its environment, it will not survive. And light is an exception among the stimuli because in most environments it is always there. One cannot ignore light. But equally important to this book is the issue of evolutionary dynamics. It is one thing to know what happens during the course of evolution, or the design of the evolutionary tree, but something altogether different to explain why it happens. In this chapter it has been demonstrated that adaptation to light can be the why of evolution. And the next questions to emerge are, of course, ‘Was light a selection pressure at the time of the Cambrian explosion?’ and, if so, ‘How strong a selection pressure was it compared with others?’
The disparate subjects of colour and animal evolution have emerged as compatible. This chapter signals the dawn of a relationship that will mature as subsequent chapters unfold. The perseverance with colour is beginning to pay off, as clues begin to gather towards solving the Cambrian enigma that this book is attempting to understand. But these are still early days in the Cambrian trial, and evidence also needs to be sought from other avenues.
So far we have examined colour in living animals and predicted the course of colour evolution. But is there also real evidence of colour in the past? Can we return now to the fossils and hope to unearth their true colours? If so, this may be a step towards finding the answers for the above questions about Cambrian light. Armed with our understanding of colour today, it is certainly worth a closer look at what was described in Chapter 2 as a void in palaeontology. In Chapter 6 I will attempt to start filling that void.
6
Colour in the Cambrian?
All species still glow in their original, almost fantastic array of colours
HERBERT LUTZ, German biologist, on the colour of 49-million-year-old jewel beetles from Messel, Germany
Today the Museum of Antiquities in Leiden in the Netherlands, houses a statue of the Egyptian god Osiris. This statue is about a foot tall with well-preserved features, and also a fair amount of its original paint - it has seen little sunlight, being mostly preserved within a tomb. Here, Osiris has a blue-green face and wears a red skirt. And another obvious feature of this statue is that it is hollow . . . but why? Without the preserved colour this question would remain unanswered. Numerous statues of Osiris have been excavated but the hollow inside and colouration make this particular representation different.
The interpretation of hieroglyphics, and the preservation of yet more pigments in the form of ancient Egyptian scripts, inform us that blue-green was the colour used to represent the afterlife and red was used for festivity. So now we can interpret this statue of Osiris as being a celebration of the afterlife. From this, and the knowledge that hollow Egyptian figures were filled with papyrus manuscripts, we can infer that our statue once contained a copy of the Egyptian Book of the Dead.
The ancient Egyptians were, in fact, skilled artists. They used colour to represent personality and status, but they knew it would fade with time. Consequently much of their art was sculpted and then painted, so that at least the physical sculpture would remain long after their death (as was their intention). But they also had gold leaf at their disposal. The cause of the gold effect in this case lies somewhere between a pigment and a structural colour. Gold leaf is a thin layer of metal that reflects a beam of sunlight in a single direction, like a mirror. It reflects all the wavelengths in sunlight except blue, all of which add up to gold. As a physical structure it outlasts the pigments of ordinary paint through time. So gold leaf was used on many Egyptian statues, since the Egyptians were conscious of the short-term prospects of their pigments. And gold leaf is indeed evident in numerous Egyptian artefacts today, as in another statue of Osiris housed in Leiden. The gold in this case is symbolic of eminence.
Chapter 3 demonstrated that colour alone tells us about where and how an animal lives today. Considering the information acquired from the colour of the pigments in the Egyptian statue of Osiris, a question relevant to this chapter now begins to form: ‘Can we bring Cambrian fossils to life in the same manner?’ The excellent preservation of gold leaf in the statue of Osiris signals hope of unearthing structural colours of geologically ancient times.
We know that animal body shapes and forms were as complex in the Cambrian as they are today, so perhaps we can also expect Cambrian animals to have been sophisticated in terms of their colour. But we have learnt not simply to predict colour based on animals today. We must find traces of the original colours themselves in ancient, extinct animals. And the best place to look is in those fossils that have been preserved under the most favourable conditions. Work in this field is already underway.
Trilobites that lived 500 million years ago, just after the Cambrian, have been found with signs of pink colouration, not something that is easily explained given the type of rock in which they were preserved. It is therefore believed that these randomly arranged pink pigment granules are remnants of a colour that once covered the entire trilobite. That would be interesting. Below the very surface waters, and in the environment inhabited by these trilobites, red light does not exist. Here, pink becomes grey and blends well into the background. So these trilobites may have been coloured for camouflage. But few experiments have been conducted in this case, and so speculation must end there. And this case of trilobite-pink also represents the end of the road for ancient pigments. Unfortunately, pigments, and also bioluminescent organs, do not take us back to the Cambrian, and so can be of little use to the subject of this chapter. But structural colours are another matter altogether. Could they tell us anything about colour in the Cambrian?
As outlined in Chapters 3 to 5, physical devices that cause ‘structural’ colours are a significant means of light display today. Like pigments, structural colours rely on a source of incoming light, usually in the form of sunlight, from which certain wavelengths, or ‘colours’, are reflected.
Structures can be preserved in the fossil record - at least their shapes and sizes can be, even if the original materials become altered or replaced. Fossils themselves, whether the whole bodies of trilobites or the bones of dinosaurs, are indeed structures. Although on a much smaller scale, it is therefore not surprising that structures responsible for colour can also be preserved within fine sediment - these are, after all, just structures. Obviously micron-sized reflectors could not be preserved in sediment of 1 millimetre sand grains - apart from the obvious physical problems they would be consumed by the bacteria infilling the spaces between grains. This was the reason why we cannot find minute sensory detectors in the Australian Ediacaran (Precambrian) fossils. Shapes of the entire animals can be seen with the naked eye, but under a microscope nothing more than piles of sand grains can be distinguished. Similarly, the chemical components in the embryonic rock must be right to replace organic parts. But there is certainly greater potential for structures to be recorded in the fossil record than for pigments. And if the conditions are right, theoretically there is no lower limit to the size of a structure that can be preserved as a fossil.
Before moving directly to the Cambrian fossils themselves, we should take a look at the methods at our disposal for unearthing ancient hues. We should be aware of the variety of structural colours that may be preserved along with some of the pitfalls one may encounter along the way to the Cambrian.
Ammonites - multilayer reflectors and modifications
We know that multilayer reflectors are the most widespread cause of structural colours in animals today. Like pigments, these occur within the bodies of animals, below the surface. Again, the scanning electron microscope is not appropriate here because it can only scan surfaces. So to search for multilayer reflectors, we must look at thin sections of fossil skin or shell - the outer layers of an animal. Some years ago I tried exactly this, using ammonites and ancient beetles as my guinea pigs.
Ammonites are among the few groups of animals whose original, transparent, thin layers have survived in fossils, and colours radiate from some of them today as they may have appeared millions of years ago. But this cannot be assumed for every case of iridescent fossils. There are warnings to heed from opal - all that glitters may not be old, or at least not as old as the animals that have been fossilised.
In Chapter 5 I described my discovery of structural colour in seed-shrimps, almost the first structural colour known in seed-shrimps. A couple of years earlier, while sorting through a large sample of small crustaceans, I had noticed a single flash of colour from one seed-shrimp. There were many other individuals of this species, and all were quite transparent, but while I moved the sample one individual was flashing red one minute, and green and blue the next.
The seed-shrimp was the size of a tomato seed, and the source of the colour much smaller, but it was large enough to be identified under the microscope. The identification also solved the problem of why only one individual should reveal colours. The source of the colour was not a feature of the animal itself, but a tiny opal, and the seed-shrimp had eaten it. The opal lay in the stomach of the transparent animal.
Opal is a form of silica dioxide. It is made up of tiny spheres, around half the wavelength of light in diameter. It reflects light in a complex manner, which has only recently been understood by optical physicists. But it is the physical nature of the structure that provides the optical effect rather than a chemical pigment, and so opal is said to produce structural colours. In fact the bright, iridescent effect of opals is similar to that of the seed-shrimp diffraction gratings.
The original chemicals that make up fossils, at whatever stage in the fossilisation process, can be replaced by other chemicals. Sometimes, the replacement chemicals can be silica dioxide and water, in which case opal is formed in the mould that is the fossil. At Lightning Ridge in Australia, opal miners often excavate dinosaur bones and teeth, and the parts of other a
nimals, which display the characteristic iridescence of opal. These fossils are so well known that most palaeontologists think of them when we mention ‘colour in fossils’. But unfortunately this adds no evidence to the original colours of ancient life - opal has nothing to do with living animals (other than that single seed-shrimp).
Ammonites are the shells of ammonoids, those long-extinct molluscs related to squid as described in Chapter 2. Some ammonites appear coloured, but like opal their hues are non-biological. Particularly striking for their visual effect are the ammonites from Alberta, Canada, which flash spectacular colours as their rocks are cracked open.
In view of the Canadian Rockies lies the small town of Magrath, and the familiar wheat fields and ranches of the Canadian prairies. Seventy-one million years ago, this land was beneath a sea which stretched from the Gulf of Mexico to the Arctic Ocean. And in this sea lived ammonoids - lots of them, ranging from the size of a compact disc to that of a car tyre. Today, one particular ranch near Magrath, of about 800 hectares, is different from the others. Its foundations contain ammonites.
These ammonites were first covered not with sand but with ash from the huge volcanic eruptions - which played a part in the creation of the Rocky Mountains. The ammonites became sealed in a waterproof layer of shale, but this did not prevent quartz, copper and iron from the volcanic ash infiltrating the shells. During the Ice Age, a layer of ice close to 2 kilometres thick covered the region. The weight of this ice served to compress the ammonites and their component chemicals, and ‘Ammolite’ was formed.
In The Blink Of An Eye Page 20
