In The Blink Of An Eye (5 page)

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.

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.

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.

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.

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?