THE INFINITE VARIETY: THE BEGINNING OF LIFE
The world is rich in animals and plants, some of which still remain to be discovered. A small area of the Tropical Forests of South America will still yield insects that have never been described, the difficulty is finding a specialist whose is able to classify them. The understanding of such diversity would have been almost impossible, if it had not been for Charles Darwin and his trip around the world. For example Darwin described the adaptations of the Giant Tortoises (Testudo elephantopus) that occur on the Galapagos Islands in the South Pacific. Tortoises occurring on the well-watered islands, with short, cropped vegetation had gently curved front edges to their shell, whereas those individuals occurring on more arid islands had to stretch their necks to reach branches of cactus and other vegetation. Consequently, these later individuals had longer necks and a high peak to the front edges of their shells, which enabled them to stretch their heads almost vertically. Observations such as these were the foundations for the theory of evolution, which suggests that species were not fixed for ever but changed with time and thereby contribute to the immense diversity of life.
Darwin's argument for the evolution of different necks in these tortoises was as follows:- all individuals of the same species are not identical. In a single clutch of eggs there will be some hatchlings, which, because of their genetic constitution, will develop longer necks than others. In times of drought such individuals will be able to reach leaves higher off the ground than their siblings and therefore will survive. The brothers and sisters in the clutch who possessed shorter necks would be unable to stretch and reach food and therefore would starve to death. Since this time natural selection has been debated and tested, refined, quantified and elaborated. Later discoveries about genetics, molecular biology, population dynamics and behaviour have developed the theory of natural selection still further. It remains the key to our understanding of the natural world and it enables us to recognize that life has a long and continuous history during which organisms, both plants and animals, have changed, generation by generation, as they colonized all parts of the world.
Occasionally some animals after dying may be covered in mud, where their bones can be preserved. Dead plant material may also accumulate and is turned to peat, in time peat is compressed and turned to coal. Great pressure from overlying sediments and mineral-rich solutions that circulate through them cause chemical changes in the calcium phosphate of the bones. Eventually they are turned to stone giving an accurate representation of the original bones. This process is called fossilization. The most suitable sites for fossilization are in seas and lakes were sedimentary deposits like sandstone and limestone are slowly accumulated. Fossils are exposed when such deposits erode away. Fossils can often be dated with the discovery of radioactivity in the surrounding rocks. Some chemicals in rocks decay with time producing radioactivity, for example potassium turns to argon, uranium to lead and rubidium to strontium. The amount of change from one chemical to the other depends on the amount of elapsed time. Consequently the proportion of the second element to the first can be used to calculate the time when the rocks were first laid down around the fossil.
When rocks occur as undisturbed layers, we find that the lowest layers will be the oldest and topmost layers will be the youngest. Frequently rivers cut incisions into the earths's surface and expose such layers. The grand canyon in the U.S.A. is the deepest cleft on the earth's surface. The upper rocks of this canyon are about 200 million years old and contain traces of reptiles, impressions of fern leaves and wings of insects. Halfway down the canyon you find limestone of about 400 million years old which contains the remains of primitive armoured fish. Further down the canyon there are no traces of vertebrates. Three-quarters way down there are no apparent traces of life. Close to the bottom of the canyon the rocks are more than 2000 million years old.
Rocks as old as those of the bottom of the Grand Canyon have been found to contain a fine-grain flint-like substance called chert. Contained in this chert are simple organisms some of which resemble filaments of algae and others resemble bacteria. These are the earliest known organism and are referred to as cyanophytes or blue-greens. These organisms are able to extract hydrogen from water and thereby produce oxygen which is essential for other organisms to survive. The chemical agent responsible for this process is called chlorophyll and process is called photosynthesis, and occurs in true algae and higher plants.
Even before these blue-greens existed organic molecules must have evolved. The original atmosphere of the earth was very thin and contained hydrogen, carbon-monoxide, ammonia and methane, but no oxygen. This chemical mixture, together with ultra-violet radiation and frequent electrical discharges causing lightening was simulated in an experiment in the 1950s. After a week's treatment complex molecules were found to have formed in the mixture, including sugars, nucleic acids and amino acids, which are the building blocks for proteins. It is likely that molecules such as these were formed in the seas of the earth at the very beginning of its history. With time these substances probably increased and interacted with each other to form more complex molecules. Eventually one substance essential to life as we know it appeared. This substance was called deoxyribonucleic acid or DNA. This molecule can act as a blueprint for the manufacture of amino acids and has the capacity to replicate itself. Such properties occur in all life as we know it including the simplest forms such as bacteria. DNA's ability to replicate itself is due to its double helix structure. During cell division, the DNA molecule splits longitudinally, and each side acts as template to which simpler molecules become attached until each half has once more become a double helix. The simpler molecules from which DNA is mainly built are of four kinds, but they are grouped in trios and arranged in particular and significant orders. These sequences of amino acids on the immensely long DNA molecule specifies how various amino acids are arranged in a protein, and how much protein is to be synthesized. A length of DNA bearing the information for an unbroken sequence of manufacture is called a gene. Occasionally, the DNA copying process goes wrong. A mistake may be made at a single point on the length of the DNA and a particular molecule may become temporarily dislocated and be re-inserted in the wrong place. The copy is then imperfect and the protein that it synthesizes will be different. Such mistakes are sources of variation from which natural selection can produce evolutionary change. We know from micro-fossils that distinct forms of bacteria-like blue-greens had evolved as long ago as 3000 million years.
The arrival of blue-greens dictated the rest of the development of life. The oxygen they produced accumulated and created the atmosphere as we know it today. Atmospheric oxygen and ozone forms the screen which filters ultra-violet rays which provided the original energy to synthesize the first amino-acids and sugars. From primitive blue-greens the first single-celled organisms evolved. Such organisms are called protista. Each celled organism is more complex than any bacteria and includes a DNA filled nucleus and elongated bodies called mitochondria which provide energy from burning oxygen. Some of these unicellular organisms have tail or flagellum which resemble the filamentous bacterium called a spirochaetae. These unicellular organisms may also contain chloroplasts (packets of chlorophyll which like blue-greens use energy from sunlight to assemble complex molecules as food for the cell). Consequently each of these tiny organisms appear to be a committee of simpler organisms. It is even possible that the first cells engulfed and incorporated bacteria and blue-greens to form a communal life. Cells of this complexity first appeared about 1200 million years ago.
These protistans like bacteria can reproduce by binary fission, but since their internal organization is more complex, the division process is more complex and includes the division of the separate structures within the cell. The division of mitochondria and chloroplast (each with their own DNA) may be independent of division of the main cell. There are, however, other means of reproduction which involves the exchange of genetic material when two individual cells conjugate. Some protistans contain two complete sets of genes which after exchange of genetic material divide to make new cells with only one set of genes. These cells are of two types, a large and comparatively immobile one and a smaller active one that possesses a flagellum and are referred to as egg and sperm cells. When the two types unite in a new amalgamated cell the genes are once again in two sets but with new combinations of genes that occur from two parent sources. This sexual reproduction increases the possibilities for genetic variation and an accelerated rates of evolution.
There are thousands of species of protistans, some possessing cilia or flagellum, whereas others use pseudopodium for locomotion. Some protistans secrete shells of silica or lime, whereas others have combined individual cells to produce a colony (eg Volvox). The constituent cells of Volvox, however, are co-ordinated, for all the flagellum around the sphere beat in an organized way and give direction to locomotion.
Increased co-ordination between colonial cells appeared with the evolution of the sponges (Porifera). Sponges may be formless lumps on the sea floor reaching two metres in size. Their surfaces are covered with tiny pores through which water is drawn into the body by flagella and then expelled through larger vents. The sponges feed by filtering particles from this stream of water passing through its body. Some sponges produce a soft flexible silica-based substance which supports the whole organism, whereas other sponges secrete lime or silica to create a hard "skeleton" for support. Despite the elaborate skeletons that some sponges are able to produce they cannot be considered as an integrated multicellular animals since they have no nervous systems nor muscle fibres.
The simplest organisms to possess such structures are the coelenterates which are represented by the jellyfish, sea anemones and their relatives. These animals have two cellular layers, each layer one-cell thick. The outer layer of cells is the ectoderm whereas the inner layer is the endoderm. The individual cells of the ectoderm are specialized for various functions such as protection, secretion, defence and cell replacement whereas the endoderm is specialized for digestion, absorption and assimilation of food. The stinging cells (nematocysts) of the ectoderm are highly specialized and contain coiled threads inside. When food or an enemy comes near, the cell discharges the thread which is armed with spines like a miniature harpoon and often loaded with poison. These cells are often concentrated at the ends of tentacles. Coelenterates reproduce by releasing eggs and sperm into the sea. The fertilized egg first develops into a free swimming creature that is quite different from its parents. It eventually settles down at the bottom of the sea and develops into a tiny flower-like organism called a polyp which filter-feed with the aid of tiny-beating cilia. Eventually, the polyps bud in a different way and produce miniature medusae which detach themselves and once again become free-swimming. True jellyfish spend most of their time as free-floating medusae with only the minimum period fixed to the rocks as solitary polyps, whereas sea anemones do the reverse with most of their life spent attached to rock as solitary polyps. Yet other coelenterates exist as colonies of polyps which have given-up a sessile life and have become free-floating eg Portuguese Man O'War (Physalia).
Although Coelenterates are relatively simple organisms and appeared fairly early in the history of life, fossil evidence for them was only recently found (1940's) in the Flinders Range, southern Australia in rock strata that has been dated at about 650 million years. Not all coelenterates are soft-bodied, and some produce skeletons of limestone in a similar way to the sea sponges and are better known as corals. These animals secrete their skeletons from their base. Each polyp is connected with its neighbours by strands that extend laterally. As the colony develops new polyps form, leaving a limestone skeleton that is riddled with tiny cells were polyps once existed. Live polyps are restricted to a thin surface layer. The size of these colonial polyps are enormous and create entire coral islands such as the great barrier reef running parallel to the east coast of Australia. This coral reef extends for over a sixteen hundred kilometres and is the greatest animal construction prior to man's artifacts.
IN YOUR OWN WORDS WRITE A ONE TO TWO PAGE ESSAY ON THE FOLLOWING TOPICS
Discuss the structure and significance of DNA to life as we know it.
Describe the process of fossilization and its significance in the interpretation of evolutionary events.
Describe how cells have become specialized to perform different functions in a multicellular organism.
BUILDING BODIES: INVERTEBRATES OF THE OCEANS
Living in association with the Great Barrier Reef is a multitude of higher animals which include shelled animals of the phylum Mollusca (clams, cowries, mussels and sea-snails), radially symmetrical creatures of the phylum Echinodermata and includes sea urchins and starfish, elongated animals with segmented bodies occurring in the phylums Annelida and Arthropoda which includes bristle worms, shrimps and crabs as well as the vertebrates (phylum Chordata) which includes cartilaginous and bony fishes and marine mammals such as dolphins and seals.
To trace the invertebrate lines we must also look for fossils where animals were deposited continuously and the fossil remains to have survived in a relatively undistorted condition such as has occurred in the Atlas Mountains of Morocco. From this fossil record and from other sites scattered around the world there appears a clear dichotomy in the history of earth where fossils are found and they are not found. This period of transition corresponds with about 600 million years and records the first annuals which are characterized by the presence of shells. It is conceivable that before this period the animals were soft bodied and did not fossilize. It has also been suggested that seas were not at the right temperature and or chemical composition to favour deposition of lime from which most marine shells and skeletons are constructed.
Simpler animals than those first found in the fossil records still inhabit the earth and its oceans and their ancestors may have represented the predecessors for the shelled invertebrates that are found in the fossil records. These soft-bodied animals belong to the phylum Platyhelminthes. The most basic of these animals is the flatworm, a flat-leaf shaped worm which like jellyfish have a single opening to their gut through which food is ingested and waste is ejected. Their bodies have differentiated into three layers, the ectoderm, mesoderm and endoderm. Cells with a different structure and function have aggregated to form a primitive system (eg nervous system which consists of a network of nerve fibres). Nevertheless, they have no breathing system with oxygen diffusing directly through the skin. Their undersides are covered with cilia which, by beating, permits them to glide over surfaces. Their front end has a mouth on the under-surface and a few light sensitive spots above. There are some 3000 species varying in size from microscopic to 600 mm, and although most are marine some species have managed to inhabit humid terrestrial environments and move on a bed of mucus. Many species in this phylum have become parasitic and live on the surface and inside bodies of other animals including man. Some of these parasitic forms such as liver flukes still resemble a basic flatworm form whereas others such as the tape worm have a highly modified morphology with hooks on their heads and an ability to detach egg-bearing sections of their posterior body parts.
It is hypothesized that the period between 600 and 1000 million years considerable erosion of the continents was producing great expanses of mud and sand adjacent to the continental shelf. This environment may have contained abundant quantities of organic material. However, in order to give protection and concealment in this environment burrowing would be a pre-requisite, and more tubular body plan would become necessary. It is possible that under such conditions the segmented worms evolved. Some of these animals became active burrowers who tunnelled through mud in search of food, whereas others lay half-buried, with their mouthparts filtering food above the sediment. Some of these animals lived in secreted protective tubes, whereas others evolved two flat, protective shells which represented the first Brachiopods descendants which exist belong to the genus lingula. Brachiopods had great variations in their design, including heavy lime shells, and large tentacles contained inside, whereas others developed a hole at the hinge end of one of the valves through which a stalk emerged and fastened the animal onto the ground.
Other kinds of annelids also developed in which the animal did not attach itself to the sea floor but continued to crawl and secreted a small conical tent under which it could escape from predators and probably represented the prototype for the Mollusc group, with a primitive representative being Neopilina. Today there are at least 60 000 different species of mollusc. Anatomically these animals usually possess a foot which may be used for locomotion, a shell, a mantle composed of thin sheets of body tissue that covers the internal organs, and an internal cavity that coats the central part of the body, in which most species have gills which extract oxygen from water.
The shell is secreted by the upper surface of the mantle, with limpets producing shell at equal rates along the edge of the mantle, in other animals the front end of the mantle secretes at a faster level than the rear end and produces a flat spiral. The maximum secretion may be to one side and develops twisted or turreted-shaped shells, or in the case of cowries the secretion is concentrated along the sides of the mantle creating a shell resembles a clenched fist. Molluscs may have either single shells (limpets), two shells or bivalves (mussels) or a number of shell plates (chitons). In some molluscs the shell has become reduced and totally internal (cuttlefish) whereas in others it is total absent (octopuses).
Molluscs have a variety of different feeding mechanisms. The bivalve molluscs can filter-feed fine particles form the water. Some of the single-shelled molluscs (limpets) possess a ribbon-shaped tongue or radula, covered with rasping teeth, which enables the animal to scrape algae from the rock. Whelks have a radula on a stalk that can extend beyond the shell and be used to bore into the shells of other molluscs. Through these holes that they have bored they poke the tip of the radula and suck out the flesh of the victim. The cone-shells also have a stalked radula which is modified into type of harpoon with which they secure their prey before injecting it with poison. In still more active carnivores the heavy shell is reduced in size and may even be lost as has occurred in the sea-slugs which have an upper surface covered with tentacles. One species of sea-slug actively hunts jelly fish and ingests these animals stinging cells which it then concentrates in the tentacles and uses them for protection.
An early group of molluscs retained the protection of a shell yet were still able to maintain a high degree of mobility. This was achieved through the development of a gas-filled floatation tanks. The prototype forms had a flat-coiled shell with an end walled-off to form a gas chamber. As the animal grew it added buoyancy with the development of new chambers. Such animals survive today and are known as nautiluses. A tube runs from the body chamber of the nautilus to the floatation tanks in the shell. The nautilus is an active carnivore eating animals such as crabs and moves in a form of jet-propulsion where water is squirted through a siphon. In this animal the original muscular foot is divided into long grasping tentacles with which it secures its prey. The mouthparts are modified to form a horny beak with which the nautilus is able to crack shells of other animals. Variations on the float chamber theme gave rise to the enormously successful group of animals called the ammonites whose circular shells were up to 2 meter in size.
One of these group of molluscs took the same path as the sea slugs and disposed of its shell entirely (octopuses and squids) whereas relict of the ancestral shell persist as the cuttlebone found in the cuttlefishe. One species of octopus (Argonauta) secretes a paper-thin replica of the nautilus shell, the chambers of which are used to lays its eggs.
Both squids and octopuses have reduced the number of tentacles (10 and 8 respectively), but squids have become more mobile with the development of undulating lateral fins. The brains and eyes of these animals is the most advanced of any invertebrate, eyes greater than 400 mm in size have been recorded for squid. Squids, in particular can reach immense sizes with one individual 21 m long (found in New Zealand in 1933).
Another group of animals that had diverged from early stage and also reached immense sizes are the crinoids or sea lilies which belong to the phylum Echinodermata. These animals have an architecture plan that is based on a five-fold symmetry and possess large lime plates that occur just below the skin. Fossil crinoids were up to 20 m long, although their present day counterparts are considerably reduced in both size and species diversity. The bodies of all members work on a unique hydrostatic principle. The hydrostatic skeleton is closed fluid-filled system that terminates as a series of blind tubes called tube-feet. Each tube feet ends in a sucker. Changing the local pressure within the tube feet allows to be extended and contracted. Extensions and contractions of these tube feet occur as waves down the length of the arms (or ray) and this allows the animal to move itself and to move particulate matter down the arm. The water from this system circulates separately from that in the body cavity. It is drawn through a pore into a canal surrounding the mouth and circulated throughout the body into the myriads of tube feet. When suspended particles of food touches an arm, the tube feet fasten on to it and pass it from one to another until it reaches the groove that runs down the upper surface of the arm to the central mouth. Although stalked, sessile sea-lilies were the most abundant crinoids in the fossil records, the most common form today is the stalkless feather stars.
Five-fold symmetry and hydrostatically operated tube feet also occur in the starfish and the brittle stars, however their body plan has become inverted and the mouth is on the undersides. Yet in another group of echinoderms the five-fold symmetry is less conspicuous and the body plan is elongated with a mouth and anus at the two ends. At the mouth the tube feet have become modified into tentacles which filter fine food particles. The five-fold symmetry and hydrostatic mechanisms did not develop further and the group is generally considered to be an evolutionary cul-de-sac.
The third major line in the evolution of invertebrates was the development of the segmented bodies (Arthropoda) which evolved at a very early stage and are contemporary with the jellyfish fossil patterns found in Flinders, Australia. This group of animals shares one important feature with the molluscs, and that is a spherical larvae possessing a belt of cilia, whereas the echinoderm larvae have a twisted morphology with winding bands of cilia. This suggests that molluscs and arthropods evolved from flatworms (Platyhelminthes), with the echinoderms having an independent evolutionary line.
Segmentation may have increased the efficiency for burrowing in mud. A line of separate limbs that are repeated down the length of the body seems to have been the most primitive form. Each segment is equipped with its own set of organs - on either side, leg-like projections sometimes accompanied by bristles and feathery appendages through which oxygen could be absorbed, and within the body wall, a pair of tubes opening to the exterior from which waste is secreted. A gut, a large blood vessel and a nerve cord run through all segments from the anterior to the posterior end of the organism and co-ordinates the segmentation. a great variety of these segmented animals have been almost perfectly fossilized in the Burgees shale of the Rocky Mountains in British Columbia, Canada.
An early segmented animal was the trilobite. These animals had a bony armour composed of lime and a horny substance called chitin. The armour was not expandable and therefore shed periodically. Many of these shed exoskeletons have been preserved as fossils. Where the entire animal is preserved you can observe the jointed legs that are attached to each segment of the body, the feathery gill next to each leg, two feelers at the front of the head, the gut running the length of the body, and even muscle fibres along the back which enabled the animal to roll itself into a ball. Comparatively high resolution eyes composed of mosaics of separate cells and a crystalline calcite lens. The very thick lens of some trilobites may have reflected their colonization of deeper water where light is considerably reduced. However, the optimal properties of the calcite lens operating in water would not have permitted a fine focus. This shortcoming was compensated by the evolution of the two-part lens with a waved surface at the junction of the two lens elements. Although they radiated throughout the oceans, only one descendent of this group survives today, the horse-shoe crab (Limulus). This animal is larger than its ancestral trilobites, and segmentation of its armour have fused to form a large domed shield. These animals generally live at great depths but each spring they migrate towards the coast and during full moon and high tides they drag themselves onto the beach where they copulate. Today the similarities between the horse-shoe crabs and the trilobites are only evident in the larval stage where segmentation of the armour plates are clearly discernable in the horse-shoe crab larvae.
Another group of armoured animals also evolved from the original segmented worms the crustaceans which exist today in the form of some 35 000 species. They may prowl around rocks and reefs as crabs, shrimps, prawns, lobsters and crayfish, they may become sessile such as barnacles, or congregate and swim in vast shoals such as krill. The size of the crustacean and the form of the exoskeleton varies considerably from the paper-thin exoskeleton of the almost microscopic water flea (Daphnia) to the carapace of giant Japanese spider crab (Macrocheira kaempferi) which measures 3 m from claw to claw. In the crustaceans the paired legs have become modified for a variety of purposes. At the anterior end they have become modified into pincers or claws, those in the middle are paddles, or walking legs or tweezers. Some have feather branches acting as gills through which oxygen can be absorbed. All limbs are jointed, tubular and operate by way of muscles. Like the primitive trilobites for crustaceans to grow they need to dispose of their calcareous carapace. As time approaches for moulting the animal absorbs as much calcium carbonate from the carapace into the blood stream, and begins to secrete a new soft wrinkled skin under the carapace. The outgrown armour splits and the crustacean swells its body by absorbing water, and wrinkled new skin stretches and hardens into a new carapace.
This exoskeleton may work to advantage for animals to colonize land if a mechanism of breathing in air as opposed to water can be secured. By developing almost closed air chambers lined with folds of moist skin crustaceans are able to absorb oxygen from air. In this way sand shrimps, beach hoppers and wood lice have been able to colonize land that retains a moist environment. The most spectacular of land dwelling crustacean is the big robber crab Birgus which exploits coconuts.
Other descendent of the invertebrates have left the sea for a terrestrial life style the first of which were probably derived from segmented marine worms, but more recently included the familiar snails and slugs. These changes started about 400 million years ago and gave rise to the most numerous and diverse of land animals; the insects.