For some reason the early members of each animal phylum did not acquire their hard parts, and hence their characteristic external parts, until the Cambrian. This poses a different question - the why of the Cambrian explosion. Why did it take place? The evolution of hard, external parts was not a chance occurrence. It took place simultaneously in all phyla, after a considerable period during which nothing happened. This extensive correlation must have been forced by an external factor. But what factor? What caused the Cambrian explosion - why did it happen? This is the problem we are left with, and the aim of this book is to solve it.
Why did the characteristic external parts of animal phyla not evolve when the genetic identities were laid down in the Precambrian? Perhaps they simply did not need to. The development of complex, hard external parts from an embryo requires more energy than a simple sausage-shaped sac - why spend more energy than is necessary? And for some 120 million years or so they did not make the leap to external part development. The factor, then, that caused this leap, and made the expenditure of additional energy necessary, must have been monumental. In this book I aim to reveal the identity of this factor and hence the cause of the Cambrian explosion - the reason why it happened.
The answers proposed
A number of explanations for the why of the Cambrian explosion have been put forward. Unfortunately, there is strong evidence against all of them: none can stand up to scientific scrutiny. The simplistic explanation is that the general environmental conditions were uniquely befitting for evolution during the Cambrian. That is, this was simply a nice time and place for animals to evolve. This includes both the physical (non-living) and biological (living) factors within the environment. But recent finds of embryos of nonskeletised animals from the Cambrian have provided evidence against this rather circular argument. The eggs of two Cambrian animals, a jellyfish and a bristle worm, are large compared with those of their living ancestors. The considerable elbowroom within the egg, and the close resemblance of late embryos to their adult forms, are clues that Cambrian embryos hatched fully equipped to depart into the environment rather than passing through a series of less-than-proficient juvenile stages. This strategy, known as direct development, is common under harsh or unpredictable environmental conditions today. It ensures that offspring will survive rough times. For example, crabs usually hatch from their eggs as slow moving planktonic forms that drift around in the water. These young forms are easy prey for many fish and when times are hard even these meagre morsels become fish food. But if the young crabs hatch so that they can live on the sea floor, and possess colour pigments and shapes that blend into their backgrounds, they may escape the attention of predators and survive to become adults. This is not the usual method of development because a highly developed hatchling comes with a high energy cost to its parent. Direct development in the Cambrian is perhaps a surprise because it indicates that this period was not so hospitable after all. Out goes the ‘nice conditions’ hypothesis.
Some other explanations of the cause of the Cambrian explosion have been victims of a general misunderstanding of what the Cambrian explosion really is. Many scientists have launched their research to expose the why armed with a very misleading explanation of this event - simply, the spontaneous evolution of all animal phyla. This is not a fair summary of the Cambrian explosion, and one which I will name the ‘misleading’ explanation. Now we know that the Cambrian explosion was the spontaneous evolution of external body parts in all phyla, where the internal body plans of all phyla are already in place. To be fair, scientists in the past have misunderstood the Cambrian explosion through no fault of their own - the genetic evidence that tells the story of internal body plans is a recent finding.
Figure 1.8 Tow versions of the history of the animal phyla. From the first soft-bodied form, evolutionary branching is equivalent in both models. (A) indicates that both internal body plans and external parts diversified throughout this branching, and most theories on the cause of the Cambrian explosion have been based on this model. (B) is the correct model and properly identifies the Cambrian explosion - that it was the simultaneous evolution of external forms in all phyla.
Current evidence suggests that the Precambrian ‘event’ - the evolution of internal body plans - was not explosive but gradual, lasting tens or hundreds of millions of years. This is likely because the Precambrian ‘event’ concerned one animal form evolving from a previous form, and so on - a condition not affiliated with the Cambrian explosion. The Precambrian ‘event’ was more a surge in evolution than an explosion. It is possible for the Cambrian explosion to happen at one moment in time, but not so the Precambrian ‘event’. To summarise, the old interpretation of the ‘Cambrian explosion’ is actually the combination of the Cambrian explosion and the Precambrian ‘surge’. In general, the surge was the major genetic event and the explosion was rather more driven by some external factor.
The next proposals for the why of the Cambrian explosion suggested that the physical environmental conditions changed at the end of the Precambrian. We have already learnt that it was not a change in the environment as a whole (physical and biological factors) that caused the Cambrian explosion, but some other explanations centre on just one part of the environment. One explanation is based on a rise in atmospheric oxygen to a critical level, another on a decrease in atmospheric carbon dioxide. Oxygen and carbon dioxide are factors affecting the breathing and circulatory systems of animals. These systems are part of the internal body plans in most animal phyla, and generally affect external parts in a minor way. So oxygen and carbon dioxide levels could not have played a part in the Cambrian explosion; maybe they were involved in the evolution of internal body plans. Also, there is geological evidence indicating that oxygen levels peaked at various times before the Cambrian. Some of this evidence comes from cosmic spherules - small rocks that landed on Earth from outer space throughout geological history. Cosmic spherules contain chemicals that react with oxygen, and the degree of reactivity indicates the level of oxygen present in the Earth’s atmosphere at the time of landing. And they reveal a series of peaks in oxygen levels, before, during and after the Cambrian.
Staying with the chemical theme, an additional physical environmental factor that may have changed during the Cambrian is the availability of phosphorus. Phosphorus facilitates the development of calcium phosphate skeletons, and an increase in phosphorus levels could have led to an increased development of hard external parts. But it is not just calcium phosphate that makes up the external parts of animals - other chemicals are involved too. The phosphorus argument takes no account of these. And like oxygen, there is evidence that phosphorus levels peaked before and during the Cambrian.
Another physical environmental suggestion for the cause of the Cambrian explosion is that continental shelf areas (‘shallow’ water habitats) increased at the beginning of the Cambrian. This condition may have been forged as seawater encroached on the land masses worldwide. But even if continental shelf areas increased, they were present to some degree long before the Cambrian explosion. Hence this event doesn’t add anything new to the system, just more of the same.
The most recent physical environmental bid for an explanation of the why of the Cambrian explosion is linked to the ‘Snowball Earth’ hypothesis. It is thought that before the Cambrian there were spells when the Earth looked like a giant snowball. In some Precambrian times, the sun was probably some 6 per cent fainter than it is today. The consequent drop in both temperature and concentrations of carbon dioxide in the atmosphere allowed polar ice caps to grow. Ice reflects sunlight and the infra-red radiation from the sun that heats the Earth’s surface. So the more ice that formed, the cooler the planet became and the greater was the potential for further ice to form. A hardline view of this idea is that all the Earth’s oceans eventually froze to a depth of about 1 kilometre. A softer view is one of greatly extended polar ice caps, leaving open water to circulate around the equator. In either case, normal conditions would resu
me after volcanic events filled the Earth’s atmosphere with enough carbon dioxide to kick-start the greenhouse effect, or global warming, causing the ice to melt. These Snowball Earth events may have taken place regularly some 2,000 million years ago, but did so at least twice during the late Precambrian, between 850 and 590 million years ago. Inevitably, for an event taking place near to the Cambrian, Snowball Earth has been nominated as the cause of the Cambrian explosion. One problem is that the scientific jury has yet to decide on the hard or soft view of Snowball Earth. The soft version provides no explanation for the Cambrian explosion for the same reason as the sudden increase in continental shelf hypothesis - namely there remained ‘normal’ watery environments available to host evolution. But even the hard view is open to criticism as a cause of the Cambrian explosion.
Firstly, this idea has a teleological foundation. It assumes the course of evolution was predetermined from the beginning. We are given a situation where the Precambrian worm-like bodies of all animal phyla are just itching to take on their Cambrian forms, but ice puts everything on hold. Then, when the ice has gone, it is time for evolution again. This is not an objective view. As we have considered before, why should a convenient worm shape have to change? If the course of evolution was predetermined, why did it not continue in the water under the ice? The second major doubt cast over this laboured explanation for the why of the Cambrian explosion is that the figures simply do not balance. The Cambrian explosion took place between 543 and 538 million years ago. The last Snowball Earth event ended 575 million years ago at the latest. So there is a difference of at least 32 million years between these two events. This is fact. So a Precambrian Snowball Earth event cannot explain the Cambrian explosion, although it could have played a role in the Precambrian ‘surge’.
We are trying to explain an explosion in diversification, or a macro-evolutionary event. In terms of external parts, changes in physical environmental conditions lead only to micro-evolution, or gradual transitions. To explain the cause of an explosion we need a factor that is a matter of life and death. Such a factor must be part of the biological environment - a change took place in the animals themselves. And biological environmental explanations for the cause of the Cambrian explosion have also been proposed.
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.
Preview
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 example
s 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.
In The Blink Of An Eye Page 6
