Figure 1.3 An amoeba - a cell with a nucleus and organelles.
Protists reproduce by splitting into two, an action known as binary fission. But there is much more to the binary fission of protists than there is to the binary fission of bacteria because, unlike in bacteria, in protists most of the separate internal structures have to split. The DNA of the nucleus divides itself in a particularly intricate manner so that its genes are copied and one complete set is passed to each daughter cell. Although methods vary within the group, the key feature of nucleated cell reproduction is that genes are shuffled around. One of the mechanisms employed involves two cell types: an egg and a sperm. This is the origin of sexuality. Here, genes are distributed to daughter cells from two parents rather than one. The daughter gene sets reflect the new combinations of parent genes, and occasionally these new combinations are so divergent that they produce a slightly different organism with new characteristics. This is another form of evolution. And with the establishment of sexuality, the possibilities for genetic variation increased and evolution accelerated.
There is a limit to the size of a single-celled organism. As the cell becomes larger, the internal chemical processes become less efficient, and eventually reach a point where the organism is no longer viable. The next step in evolution, occurring in chapter five in ‘The History of Life’, was to bypass this limitation by grouping cells together in an organised colony. Volvox is a species that has done exactly this. Volvox is a hollow sphere, about a millimetre in diameter, where the wall is made up of cells, each with a rhythmically beating hair appearing like a tail. The movement of the hairs is coordinated to move the entire sphere in one direction. The next group of cells to evolve had an additional character - a cuticular stalk that is branched to unite small colonies of cells. But the following, very important, step was the division of labour between the component cells of a colony, around 1,000 million years ago. This step was manifest in the beginning of chapter six in ‘The History of Life’ - the appearance of the sponges, the first true multicelled animal phylum.
Chapters 6 to 8 in ‘The History of Life’ - appearance of the true multicelled animals
Sponges have only a few cell types modified to perform specialised functions, and the sort of cell-to-cell junctions that form sheets of tissues in higher forms are absent. In general, sponges have open-topped, sack-like bodies which are fixed to the sea floor. Water is pulled through the body and food is filtered out. They are the only multicelled animals with cells capable of independent survival. If a sponge is passed through a sieve the individual cells separate but continue to survive and even reproduce. Sponges also lack a nervous system and muscle fibre, characters possessed by the next two most derived phyla, Cnidaria (with a silent ‘c’ - this phylum includes jellyfish, corals and sea anemones) and comb jellies. Cnidarians and comb jellies have two thin but clearly modified tissue layers separated by a gelatinous material. One layer is protective and surrounds the body; the other has a digestive function and forms the lining of a gut. Cnidarians and comb jellies have a basic body plan that is also a sack-like form, but at one end there is a mouth which can be opened and closed and tentacles which direct food to the mouth.
Chapter seven in ‘The History of Life’ opens with the evolution of a body plan where three primary tissue layers exist but a blood space between tissue layers is absent. The animals with such an internal organisation are the flatworms. Flatworms have an inner tissue layer that produces muscles and some other organs - obviously a layer with a future - but they are without a blood circulatory system. This means that oxygen has to be transported to the inner tissue layer by diffusion, which works very slowly and its efficiency decreases as one thickness of tissue increases. This means that the animals must be flat, which indeed they are. Like jellyfish, flatworms have guts with only one opening, which is a port for both incomings (food) and outgoings (waste). But the evolutionary position of flatworms is uncertain, and so too is the relationship between chapters seven and eight in ‘The History of Life’. Controversy aside, in chapter eight of ‘The History of Life’ the next evolutionary innovation takes place - a body with again three modified layers of tissue but also an open blood space. This is followed, in the same chapter, by the appearance of a body plan with three modified layers of tissue, a blood space in the form of blood vessels and an internal body space, in which the gut is suspended. But the appearances of a blood space and a body space were no run-of-the-mill evolutionary innovations. They paved the way for the evolution of the further internal variations that discriminate the remaining thirty-four animal phyla, including arthropods (crabs, insects and spiders), molluscs (snails and squid), echinoderms (starfish and sea urchins), chordates (fish and mammals) and many other weird and wonderful phyla that have not made household names. The obvious question to be posed is: ‘When exactly did the step take place from about three to thirty-eight phyla, still within chapter eight in “The History of Life”?’ The similar question posed earlier in this chapter has now become refined and more understandable. But before attempting to answer this new enquiry, we should take a moment to pause and reconsider what life’s history book has taught us so far. It is important to remember that this question does not refer to the Cambrian explosion, but rather to prior events.
We know that phyla are defined by internal body plans, and we have now reached a stage in life’s story where all thirty-eight body plans of multicelled animals are in place on Earth. But we have not yet considered the external appearances of these animals. The most advanced animal whose shape has been considered up till now is the comb jelly, or possibly the flatworm. So what we have in chapter eight are three primitive phyla - sponges, jellyfish and comb jellies - with their own distinctive body plans and body shapes, and a bunch of worm-shaped, or soft-bodied forms, each with one of thirty-five different internal body plans, including that of the flatworms. Is this picture accurate? Were the internal body plans of crabs and starfish really once hidden within the soft body of a worm? This ‘all-worm’ scenario does not seem so far-fetched when we consider that many different phyla still possess a worm-like body today. Remember the ribbon, peanut, arrow and acorn worm phyla? Also we know that the most primitive forms of some phyla, including the chordates to which we belong, had the shape of a worm. But this is far from conclusive evidence. If the ‘all-worm’ scenario is correct, we are faced with a chapter nine in ‘The History of Life’ that deals with the evolution of external body forms, leading on from a chapter eight where only the internal body plans of phyla are in place. What does chapter nine have to say? At what points in geological time does it begin and end? Using their genetic dating techniques, or molecular clocks, the biologists tell us that the internal body plans of all phyla evolved between 1,000 and 660 million years ago, in chapter eight of life’s history book. To learn about external body history and the Cambrian explosion making the intrepid leap to chapter nine, we must turn to the fossil record.
Figure 1.4 Sections through representative bodies of different phyla showing simplified examples of internal body plans.
Chapter 8 in ‘The History of Life’, continued - the Ediacaran enigma
The Flinders and Mount Lofty Ranges are dominant features of the state of South Australia. They extend like a backbone from the coast, near Adelaide, to distant inland regions. These ranges became the subject of some renowned geological study, which resulted in a thorough explanation of the eventful geological history of the area.
Sediments were deposited into an elongated trough in the ranges, and a sequence of rocks 24 kilometres thick gradually accumulated. When the stresses in the Earth’s crust subsequently changed, the entire mass of sediment was folded and pushed up to form a predecessor of the present ranges. Some of the oldest known cells with a nucleus, in addition to stromatolites up to 1,600 million years old, have been found fossilised in this sediment. But more importantly, sometime between 1,400 and 900 million years ago a sandy beach developed on the pre-existing crystalline rocks that had formed a
coastal trough. Fortunately for palaeontologists, the sandy beach environment continued into the Cambrian period, up until 540 million years ago.
In 1947 Australian geologist Reginald Spriggs collected fossils of multicelled animals from the Ediacaran Hills in the Flinders Ranges. These fossils were from the Late Precambrian epoch, about 570 million years ago. But because everyone ‘knew’ there could be no fossils from this geological period, Spriggs’ professor duly placed the rocks next to the dustbin. Spriggs’ enthusiasm got the better of him and he rescued his fossils to give them closer inspection and reprieve from an undignified end. Such an end would have been inappropriate for the spoils of Spriggs’ labour, which has resulted in the universal term ‘Ediacaran fauna’. This is the name given to collections of the earliest known multicelled animals, the first of which was found in Spriggs’ enigmatic rocks. Although the Australian site has yielded the greatest variety of Ediacaran organisms, they have since been discovered in Africa, Russia, England, Sweden and the USA. The oldest Ediacaran fossils derive from the remote Mackenzie Mountains in Canada’s Northwest Territories. These impressions are interpreted as soft, cup-shaped animals that lived on a muddy sea floor around 600 million years ago. They have become accepted as the oldest known multicelled animal fossils in the world.
The first Ediacaran fossils discovered look like flower impressions. These may have been blobs of living matter that were washed up on to a beach, baked in the sun and then covered by a wash of fine sand by the next tide. Walking along the beach on Heron Island, in the Great Barrier Reef, one can find the divided, circular shapes of jellyfish that have become beached and are about to go through a similar process of eternal preservation. The chances are that these too will soon become similar flower-like impressions. Were the Ediacaran organisms jellyfish? Other Ediacaran fossils appear frond- or feather-shaped. Once underwater off Heron Island, similar shapes can be seen waving from the sandy bottom in the form of sea pens that share a phylum with jellyfish. At least sixteen different species of Ediacaran organisms have been identified, but what kind of animals were these ancient creatures? Were there really sea pens and jellyfish among them?
Figure 1.5 The Ediacaran animals Tribrachidium, Mawsonites and Parvancorina.
At first sight, many of the Ediacaran organisms look like species living today. But some of these similarities may be explained by our old stumbling block - convergence. Take Dickinsonia, for instance. The Precambrian species Dickinsonia appeared elliptical in its shape from above. It grew to about a metre in length yet was less than 3 millimetres thick. Several hundred specimens of Dickinsonia have been collected from the Ediacaran Hills - it must have been quite common. It is also known from northern Russia, and presumably inhabited a large proportion of the Late Precambrian globe. Dickinsonia shows dividing lines radiating from a central region to the edges of the animal. If these lines are interpreted as segment-dividers, a body made up of separate, connected segments is conceived. Then Dickinsonia could be assigned to the phylum of segmented worms, to which living bristle worms, earthworms and leeches belong. But there is another possibility even if we continue down the segmentation path. If the ‘segmentation’ of Dickinsonia evolved as a means of increasing its body size it could belong to another phylum, because the equivalent character in segmented worms evolved in response to soft layers of sand, to facilitate burrowing. So for what purpose did the ‘segments’ of Dickinsonia evolve? There is more evidence that the fossil record can provide towards this question, but it doesn’t involve fossils of the animals themselves.
Despite the number of fossilised specimens known of Dickinsonia, no fossilised burrows have been found that could have accommodated this species. Yet burrows are known to preserve as fossils - such marks left by ancient animals are known as trace fossils. But surface trails rather than burrows are the characteristic trace fossils of the Ediacaran epoch. This suggests that some Ediacaran animals crawled around on the sea floor and none burrowed into it. Now we can cross segmented worms off the Precambrian list, at least in their segmented forms, with their burrowing lifestyles. So Dickinsonia was not a segmented worm.
Figure 1.6 The Ediacaran animal Dickinsonia costata.
Recently, researchers have been able to unravel the ancient ancestors of Precambrian animals by comparing the squiggles and swirls that mark their trails with the wriggling trails left by worms and other soft-bodied animals today. Thus, problems that had been unsolved following years of anatomical study are now being resolved.
Signs of internal features of the Ediacaran animals are not evident, but theories relating them to jellyfish and sea pens (cnidarians) have become accepted as serious propositions. Indeed, we have found probable embryos of jellyfish (and sponges) in Chinese rocks 570 million years old. This would place the phylum Cnidaria in the Precambrian with a diversity of external forms or body shapes. Also, it is now believed that many of the Ediacaran fossils represent animals from more derived phyla, albeit before they possessed their characteristic shapes of today. This will be addressed later.
The gap in geological time that once separated the Ediacaran fossils from the next suite of fossils to be found, and once provided evidence that the Ediacaran organisms were a ‘failed’ first attempt at evolving animals and consequently died out, has been filled. Now it is known that the Ediacaran organisms lived right up to the next major event in animal evolution. Not only that, but the last six million years of their existence appears to have been the period of greatest Ediacaran diversity. No one knows why they died out, although this probably has a good deal to do with the sudden, ‘blitzkrieg’ appearance of the next dominant forces on Earth. It is time now to consider the Cambrian explosion.
Chapter 9 in ‘The History of Life’ - the Cambrian explosion
The Cambrian explosion, which post-dated the Ediacaran fauna, is a milestone in evolution that can be matched in significance only by the beginning of life itself. It paved the way for the emergence of the vast diversity of life found today, whether in Australia’s Great Barrier Reef or Brazil’s tropical rainforests. It involved a burst of creativity, like nothing before or since, in which the blueprints for the external parts of today’s animals were mapped out. Animals with teeth and tentacles and claws and jaws suddenly appeared. Explanations for this grand event do not have such a deep history as theories on the origin of life, only because the Cambrian explosion is a recent realisation. Darwin, among others, became puzzled by the sudden appearance of hard-shelled fossils at the beginning of the Cambrian period, about 543 million years ago, and by their apparent lack of evolutionary ancestors. Darwin and his contemporaries hypothesised that early forms of each animal phylum did not fossilise or became entombed in old rocks that were unsuitable to provide preservation as fossils. But as we have seen, Darwin had only micro-evolution to work with. Now we have so many examples of well-preserved sedimentary rocks (suitable for fossil preservation) from before the Cambrian that it is no longer reasonable to claim that conditions only became suitable for preservation during the Cambrian. Today’s view of the fossil record invokes a Cambrian ‘Big Bang’ in the evolution of external body parts from soft, worm-like forms.
The Cambrian is a relatively brief period in the history of the Earth yet outstanding in the history of life. Spanning just forty-three million years it was a period of monumental change. The earliest hard-shelled fossils that Darwin pondered over were later revealed to have appeared even more suddenly. They were narrowed down to the Cambrian period on the discovery of the fossils of the Burgess Shale. These abundant fossils, of the fauna and flora communities that existed 515 million years ago, have been the subject of many spirited scientific discussions and are deserving of their place in the history of science. They have been regarded either as the predecessors of today’s fauna and flora or as enigmatic species that belong to phyla that did not survive the Cambrian. We now prefer the interpretation that the Burgess Shale organisms can be accommodated within today’s thirty-eight phyla (actually a few of th
ese are extinct today).
The Burgess quarries today
Although barely cutting through the distant haze, transcendental mountains are prominent over the otherwise featureless landscape viewed from Calgary in Alberta, south-western Canada. One is instantly drawn to those geological wonders, the Rocky Mountains, and the attraction becomes stronger as they are approached via the Trans-Canadian Highway. Banff National Park is the first port of call on entry to the Rocky Mountains. Everywhere there are mountains that could be termed spectacular even when compared to any other mountain range. It is the steepness of the mountainsides, their jagged peaks, endless variety of shapes and their ‘contour lines’ running in conflicting directions that makes this place so unique. The ‘contour lines’ swirl around the landscape, continuously pulling the eye in different directions and towards different focal points. These lines are actually the boundaries of sediment layers, laid down millions of years ago by sediment in the sea settling out on to the bottom, forming a new sea floor. So although a thousand metres or two above ground today, the rock that constitutes these mountains began its history underwater. As the Earth’s plates moved around throughout geological time, and slowly crashed into each other, something had to give. The rocks that now form the Rocky Mountains were one of those things. They were forced up from below the water and into the air, and their chaotic movements produced the uneven patterns of ‘contour lines’ seen on the mountains today.
Continuing west along the Trans-Canadian Highway, and staying within the Rockies, one enters British Columbia and Yoho National Park. The small mining town of Field lies at the foot of Mount Stephen, famous for its Cambrian trilobite fossils. The rusting iron shacks of Field are gradually being replaced by wooden bungalows and small motels that blend into the coniferous surroundings. This is now a useful base for serious mountain walkers. But there is something unique about Field. Some of the best preserved, complete Burgess Shale fossils are on display at the information centre here. These fossils separate Field from the rest of the Rockies and inspire thoughts that there is something very special about this place. The fossil display is the work of Des Collins, a palaeontologist at the Royal Ontario Museum in Toronto, eastern Canada. Each year Des Collins and his team of helpers and students rent one of the utilitarian wooden bungalows in Field. On my visit to this bungalow in July 2000, I was directed to Des Collins’s new and more temporary base camp, at the site of the Burgess Shale quarry.
In The Blink Of An Eye Page 4
