The Cambrian explosion in brief
Thirty-eight animal phyla have evolved on Earth. So only thirty-eight monumental genetic events have taken place, resulting in thirty-eight different internal organisations. Members of these phyla possess a variety of appearances - or external forms - as we have explored on the Great Barrier Reef. Think of the protective spines, swimming paddles, burrowing shapes, grasping arms, eyes and colours. We have also seen that sometimes the same forms can occur in members from different phyla (convergence), but in general each phylum contains a characteristic variety of external forms.
The first fossils from the time 543 to 490 million years ago were found in the Cambrian Hills in Wales. Hence this period became known as the ‘Cambrian’ (as named by the great Cambridge geologist Adam Sedgwick). It follows that the time span prior to 543 million years ago is called the Precambrian (the Precambrian can be further divided). What if I stated that, based on external characters, 544 million years ago there were perhaps three phyla? Most people would picture a scenario where the number of phyla simply increased gradually from three to thirty-eight over the past 544 million years. Along this trail of thought, 320 million years ago there might have been some twenty distinguishable phyla. Such a steady progression involves a type of process known as ‘micro-evolution’. Darwin and Wallace thought along these lines.
Figure 1.2 The geological timescale and epochs.
Revolutions in evolutionary theory have occurred since Darwin’s time. Now we know that the history of life on Earth has been dominated by long periods of gradual evolution - ‘micro-evolution’ - or even a complete standstill. But these periods ended abruptly as they were replaced by ‘macro-evolution’ - short but prolific bursts in evolutionary activity, hence a so-called ‘punctuated equilibrium’ model for evolutionary history. Darwin and others of his time cannot be blamed for overlooking macro-evolution because its discovery was a consequence of twentieth-century fossil finds and the development of modern biochemical techniques, encompassing genetics and the biology of development from embryos to adults. Events that cause macro-evolution include a faster development from embryo to adult form, the development of sexuality in juvenile forms, and the turning on or off of major genes.
With all this in mind, I should like to change the facts and state that 544 million years ago there were indeed three animal phyla with their variety of external forms, but at 538 million years ago there were thirty-eight, the same number that exists today.1 In this case the vast diversity of body architectures observed on the Great Barrier Reef would all have appeared during a five-million-year interval (some researchers say fifteen), beginning 543 million years ago. In fact such an interpretation is closer to the truth, and this particular five-million-year interval hosts the subject matter of this book - the ‘Cambrian explosion’. The Cambrian explosion is the evolutionary episode in which all animal phyla attained complex external forms. In other words, it is the event during which animal phyla changed from all looking the same to looking different. Now that I have introduced the Cambrian explosion, can I end the first chapter of this book here? Unfortunately not. Such a simple description of the spectacular transition in evolution from Precambrian to Cambrian times does not provide a fair description of how today’s diversity of life came into being. We cannot consider only the external appearances of animals but need also to think of their internal body plans. To understand what the Cambrian explosion really is, this is essential. Previous explanations of the Cambrian explosion have been greatly simplified by the definition ‘the sudden evolution of all animal phyla’. This flippant approach to the most dramatic event in the history of life is misleading in the extreme, and has led to a number of false explanations for the cause of the event. The crux of the problem here is that internal body plans and external parts have been treated collectively, and their evolution is thought to have occurred simultaneously. This is not true. The Cambrian explosion is all about external body parts only. But we have learnt of the great significance of internal body organisations to animal diversity and should study this subject further if only to provide the outside pieces of the jigsaw puzzle to be solved in this book - what caused the Cambrian explosion? The story of internal body plan history takes us deep into the Precambrian.
Up till now we have been measuring time in units of millions if not billions of years. Such quantities are hard for us to make sense of. We think of ancient history as perhaps a couple of thousand years ago. Ten thousand years would be extremely difficult to conceptualise, a hundred thousand, let alone a million, inconceivable. So hundreds of millions of years of evolution are way beyond the realms of the most vivid human imagination. If it is of any help, I began to conceptualise one million years after seeing the immense valleys in Hawaii that have been formed by one million years of running water. These perfectly triangular valleys that terminate at the coast are over 100 metres deep. But a million years ago they did not exist, and the north-west coast of Hawaii had a continuous cliff face with a flat top. As volcanoes formed inland, so did streams or small rivers terminating at the coast. The action of this running water gradually wore a groove into the surface of the ground. And over a million years, water can form a groove 100 metres deep - this is worth thinking about. During such a time period, and without taking space into account, even outcomes with almost negligible odds can emerge. But only when the process in question can change gradually, where each step or small change is saved, and the process can then proceed from a new starting point with this change firmly in place. This line of thought will be continued in this chapter with Sir Andrew Huxley’s criticism of a ‘jumbo jet in a junkyard’.
‘The History of Life’ from the very beginning
A book on the complete ‘History of Life’ on Earth would comprise tens of chapters, where the real subject of this book, the Cambrian explosion, would occupy chapter nine in the thesis, as I will go on to explain. A summary of the chapters following chapter nine, such as those where dinosaurs first appeared, and then disappeared, would offer no help in comprehending the Cambrian explosion. But one would be somewhat lost in a book that began with chapter nine, even though it may be the most alluring, without a summary of the previous chapters. Also, I have mentioned that we would look back in the Precambrian at the story of internal body plans. So before returning to the biggest macro-evolutionary event of all, beginning 543 million years ago, I will attempt to paint a word picture of the world as it was then and before that time.
The fine details of life’s earliest history, or first chapters, are more open to debate than those of the last 550 million years. This is partly because the fossils that we have from the more ancient times are either microscopic or preserved in poor detail. But it is also because the further back in time we go, the more different the environment was from what it is today and hence the greater the inaccuracy of any extrapolations we may make. Since chapters one to three in ‘The History of Life’ deal with the longest periods of time, they will be considered in the most detail.
The Earth formed some 4,600 million years ago, and it is generally accepted that life came into existence around 3,900 million years ago, following a flurry of meteorite bombardment. But during the first 3,000 million years of life’s history, or chapters one to three, the Earth was populated only by bacteria, algae and single-celled animals. The history of life is written into the Earth’s rocks as fossils or preserved in primitive environments. To investigate the first chapters of Earth’s story, or the first stages in evolution, we must visit hot, volcanic pools or go deep into the ocean.
Chapters 1 to 3 in ‘The History of Life’ - the first cells
Thousands of metres below the ocean surface today, black smoke pours into the water from the submarine ridge known as the Axial Seamount, 300 miles west of the coast of Oregon. As dramatic flashes of colour and air flare from the primeval cauldrons or chimneys known as hydrothermal vents, or ‘black smokers’, one can really begin to form images of a very primitive Earth. There is
justification in this imagery because black smokers would have emerged with the appearance of the first seas. They mark the separation of boundaries between Earth’s massive plates, on which we live, that float on the planet’s surface. Up through the gaps created, hot magma oozes out of the Earth’s crust to form new sea floor. The unstable concoction of chemicals ejected from the first black smokers reacted with seawater and provided conditions that could have given rise to the inorganic construction of amino acids and other prebiotic organic molecules that are the building blocks for life. Such chemical reactions compare with those found in primitive living creatures today. As one can imagine, chemicals leaving a black smoker are hot. But very primitive bacteria can tolerate temperatures of up to 110° Celcius in today’s black smokers, so heat was never a problem for early life. In fact the living representatives of all of life’s most primitive species require very hot temperatures to sustain their chemical workings. It is also interesting that black smokers are probably the only places on Earth where the energy of life is not drawn from the sun by means of photosynthesis in an oxygenated atmosphere. The small iron sulphide globules found in the chimneys of the black smokers quite possibly provided the reducing environment necessary to sustain the first life forms. All things considered, black smokers are good candidates for the cradle of life on Earth, and belong in chapter one of ‘The History of Life’.
The Murchison meteorite that hit Australia in 1969 contains around seventy-four amino acids, and at least eight of these are of the type that makes up proteins. Could life on Earth have an extraterrestrial origin? Current evidence suggests not. Space is full of organic molecules (those that make up living organisms, including amino acids). But the concentrations they form on impact with Earth, such as in interplanetary dust within the ocean, are much too low to induce life. Hoyle and Wickramasinghe, advocates of the outer space origin, once calculated that the chances of life starting on Earth independently, within a watery soup of amino acids, were roughly the same as having a junkyard spontaneously forming a jumbo jet. Andrew Huxley was among those who set the record straight by explaining a jumbo oversight in these equations. Hoyle and Wickramasinghe had calculated the probability that the right amino acids would come together by chance, in the right order, to form an active protein molecule. But they admit to leaving out two absolutely enormous factors - time and space. The calculation is reasonable enough, but the answer it gives is the probability of this protein molecule originating spontaneously at one particular moment in time and at one particular point in the Earth’s oceans. Huxley pointed out that this is of no interest to anyone; what we are concerned with is the chance of a primitive living system being formed at any moment within a period of hundreds of millions of years, and at any point within the enormous volume of the oceans. In fact the idea of omitting these enormous factors would have been unbelievable if they had not actually done it! Hoyle and Wickramasinghe also assumed that two thousand active protein molecules have to be formed simultaneously by chance to make a primitive living system. But once the first protein molecule had assembled, a self-replicating system would have come into play, which would then develop by natural selection. As Stanley Miller, who first attempted to simulate the origin of life in the laboratory, famously stated, ‘The origin of life is the origin of evolution.’
As expected for such a fundamental question, there are many explanations for the first stage of life on Earth, although most researchers now agree that a hot region was involved. Today, American scientists are investigating the problem at the undersea hot springs in the Pacific Ocean. The ocean is not a theoretical necessity. Suitable heat exists in the ground waters deep in the Artesian Basin in Western Australia. And the ‘hot’ water that exits under ground in the volcanic regions of the USA accommodates a spectacular possibility for the second stage in evolution - chapter two in ‘The History of Life’.
Just a couple of thousand metres beneath the surface at Hawaii’s Volcano National Park and parts of Wyoming’s Yellowstone National Park, there is molten rock that heats the rocks on the surface. Ground water consequently boils in some regions and flows up channels through rocks until it either bursts from the surface as geysers and vapour, or collects to form steaming pools. On its journey to the surface, the water collects minerals from the circumventing rocks. Together with those gained from the molten rock, the minerals become concentrated in the steaming pools, or are deposited as surface waters evaporate. A range of colours can be seen in and around the surface waters, and these belong to colonies of bacteria, which flourish on the minerals. These bacteria represent the second stage in life’s history, for they do not have to rely on the finite organic compounds that would have originally accumulated in the Earth’s bodies of water. Instead they make their own organic compounds within their cell walls, drawing energy from sunlight. This process is photosynthesis, and requires hydrogen. The bacteria obtain their hydrogen from hydrogen sulphide (‘rotten eggs’ gas), originating from an underground reaction between the molten rock and ground water. But such a delicately balanced diet means that these bacteria are restricted to regions of volcanic action. The third stage in evolution had further repercussions: it opened the floodgates for endless possibilities of life forms.
Chapter three in ‘The History of Life’ sees the appearance of cyanobacteria (traditionally and erroneously called ‘blue-green algae’), organisms that obtain their hydrogen from water. This was achieved by the evolution of a substance of great consequence - chlorophyll, the lifeblood of true algae and the higher plants. Unlike hydrogen sulphide, there is an extensive supply of water on the planet, and this equates with a profuse occupation of Earth by life. As the cyanobacteria removed hydrogen from the Earth’s water, oxygen remained and entered the atmosphere. Cyanobacteria include the simplest forms of life existing today, and the timing of the first cyanobacteria is known from fossil evidence.
At Pilbara near Marble Bar in Western Australia, a fine-grained mineral called chirt can be found in rocks 3,500 million years old. Slices of this chirt are cut so thin that they are translucent and can be examined with an ordinary microscope, revealing the shapes of cyanobacteria. But how do we know the fossils really are cyanobacteria like those of today? After all, the organisms are little more than minute squiggles. The answer lies in the large, unique structures that are formed by the micro-organisms, structures that are still formed today.
In the same Australian state as Pilbara, Hamelin Pool can be found within Shark Bay. Here coral reefs are replaced by stromatolites (from the Greek, meaning ‘stony carpet’), appearing like large button mushrooms carved from rocks that rise above the shallow sea. The entrance to Hamelin Pool is blocked by a sand bar and eel grass. This barrier separates the water in the pool from the ocean, and the evaporation of water increases the salt concentration of the pool. Animals that usually feed on the cyanobacteria in the pool cannot survive under such salty conditions, and so the cyanobacteria thrive. The cyanobacteria exude lime, which hardens to form the stromatolites. We know that the Pilbara chirt is actually made up of ancient stromatolites because it shares the same unique structure as the stromatolites of Hamelin Pool. Hence the Pilbara chirt comprises the first known tombstones which record the beginnings of life (though there is chemical evidence from Greenland that suggests life was present on Earth 350 million years earlier, but this has yet to be widely accepted). So Hamelin Pool may represent a scene that could have been seen on Earth some 2,000 million years ago. And importantly, thanks to the cyanobacteria, the Earth gained an oxygenated atmosphere around this time. Atmospheric oxygen not only permits breathing in higher animals but also provides a protective barrier - the ozone layer - from the sun’s ultraviolet rays, which can be harmful to animal tissue.
A long period in the history of the Earth followed where, as far as we know, nothing of any great significance happened. But, and just as mysterious, came another huge step, or chapter four in ‘The History of Life’ - the appearance of cells with a nucleus.
Chapters 4 and 5 in ‘The History of Life’ - the nucleus and the grouping of cells
The organisms found in the first three chapters of life’s history book are single-celled and have their DNA distributed irregularly throughout their cells. The new organisms to appear are also single-celled but have a distinct nucleus packed with DNA and separated from the watery fluid of the cell by a membrane. Outside the nucleus there are other units such as mitochondria, that produce energy for the cell by using oxygen in a similar manner to bacteria. The nucleus is the main organising force of the cell. The first cells with a nucleus appeared around 1,200 million years ago and belonged to a group of single-celled organisms called protists. There are around 10,000 species of protists today, including the familiar amoeba. Protists can be seen readily when a drop of pond water is viewed under a microscope. Some possess a thrashing tail or fine rhythmically beating hairs, while others contain packets of chlorophyll that, like cyanobacteria, use the energy of sunlight to produce food for the cell. These packets of chlorophyll and the mitochondria have their own DNA. Some researchers believe that the cells with a nucleus are the combination of a number of cells without a nucleus, each performing a specific function to maintain a life system.
In The Blink Of An Eye Page 3
