In The Blink Of An Eye (9 page)

One biological explanation is that collagen was universally acquired in animals during the Cambrian. Unfortunately, this only works for the misleading Cambrian explosion where collagen could have evolved in one animal phylum that was quickly to become the ancestor of all other phyla in the Cambrian. Since we know that evolution did not happen in this way, the independent evolution of collagen in all phyla would have to have happened simultaneously if this explanation is correct. The chances of this happening are extremely slim. Also, like
phosphorous, collagen is not the only material used to build the hard external parts of animals.

The American biologist James Valentine, from the University of California, Berkeley, proposed that major diversification can only take place when there is an unoccupied niche (a ‘way of life') to evolve into. This implies that the
why
of the Cambrian explosion is the sudden availability of niches in the Cambrian. Unfortunately, this explanation is a victim of the misleading version of the Cambrian explosion. We are not looking for an explanation of why four animal phyla suddenly evolved to become thirty-eight phyla; rather why thirty-eight phyla with different internal body plans only suddenly became thirty-eight phyla with different internal body plans
and
different external body forms. For some 120 million years this transition did not take place, yet all that time there were certainly new niches to evolve into. For example, one potential niche included a predatory lifestyle. The worm-like forms of this 120-million-year interval were basically slow-moving chunks of protein. But no animal filled this predatory niche, which may have exacted a body with hard, biting jaws and strong, grasping limbs. Numerous examples exist of the potential niches available prior to the Cambrian explosion, but for some reason these niches did not become filled until the beginning of the Cambrian - they remained potential niches. The consideration of niches is surely important, but it is not the basic explanation we are looking for. We are looking for a factor that drove all phyla to occupy all potential niches at one point in geological time. Something
very
unusual must have happened at the beginning of the Cambrian.

There may have been an increased availability of the free-swimming plants that lived as plankton in the Cambrian. This could have resulted from a major event in oceanic upwelling, which itself has been assigned several explanations. These plants were in turn a selection pressure for animals to evolve swimming limbs, so that the plants in the water could be reached, and to evolve specialised mouthparts to eat them. A short, fat worm with big lips and no teeth could never catch, let alone chew, some of the fleet-footed plants of the Cambrian. But this explanation focuses on the generation of just one new niche, not all of the niches occupied by the Burgess Shale animals. There are more than just
swimming forms represented in the Burgess Shale community, so this explanation alone is not the
why
of the Cambrian explosion, but we may now be on the right track.

One of the most plausible explanations of the cause of the Cambrian explosion suggested so far was reworked recently by Mark McMenamin and Dianna Schulte McMenamin of Mount Holyoke College in South Hadley, Massachusetts. Here all feeding modes, including predation, were considered as one major factor. On the one hand, McMenamin employed modern ecological methods to resurrect a century-old idea that animals developed shells as shields against predators. But at the same time he conceptualised the entire Cambrian community in terms of a food web, where every species has its own predators and food. This conceptualisation, nonetheless, has been criticised as being simplistic and anthropomorphic.

Despite, or even because of, all the explanations proposed, biologists and palaeontologists generally are not convinced that we understand the real reason for what was arguably the most dramatic event in the history of life on Earth. Jan Bergström of the Natural History Museum in Stockholm stated in 1993: ‘Why was there a radiation in the Cambrian? Our most sincere answer is that we do not know.' Four years later, Doug Erwin of the Smithsonian Institution confirmed that ‘the trigger of the Cambrian explosion is still uncertain'. With this book I aim to put an end to the uncertainty and the speculation about the cause of the Cambrian explosion. I will agree with Balavoine, Adoutte and Knoll, who independently inform us that the explanation lies in a sudden change in the ecology and behavioural system of multicelled animals. But I will be much more specific.

Often in science, learning that a theory is wrong can be almost as useful as knowing it is right. The wrong answers for the
what
and the
why
of the Cambrian explosion have gradually led us to understand where to look for the correct answers to both questions. They are themselves pieces of the complete jigsaw puzzle, albeit on the edge of
the picture. Having explained the correct answer to the
what
of the Cambrian explosion in this chapter, I will set out in the remainder of this book to present my new explanation of the
why
of the Cambrian explosion, something that has become known as the ‘Light Switch' theory. To uncover the real cause of the Cambrian explosion we need to put together
all
the pieces of the puzzle. The next seven chapters of this book will be given over to the more significant pieces. In the course of these chapters I will construct a multidimensional picture of how life works today, what happened during the course of evolution on Earth and, consequently, how life worked at different times in the past.

The following chapters will bring together the most unlikely of subjects, from ancient churches to impressionist paintings. At the turn of the twentieth century, the president of the Inventors' Association resigned his position after claiming, ‘Everything that could be invented has been invented.' He was not missed. This book will demonstrate the rewards of exploring laterally and how science can benefit from an interdisciplinary approach.

In the next chapter I will examine fossils in more detail, providing examples of the information they have yielded in the past. But an explanation of the Cambrian explosion needs more than palaeontological evidence; it needs biological evidence too. As many clues can be found from studying living ecosystems as can be found in the fossils of the Cambrian animals themselves. The solution I propose draws on clues from all over science. By moving through time to the living world and on to my own specialist subjects, I will explain, in Chapter 3, how modern animals appear coloured or invisible. I will demonstrate the sophistication of the colour-producing systems of today's animals, something we know very little about in extinct animals. A central theme will be that light is the most important stimulus to animal behaviour in the vast majority of today's environments - those exposed to light.

The case for light as a major stimulus today will be strengthened in Chapter 4 by examining the other side of the story - life in the dark, in caves and in the deep sea. Here, the importance of light will become even more apparent, not just in animal behaviour but also in evolution. In Chapter 5 I will compare the rates of evolution in two groups of
seed-shrimps which began their histories in different environments. One group lives in the open sea, the other in marine caves. By taking a closer look at the group from the open sea, it will emerge that light has driven their evolution, while those in the dark have barely changed from their primitive ancestors. The result is that the open-sea seed-shrimps are considerably more diverse than they are in dark caves. The role that light can play in evolution will also be demonstrated using marine isopod crustaceans (to which woodlice, or slaters, belong), where we will join Jim Lowry's SEAS expedition in the Pacific Ocean, and also using crabs and flies.

In Chapter 6 I will lighten the mood a little with an exploration for colour in ancient, extinct animals. Bones and other hard parts that may become fossils are physical structures. Some colours today result from physical structures, albeit microscopic. Could such micro-structures also preserve in the fossil record? Potential will be unearthed in fifty-million-year-old beetles and 180-million-year-old ammonites. Then the pages of the history book will be turned back even further … If the original colour alone of an Egyptian statue can tell us that it once housed the Book of the Dead, just think how much can be learnt from finding colour in fossils.

To balance the information provided on colour in animals, Chapter 7 will introduce the variety of eyes. It will show that all animals have to be adapted to the existence of eyes not only in terms of their colour, but also in their shape and behaviour - all factors affecting an animal's appearance on a retina. When this retina belongs to a predator, the image formed on it becomes a matter of life and death for the potential prey. But is the danger of visual appearance a recent one? The history of predation will be discussed in Chapter 8. By returning to the fossil record I will show that eyes, predators and probably the link between them go back a long way. But exactly how long? This will become a fundamental question.

By the beginning of the penultimate chapter the reader will have all the clues necessary to decipher the probable cause of the Cambrian explosion. In many ways it seems the most obvious explanation, but to reach it one must take this indirect, winding road. Encountered along the way will be a number of unfamiliar but fascinating examples of the
sophisticated and finely balanced ecosystems that exist in nature - but to begin with, it's back to the bare bones and a modern perspective on the lifeless rocks once kept safely within dusty Victorian display cases. Now we are bringing the past to life.

Nothing ever becomes real until it is experienced

JOHN KEATS

 

 

 

Beginning, as it were, with the very beginning, Chapter 1 summarised a history of life on Earth. In this chapter, the evidence used to create such a story will be examined, making a closer inspection of the rocks. But here time shall be traversed from today, travelling back to the Cambrian via some landmark attractions. And good old-fashioned fossils will provide the attractions.

Although the study of evolution is increasingly becoming consumed by genetic studies, the inferences from genetics are, and always will be, theoretical. The genes of many living species have been exposed, but the animals we see today did not evolve directly from each other. Intermediate stages were involved - species, for instance, that became extinct. So in order to reveal evolution, the genetics of the living
and
the extinct are required. And of course the extinct genes are, barring a few exceptions, subject to theoretical fabrication.

Fossils, on the other hand, are factual. They are literally hard facts that we cannot ignore. Around a decade ago, molecular sequences pointed to a Cambrian explosion that occurred way back in the Precambrian. The fossil record, which places an Early Cambrian label on the grand event, was thus contradictory and appeared to be standing in the way of progress. But palaeontologists stood firm, reminding us that fossils were
not optical illusions. When 350-million-year-old rocks are split to reveal the fine details of a bony fish, then bony fish
did
swim in Earth's waters 350 million years ago. When rocks formed under similar conditions, but from 550 million years ago, are consistently found without bony fish, eventually we must conclude that bony fish did not exist during this time. However, it would be equally foolish to ignore the genetic evidence, and indeed by reconciling the fossils and the genes a true picture of the Cambrian explosion has been painted. But whichever way they are looked upon, fossils are precious to the study of evolution. And they certainly justify a chapter of their own in this book, where the subject of fossils will surface again during discussion of seemingly unrelated topics.

It was the role of fossils in revealing the paths taken by evolution which contributed heavily to the previous chapter. The main purposes of this chapter are to expose the tricks used in creating this knowledge, but also to demonstrate that fossils have much more to say. The history book, ‘The History of Life', conceptualised here contains two-dimensional pages. The next task is to pump blood into the flattened veins of fossils and let them spring from the pages, so ancient animals can be seen doing what they once did. The application of engineering, physics, chemistry and biology can indeed transform a load of old bones into a virtual 3D world, perhaps millions of years old, where animals run, fly, gallop, burrow, eat and avoid being eaten.

Fossils can add some surprising details to the past, and they will provide considerable hard evidence towards the Cambrian enigma that this book attempts to solve. The individual cases in this chapter will provide a flavour of palaeontology in the twenty-first century, and constitute tools for the evolutionary trade. The art of Sherlock Holmes and modern forensic science will be reconciled with that of dinosaur specialists and religious artists. Fossil leaves will be employed to aid the palaeo-meteorologist. The technology of car designers will bring 400-million-year-old ‘worms' and arthropods back to virtual life on the computer screen. And the biology of living organisms and principles of Scuba diving will help to solve the ‘ammonite mysteries'. But to begin I will ask the question: ‘What, exactly, is a fossil?' The answer to this is not so obvious, especially when the remains of some extinct species are so ‘fresh' they can literally be brought back to life.

I have a colossal, antiquated book on the fauna of Earth. It is entitled
Knight's Pictorial Museum of Animated Nature
and is now in its seventh generation within my family. Between the heavy, morbid black covers exist brief descriptions, biological data and woodcut illustrations for thousands of species. Some of the illustrations are quite primitive, especially the unnatural poses of monkeys quite clearly based on stuffed museum specimens. The kangaroo drawings appear like those made by the first Europeans to reach Australia, and the story is similar for the American buffalo. A quick glimpse of a very unfamiliar form can result in a reconstruction with a more familiar form in mind. A buffalo could become cow-like, and a kangaroo could acquire some of the features of a hare. Here lies a lesson in fossil reconstructions - extrapolation can be risky, at least beyond a reasonable point. Crocodiles may be the closest living relative to certain dinosaurs. Although it may be safe to infer a similar scale-like skin texture, as we can confirm from recent finds of fossil skin, the sluggish quadrupedal form with a belly that scrapes the ground is probably a characteristic of the crocodile only. Yet pioneers of dinosaur reconstructions depicted the
Diplodocus
with its belly scraping the ground. That's fine - we need mistakes from which to learn (and mistakes are everywhere in science). Nowhere is this principle of extrapolation more dangerous than in the colour of extinct animals, as will be demonstrated later in this book.