Our planet is about 4.5 billion years old. Situated 150 million kilometres away from the Sun, it has a circumference of about 30,800 kilometres. It is almost spherical, but bulges a little at the equator and it is flattened slightly at the North and South poles.
The Earth has an inner core, which makes up about 16 per cent of its total volume, and an outer mantle. The core consists mainly of molten iron at a temperature of around 2,500°C, although there appears to be a solid part right at the centre. The mantle is about 2,900 kilometres thick and consists of relatively solid rock (Figure 2.1).
Outside the mantle there is a relatively thin layer of less dense rock called the crust. The average thickness of the crust is 35 kilometres on land and 5–6 kilometres under the oceans. The highest point on the Earth’s crust, Mount Everest, is nearly 19 kilometres above the lowest point, which is in the ocean just off the coast of the Philippines.
Ninety-eight per cent of the solid matter of the Earth’s crust is made up of eight elements. These, in order of abundance, are oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium. Oxygen accounts for 94 per cent of the crust by volume and 47 per cent by weight, and silicon accounts for 1 per cent of the crust by volume and 28 per cent by weight.
Figure 2.1 Planet Earth
Source: Stephen Boyden
There are three broad classes of rock in the Earth’s crust. First, there is the original igneous rock, which was formed when the hot molten liquid of the Earth’s core cooled and solidified. Basalt, obsidian and granite are examples of igneous rock. Second, there is sedimentary rock, which was formed by pressure or a chemical cementing action on rock fragments. Examples of sedimentary rock include sandstone and shale.
The third main category is metamorphic rock, in which the original structure has been altered by the action of heat, pressure or chemicals. Examples of metamorphic rock include schist, slate and marble.
As a result of physical and chemical weathering, some of the rock of the crust is broken up, forming a layer of particles of disintegrated rock of different sizes, like gravel, sand and clay.
The Earth’s crust is coated with an envelope of gases — the atmosphere. Apart from water vapour, the main permanent gases in the atmosphere are nitrogen (78 per cent), oxygen (21 per cent), argon (0.93 per cent) and carbon dioxide (0.04 per cent and increasing).
Another key component of the planet’s surface is water, around 97 per cent of which is in the oceans. About 2 per cent of the Earth’s surface water is in the form of ice or snow in the polar regions, and about 0.5 to 1.5 per cent in the soil and in cracks between rocks. Less than 0.03 per cent is in ponds, streams, rivers and lakes, while only about 0.001 per cent is in the atmosphere.
The surface of the Earth receives constant radiation of energy from the Sun, where it is generated by nuclear fusion. This input of energy is in the form of the shorter wavelength ultraviolet rays, through visible light, to infrared. It is eventually re-radiated back into space, mainly in the form of heat.
The energy from the Sun is largely responsible for the two great circulatory systems on the Earth’s surface — those of the atmosphere and the oceans. The flows in the atmosphere are caused by the unequal heating of large masses of air, and this leads to air movements that then set in motion the flows of water in the surface layers of the oceans. The patterns of flow in both the atmosphere and the oceans are also affected by the rotational movement of the planet. This is known as the Coriolis effect. The end result is that heat becomes more evenly spread over the surface of the planet.
Another important process that is driven partly by the energy from the Sun, but also by gravity, is the water cycle. Heat from the Sun causes water to evaporate from the surfaces of the oceans, lakes and land to form water vapour in the atmosphere. When this vapour cools, the water condenses and gravity eventually causes the droplets to fall back to Earth as rain or snow. Gravity also plays an essential role by causing much of the water that falls on land to sink down into the soil and then move into streams and rivers, from where it eventually flows back into the oceans.
Certain of the gases in the atmosphere, notably water vapour and, to a lesser extent, carbon dioxide, play a key role in keeping the temperature of the Earth’s surface at levels suitable for life as we know it. The end result of this process, which is referred to as the greenhouse effect, is a world with an average temperature of around around 15°C. If these gases were not there, the energy radiated onto the Earth’s surface from the Sun would re-radiate back into space, mainly in the form of heat, and the average temperature of the Earth would be –18°C.
The evolution of life
The first 4 billion years
The earliest living things on Earth are believed to have come into being around 4 billion years ago. They were single-celled organisms and they were the most complex form of life on Earth for approximately a 1 billion years. There were, and still are, two distinct groups of such microorganisms with different biochemical characteristics. They are classified as Bacteria and Archaea. The Archaea include microbes that live and multiply under extreme conditions, such as very high temperatures and very high salinity. It is not known which of these groups came into existence first.
It is believed that the main source of energy for the first single-celled organisms was energy-containing chemical compounds that had formed through the action of ultraviolet (UV) radiation and electrical discharges in storms. But the amount of energy from such sources was strictly limited, and there was certainly not enough of it to sustain life on the scale that exists today.
Single-celled organisms capable of photosynthesis — cyanobacteria — were in existence by around 2.5 billion years ago. This development represents one of the great watersheds in biological evolution. It changed the living world forever. In photosynthesis, light energy from the Sun is captured in the leaves of green plants and converted into chemical energy in the form of complex organic molecules. All animal and plant life on Earth is entirely dependent on this process. Photosynthesis involves the uptake of carbon dioxide and water from the environment and the release of free oxygen.
The emergence of photosynthesis had far-reaching evolutionary consequences. Among these was the fact that oxygen began to accumulate in the atmosphere, making it possible for life forms to evolve that relied on oxygen for their respiratory processes. Another outcome was the fact that some of the atmospheric oxygen was converted to ozone (O3), which formed a layer in the upper part of the atmosphere. Here, it acted as a filter, absorbing much of the UV radiation from the Sun. As a result, by the time that humans appeared on Earth, and probably by 2 billion years before this, only about half of the total solar UV radiation, and a much smaller fraction of the short-wave UV-B rays, penetrated through to the Earth’s surface. Had it not been for this effect, life as it exists on land today would not have been possible.
Although excessive UV radiation is damaging to living organisms, the UV rays that continue to penetrate beyond the ozone layer play a number of useful biological roles, including the promotion of the synthesis of vitamin D in human skin.
Like bacteria today, the earliest single-celled organisms did not possess nuclei. The first nucleated cells appeared about 1.5 billion years ago, and it seems that, around this time, a great evolutionary diversification began to take place among living forms, which suggests that a form of sexual reproduction was by then in existence. Previously, all reproduction had been asexual, involving the simple division of one cell into two. In sexual reproduction a new individual comes into existence through the union of two cells, the male and female gametes, each bringing its complement of genetic material (deoxyribonucleic acid (DNA)) from one of the parent organisms.
Around 600 to 700 million years ago, another watershed occurred in the history of life on our planet in the appearance of multicellular organisms. There is uncertainty about the timing of this evolutionary development but, by about 700 million years ago, there were flat and soft-bodied multicellular creatures in existence. They are called Ediacarans, after the Ediacara Hills in South Australia, where the first big deposits of their fossils were found.
By 500 to 600 million years ago, the Ediacarans had been replaced by very different fauna and flora, which included seaweeds, sponges, jellyfish, corals, worms, molluscs, sea urchins, starfish, lamp shells and trilobites. The various forms of life of that time, like the organisms of today, can be classified as belonging to three ‘domains’ — namely, the Archaea, the Bacteria and the Eukarya. The cells of Eukarya contain nuclei, and this domain includes Protista (e.g. amoebae), Fungi, Plants and Animals.
The next 500 million years
Five hundred million years ago, there were animals swimming in the oceans that had an internal supporting structure or backbone. The earliest of these were the so-called jawless fishes, a group represented today by the lampreys. By 400 million years ago, the so-called ‘true fishes’ were just emerging, although the oceans were dominated by arthropods, especially trilobites.
There was much less diversity among plants. At that time all plants were thallophytes, which exhibited no real differentiation into stems, leaves and roots. This group included various kinds of multicellular algae, like stoneworts and brown seaweeds.
The main plants of the oceans have changed little since that time. In contrast, spectacular evolutionary changes took place among animals in the aquatic environment. By 200 million years ago the trilobites, which had dominated the scene for so long, had entirely disappeared and were replaced by a new group of molluscs known as ammonites. At one time there were over 20 different families of ammonites, and some of them had a diameter of at least a metre. But the ammonites were also extinct by 60 million years ago.
Meanwhile, there was remarkable diversification taking place among the bony fishes, leading eventually to the immense variety of fish species that are found in ponds, streams, rivers, lakes and oceans today.
The earliest plants to grow on land appeared on the edge of the shallow water of estuaries a little over 400 million years ago. Unlike the thallophytes in the oceans, the earliest land plants had a distinct stem that provided them with support in the new environment, and some of them had rudimentary leaves. Fossilised remains have been found of two distinct groups related to the modern psilotums and club mosses. Eventually larger plants evolved. By 350 million years ago, there were great forests of seed ferns and horsetails. Because their reproduction depended on the sperm being able to swim in a film of moisture to reach the ovum, they could exist only in moist areas. This is still the case today for the mosses, liverworts, psilotums, horsetails, ferns and club mosses. Seed ferns are now extinct.
The colonisation of drier land by plants depended on the evolution of a means of reproduction that did not require the sperm to swim through a film of water. This came about in the development of a pollen tube through which the sperm passes to reach the ovum. The first plants with a pollen tube were the gymnosperms, which appeared around 300 million years ago. There were four main kinds of gymnosperms — the cordaites, which are now extinct, and the cycads, ginkgos (maidenhair trees) and conifers.
It was also around 400 million years ago that the first animals ventured onto land. Except in the case of worms, this development involved some important structural changes that enabled them to resist drying out, to breathe atmospheric oxygen, and to move around from one place to another. The first of these requirements was met by the formation of a resistant outer skin, and the second by the development of cavities in the body into which air could pass and from which oxygen could be transported to the various tissues. Locomotion on land in the crustaceans, centipedes, spiders and, later, insects was made possible by modification of the limbs that already existed in earlier aquatic forms. The five-toed limbs of the vertebrates evolved directly from the fins of their fish ancestors.
The heyday of the amphibians was around 300 million years ago, when many diverse forms existed. By 200 million years ago, however, their numbers had declined dramatically, and their place had been taken by reptiles, including the earliest dinosaurs. Birds and mammals evolved directly from reptiles.
Reptiles, including the dinosaurs, showed extraordinary diversification, with different groups becoming adapted to many different kinds of habitat. Several aquatic groups evolved, some of which looked very much like fish, although they did not have gills and they breathed air through a respiratory tract. There were also various forms of flying reptiles, with wings spanning up to seven metres and made of leathery membranes, supported and extended by elongated fingers.
Between 60 and 70 million years ago, a great crisis occurred in reptilian history and many forms became extinct, including all the dinosaurs and flying reptiles and most of the large marine reptiles. Many other forms of life disappeared during this period of reptile extinction, including various microscopic foraminifera in the oceans and many aquatic animals, including the ammonites. Whatever the cause of this wave of extinction, placental mammals, birds, lizards, snakes, turtles, crocodiles, fishes and plants were relatively unaffected.
The earliest mammals came into existence about 200 million years ago, at about the same time that the dinosaurs were emerging as a distinct group; and there were animals very like modern echidnas wandering around 150 million years ago. But mammals remained a rather insignificant group during this period of reptile dominance.
The evolutionary transition from reptiles to mammals involved three especially important changes. First, except in the case of the egg-laying platypus and echidna, a mechanism evolved by which the embryo developed within the mother’s body, attached to maternal tissue by a placenta through which oxygen, carbon dioxide, nutrients and waste products passed to and from the embryo. A somewhat similar arrangement is found in a few reptiles, such as the Australian blue-tongue skink. Second, in all mammals the newborn young are cared for by the mother and nourished by milk from her mammary glands. Third, a mechanism developed in mammals and birds that maintained a more or less constant body temperature, relatively independently of muscular activity and environment. It has been suggested that similar mechanisms may have existed in dinosaurs.
The first true flowering plants, the angiosperms, emerged about 160 million years ago and, since that time, they have undergone spectacular diversification. They are now the dominant division of plants. They are made up of two main groups — the monocotyledons and dicotyledons. The seedlings of the monocotyledons, which include grasses, lilies, irises and crocuses, have a single leaf and the stems do not thicken. The seedlings of dicotyledons have two leaves and the stems become thicker as the plant matures.
After about 60 million years ago, an amazing evolutionary diversification took place among birds and mammals. The primates, for example, which had emerged during the last part of the dinosaur era, evolved into four main groups. The most ancient group is the prosimians, which includes lemurs, aye-ayes, lorises and tarsiers. The second group, the ceboids, consists of the monkeys of South America. These animals have tails by which they can hang from branches of trees, and the group includes marmosets, howler monkeys and spider monkeys. The third group is the ceropithecoids, the monkeys of Africa and southern Asia, and it includes baboons, mandrills, langurs, and macaques. These animals also have tails, but they cannot hang by them. The fourth group, the hominoids, which includes gibbons, orangutans, chimpanzees, gorillas and humans, do not possess tails.
The evolutionary sequence of life on Earth is depicted diagrammatically in Figure 2.2.
Figure 2.2 Some major developments in the history of life
Source: Stephen Boyden
The mechanism of evolution
According to the Darwinian explanation, evolutionary change comes about through natural selection. This process depends on the fact that, at any given time, the individuals in a population of living organisms are not genetically identical. This genetic variability is due partly to changes, or mutations, that occur spontaneously from time to time in the genetic material of the sex cells (gametes), and partly to the fact that genes occur in different combinations in different individuals.
Because the members of a population are not genetically the same, some of them are likely to be better suited than others to the prevailing conditions. These better suited individuals tend to be more successful in surviving and reproducing, and are therefore likely to contribute a greater number of individuals to the next generation. Their progeny will carry the genes that rendered their parents at a biological advantage. Consequently, generation by generation, a population can become increasingly well suited to the environment in which it lives.
Similarly, when a significant and lasting change occurs in the environment of a population, some individuals, because of the genetic variability in the population, may be better suited than others to the new conditions. These individuals are more likely to survive and successfully reproduce, passing on their genes to subsequent generations.
Not all populations adapt successfully in this way to environmental change. Indeed, the great majority of species that existed in evolutionary history eventually failed to adapt to new environmental conditions and became extinct.
The rate at which evolutionary adaptation occurs in a population following environmental change depends on a number of factors. Especially important among these is the frequency in the initial population of ‘favourable’ genes associated with resistance to the threats inherent in the new situation, and the extent to which such genes confer an advantage on the individuals that carry them (i.e. their selective advantage).
The mutation rate for individual genes is estimated to be around one mutation per 100,000 spermatozoa or ova, and most mutations are harmful rather than beneficial. The chances of a suitable or helpful mutation arising in an appropriate gene in a small population that is suddenly exposed to a new detrimental environmental condition are, therefore, negligible. In the long term, however, all major evolutionary change depends on the introduction of new genetic characteristics through random mutation.
The fossil record shows that the evolution of life has not been an entirely gradual process. There have been five periods of mass extinction resulting from major changes in global conditions. These occurred around 450 million years ago, 360 million years ago, 250 million years ago, 205 million years ago and 65 million years ago.
In the most severe of these mass extinctions — the one that took place about 250 million years ago — 57 per cent of all families became extinct, as did 83 per cent of all genera and 96 per cent of all species.
After a wave of extinction, many ecological niches are left vacant, and this encourages relatively rapid evolutionary change and diversification among surviving populations. This is what happened after the disappearance of the dinosaurs 65 million years ago. Before many millions of years had passed, the ecological niches that these creatures had vacated were occupied by new kinds of animals.
Energy and ecology
The rich diversity of plants and animals that exist in the modern world, including humans and their civilisation, could not exist without photosynthesis.
Plants use about half the energy they capture from sunlight in their own vital processes, eventually releasing it into the environment in the form of heat. The remaining energy takes one of several pathways. Dead plant tissue containing stored energy is broken down by bacteria or fungi, which make use of the energy in their own vital processes and eventually release it into the environment as heat. Some plant tissue is consumed by plant-eating animals, providing them with the energy they need for their life processes. Some of this energy is given off in the form of heat, while some of it is retained in the animals’ own tissues, eventually to be consumed either by carnivores or by microorganisms, and ultimately returned to the environment in the form of heat. This sequence of events is referred to as a food chain, with plants playing the role of producers, animals the role of consumers, and microorganisms and fungi the role of decomposers (Figure 2.3). Sometimes the chemical energy stored in plants is converted directly to heat through the action of fire.
Figure 2.3 The nature of food chains
Source: Stephen Boyden
A very small fraction of dead plant tissue avoids these various fates and only partially decomposes. Under certain conditions, like those that are likely to exist in swamps or bogs, decomposition of dead plant material may be incomplete due to lack of oxygen in the stagnant water and acidity resulting from the decay process. The soft, fibrous energy-containing material formed in this way is called peat. Downward pressure resulting from the accumulation of sediments above may eventually transform peat into coal.
Petroleum and fossil gas, which are also of organic origin, are produced by the breakdown of vast quantities of microscopic plants and animals in the oceans. Unlike coal, the liquid and gaseous hydrocarbons often migrate from their place of origin to become concentrated in distant reservoirs. The formation of the deposits of these fossil hydrocarbons spanned several hundred million years. They are now being used by humans as sources of energy at a rate that is several million times faster than the rate at which they were formed.
While the energy on which life processes depend comes from outside the biosphere in the form of light and is eventually returned to outer space as heat, the material components of living organisms come from the planet itself. An essential characteristic of life on Earth is the cycling within the system of chemical elements that are taken up from the environment, built into the tissues of living organisms, and then eventually released again into the environment — to become available for incorporation into new life.
Plants take up the various nutrients that they need for their growth from their immediate environment. Carbon is taken from the atmosphere and oxygen from the atmosphere, soil and water. All other essential nutrients are taken from the soil and water.
These nutrient cycles are essential for the sustainability of life in all natural terrestrial ecosystems.
The different nutrient cycles vary in complexity. The carbon and oxygen cycles are intimately connected and are relatively simple (Figure 2.4). The nitrogen cycle is more complicated. Plants take up the nitrogen that they need for growth from the soil and water in the form of sodium nitrate. This is made available largely through the activities of certain bacteria in the soil, some of which manufacture it from breakdown products of decomposers, and some by fixing free nitrogen from the atmosphere so that it becomes assimilated into organic compounds.
Indeed, the whole of multicellular life on Earth is ultimately dependent on the activities of microbes because of the essential roles that they play in the breakdown of the tissue of dead plants and animals and in the cycling of nutrients.
Figure 2.4 Carbon and oxygen cycles
Source: Stephen Boyden
The term ecosystem is applicable to any system of interconnected living organisms and the physical environment with which they interact, from a small pond or vegetable garden to a forest, a continent or the biosphere as a whole.
At a basic level we can recognise two broad classes of ecosystem, aquatic and terrestrial, although many ecosystems incorporate both aquatic and terrestrial elements. The biological characteristics of ecosystems are largely determined by their latitude and altitude and by temperature and rainfall.
Terrestrial ecosystems are conventionally classified as follows: desert; tropical grassland and savannah; tropical scrub forest; tropical rainforest; temperate grassland; temperate forest (deciduous, or eucalyptus and acacia); northern coniferous forest; and tundra (a treeless zone lying between the ice cap and the timber line in the northern hemisphere and also at the southern tip of South America). Cities can also be seen as ecosystems, although they are totally dependent on inputs from ecosystems elsewhere for their survival.
Individual species in an ecosystem are said to occupy a particular ecological niche, a term that refers not only to the physical space in which a species lives, but also to its functional role in the ecosystem.
Soil has been defined as the unconsolidated portion of the Earth’s crust that supports plant life, and it is made up of debris resulting from the weathering of rocks as well as organic matter. The fragments of rock may range in size from largish lumps, through sand, down to fine clay.
The chemical composition of soil influences plant growth and is largely determined by the kind of rock from which it is formed. Soil quality is also affected by the vegetation that it supports. Thus, the soil and vegetation develop together, each influencing the other, and each being influenced by, and to some extent influencing, the climate.
The organic content of soil consists of decomposing plant and animal matter, and the living microorganisms involved in this decomposition. It also contains numerous other microorganisms, such as those participating in the fixation of nitrogen from the atmosphere. Some components of decaying organic matter, like waxes, lignins and fats are relatively resistant to decomposition, and together they form a colloidal substance known as humus. Humus has an important influence on the capacity of the soil to support plant life.
Soil also contains many animals, including nematodes, millipedes, mites, insects, earthworms and burrowing amphibians, reptiles and mammals. There are believed to be around 1 million species of nematodes, most of them living in soil.
The living organisms in soil play a crucial role in the nutrient cycles on which all life depends. Although the organic component of soil may represent less than 0.1 per cent of the total soil mass, it can still amount to several tonnes per hectare. In the case of grassland, for example, the weight of organic matter in the soil in a given area is many times the weight of cattle, sheep, kangaroos or other herbivores that could be supported on that land.
Uniformity and diversity in nature
We have only to look around us anywhere in the natural environment to be struck by the amazing diversity among living organisms — diversity in habitat, size, shape and colour; and, among animals, diversity in means of locomotion and patterns of behaviour. Animal species also differ widely in their food sources and in their resistance to heat, cold, dryness and wetness; some are at home on the land, some in the water, some in the soil, and some in the air. Each is adapted, in its inheritable characteristics, to its particular ecological niche.
It is impossible to state precisely how many different kinds of life now exist on Earth, but it has been roughly estimated that there are some 7 to 10 million species of eukaryotic organisms (i.e. all organisms excluding bacteria, archaea and viruses).
Underlying all this diversity, however, there are some remarkable and essential uniformities. One of these fundamental universals is the fact that all forms of life depend on a continual supply of energy. Except in the case of a small proportion of microbial organisms, this energy was initially captured from sunlight by photosynthesis in green plants, converted into chemical energy and stored in organic molecules.
There is also a basic similarity in the complex chemical processes by which this energy is used in living cells, be they animal, plant or microbial. For example, a common denominator at the molecular level is adenosine triphosphate (ATP). In every kind of living organism this substance plays an essential part in the chemical reactions involved in the storage of energy and its eventual release, for instance, in the synthesis of complex molecules or the contraction of muscles.
The organic molecules of which organisms are made up also share the same basic characteristics. These molecules fall into four classes: carbohydrates, proteins, lipids (fats) and nucleic acids. Within these four main classes, however, there is immense diversity. In the case of proteins, for example, every species of animal and plant has many different proteins with different functions, like enzymes and hormones, or playing specific structural roles; and the proteins of each species are distinguishable from those of all other species. Indeed, subtle differences exist in the structure of proteins between individual members of the same species. This is why skin, or other organs, can only rarely be successfully grafted from one individual to another (except in the case of identical twins) unless special steps are taken to depress the immune system of the recipient. The rejection of a tissue graft from another individual is due to the fact that the immune system recognises the cells of the donor as ‘foreign’, and consequently sets up an inflammatory response, which ultimately destroys them.
Another universal is the fact that all life forms, with the exception of sub-microscopic viruses, have a cellular structure — ranging from single-celled organisms, like bacteria and amoebae, to the large multicellular plants and animals, which may be made up of hundreds of billions of separate cells with many different functions. But every one of these multicellular animals, and most of the multicellular plants, begin life as a single cell, formed by the union of two cells, the ovum and the sperm.
Also universal among cellular organisms is the means by which genetic information is passed from parents to their progeny, providing the instructions that result in the new organisms developing and functioning as members of the species to which their parents belong, and that determine all their other inherited characteristics.
The essential agent in this process is the genetic material of the cell, deoxyribonucleic acid (DNA). In animal and plant cells, chains of DNA are located in the cell nucleus and, in this situation (and, in some laboratory situations, outside the living cell), DNA is itself capable of self-replication. It contains, in coded form, most of the information that is necessary for the formation of the new individual.
The inheritable characteristics of all organisms are determined by the arrangement of four nucleotides (cytosine, thymine, adenine and guanine) in the genes, which are discrete areas or regions on the DNA chains.
Almost universal among plants and animals is the involvement of the sexual process at some stage in the reproductive cycle. This consists of the fusion of two separate cells (gametes) that, in the case of multicellular organisms, usually come from two different individuals, although in some species, they come from different parts of the same individual. In some very simple organisms the two gametes may be identical, but in all higher species of plants and animals they are clearly different. One, the male gamete, or sperm, is motile. The other, the female gamete, or ovum, is larger and sessile. The fusion of the two cells results in the new fertilised egg, or zygote, which contains twice the amount of DNA contained in each of the gametes. A mechanism known as meiosis results, however, in the amount of genetic material being halved, so that the total amount of DNA does not double at each generation.
The fertilised egg thus contains genetic material from two different individuals. Since it is unlikely that the material from each parent will be identical, it follows that the offspring will be genetically different, even if only slightly, from either parent.
The sexual process means that the genetic material in a population is being constantly reshuffled. From the evolutionary point of view, the importance of sexual reproduction lies in the fact that, unlike in asexual reproduction, the precise genetic make-up of the new individual is different from that of either parent. This has the effect of maximising the number of genetic combinations in the population, and so increasing the potential of the population to adapt to environmental change through natural selection.
While the mechanism of sexual reproduction explains the continual rearranging of genetic material in populations, it does not tell us how entirely new genetic characteristics come into existence. This happens through the process of mutation, which consists of a chemical change in a gene that is perpetuated when the gene replicates in cell division. The change then affects the particular characteristic of the organism for which the gene is responsible. Mutations are normally rare events, but their frequency can be increased by certain physical and chemical agents, such as ultraviolet light, radioactive radiation and mustard gas.
The great majority of mutations are deleterious, so that cells that carry them do not survive. Occasionally, however, a mutation arises that, by chance, increases the likelihood of the organism surviving and successfully reproducing in the habitat in which it lives.
Sometimes reproduction occurs without the involvement of a male organism. In this process, which is known as parthenogenesis, a new individual develops from an unfertilised egg. It is not uncommon among plants but rare in animals, although it has been recorded in some invertebrates, including nematodes, water fleas, aphids, some bees and parasitic wasps and in a few vertebrates, including some lizards, geckos and fish.
Despite these fundamental uniformities, evolution has given rise to a fantastic variety of structural forms, physiological mechanisms and ways of life. Let us look at just a few examples to illustrate the extent of this diversity, focusing on two key aspects of life — food intake and reproduction.
Food intake in plants
The range of ecological niches exploited by plants is vast, and very different forms of vegetation can be found in deciduous and coniferous forests of the northern hemisphere, dense evergreen forests of the tropical zones, eucalyptus forests of Australia and in mountainous terrains, savannah, marshlands, deserts, heathlands and sand dunes across the globe.
Each plant form is adapted, through evolution, to certain conditions of temperature, humidity, soil quality, soil wetness, light and wind.
While some water is essential for the survival and growth of all plants, enormous variation exists in the amount of water that different plants need. Some forms, like most reeds and bulrushes, cannot survive in soil that does not have a high water content, while others are adapted to extraordinarily dry conditions. Plants found in dry habitats often have small, leathery leaves. An extreme example is the desert cacti in which the leaves are hard, spiny structures that do not support photosynthesis. In these plants the photosynthetic process takes place in the fleshy stems, which are also organs for storing water. Their water content may account for up to 98 per cent of their weight.
There are many other kinds of adaptation to dry conditions in plants. One of these takes the form of short life cycles. Parts of the Australian desert may receive a reasonable rainfall only once in every few years. When this occurs, the previously parched and apparently lifeless ground suddenly becomes a mass of small flowering plants and, in a very short time, seeds are produced. If there is no further rain, the soil soon returns to a state of desiccation that contains myriads of drought-resistant seeds lying dormant until the next time it rains.
In most leafy plants the size of the pores, or stomata, on the leaves varies in response to changes in the moisture content of the soil and the humidity of the atmosphere, so controlling the rate of water loss by evaporation. In some plants that live in dry regions, the stomata are permanently sunken into the surface of the leaf, thus minimising evaporation, while in others the leaves are covered with hairs that have the same effect. In many plants the leaves fold up when conditions become dry and, in some forms, the leaves fold regularly after dark and sometimes in the late afternoon. In most plants only about 1 or 2 per cent of the water taken up by the roots is used in photosynthesis and the rest is released by the stomata into the atmosphere, through the process of transpiration.
A particularly interesting adaptation to nutrient deficiency in soils is seen in the carnivorous plants, of which there are at least 350 different species. These plants are usually found in swamps, bogs and peat marshes where acids have leached the soil of nutrients. Their prey may consist of insects and other invertebrates and sometimes even small birds and amphibians. Sundews, for example, are very small plants, usually not more than five centimetres across, and they have tentacles on the upper side of the leaf that secrete a clear sticky fluid to attract insects. As soon as an insect is caught by one tentacle, the others bend inwards towards it, so that the animal is thoroughly trapped. The tentacles then secrete enzymes to digest the insect tissues, and the soluble nutrients are absorbed by the leaf surface. Among other carnivorous plants is the well-known Venus flytrap, which occurs naturally on the coastal plain of North and South Carolina, in North America. Unlike carnivorous animals, carnivorous plants do not use their prey as a source of energy, but rather as a supplementary source of nutrients, especially nitrogen and phosphorus.
A more common way of acquiring nitrogen operates in legumes, such as clovers, vetches, lucernes, peas, beans, and acacias, and involves a symbiotic relationship between the plant and certain nitrogen-fixing bacteria known as rhizobia. When the plants are seedlings their root hairs are invaded by the rhizobia, and eventually these give rise to small nodules in which the bacteria live and multiply. These microorganisms fix free nitrogen and release it in the form of ammonia, which combines with carbon compounds in the plant cells to produce amino-acids. In agricultural systems the beneficial effects of growing legumes has been appreciated for over 200 years. Some of the fixed nitrogen is released into the soil around the legumes and so becomes available to other plants. If leguminous plants are ploughed back into the soil, much of the incorporated nitrogen becomes available for other crops. A crop of lucerne ploughed back into a field may add as much as 350 kilograms of nitrogen to the soil per hectare.
Turning to the procurement and assimilation of food in animals, the basic arrangement of the digestive tract — that is, a single mouth, a stomach and intestines containing digestive juices, and a single anus — is common to all multicellular animals, from mosquitoes to elephants, with the exception only of some simple forms, like sponges and flatworms. The extent of variation on this common theme, however, is enormous.
First, the great range of different kinds of food sources has resulted in wide variation in the structure of the mouth parts in different animals. Some examples are the grinding molars of herbivores (e.g. ox, horse); the sharp cutting teeth of carnivores (e.g. dog, tiger); the beaks of sparrows and pelicans; the sucking mouthparts of leeches; the powerful biting and chewing jaws of the praying mantis; the proboscis of mosquitos; the fly-catching tongue of chameleons; and the simple oral cavity of earthworms.
There is also great diversity in the various organs concerned in the digestive process and in the biochemical properties of the digestive juices. Because of the specificity of these adaptations, if animals are forced to consume a diet that is significantly different from that to which they are adapted through evolution, they are likely to show signs of ill health. Tigers will not last long on a diet of honey, and bee larvae could not survive on a diet of meat.
The following few examples illustrate the range of adaptations in the internal digestive organs. Termites eat mainly wood. Like other animals, however, they do not produce any enzymes in their digestive tracts that are capable of breaking down the lignin of which wood is made. They are entirely dependent for their nutrition and survival on certain microorganisms that live in their stomachs and which produce an enzyme to split lignin into soluble carbohydrate molecules that can be utilised by the termite.
There is wide variation in the structure and physiology of the gastro-intestinal tract among birds. In most species, the lower end of the oesophagus swells into a large storage chamber, the crop, where the food remains, sometimes for as long as two days, until the stomach can accommodate it. Crops are prominent in many grain-eating birds, allowing them to swallow a large volume of food in a hurry, so shortening their time of exposure to predators. In pigeons, the crop takes the form of a large double sac that not only stores grain, but which also secretes ‘pigeon’s milk’ for feeding the young birds.
In grain-eating birds, the stomach itself consists of two parts, the anterior glandular stomach, which secretes digestive juices, and the posterior muscular stomach, or gizzard. The gizzard is especially well-developed in grain-eating birds and is lined with horny plates or ridges that serve as millstones for grinding the food. This process is often furthered by the abrasive action of small pieces of grit that the birds have swallowed. The gizzard of the domestic goose may contain 30 grams of grit.
In carnivorous birds, the gizzard usually has much thinner walls and has a completely different function. In owls, gulls, swifts, grouse and some hawks it operates as a trap that stops sharp bits of bone and other non-digestible fragments from passing on through the alimentary canal. This material is rolled up into elongated ‘pellets’ which are regurgitated through the mouth.
A further example of an alimentary adaptation to a specific kind of diet is provided by the four ‘stomachs’ of ruminants such as cattle and giraffes. These animals tear the leaves off the plant they are eating with their incisors and swallow them almost immediately, without making any attempt to chew them up. The food bypasses the ‘first stomach’, or rumen, and goes directly to the smaller ‘second stomach’ or reticulum, where it is compacted into balls. At a later time, when the animal has stopped feeding, these balls, known as the cud, are regurgitated to the mouth. The cud is then properly chewed by the grinding action of the animal’s molars, before being swallowed a second time, this time to be retained in the rumen. This large organ represents about 80 per cent of the total volume of the four stomachs. It is colonised by bacteria and protozoa, which not only break down cellulose, as in termites, but also synthesise proteins, using urea and ammonia as nitrogen sources. Some of these microorganisms pass down the alimentary canal and are themselves digested, so contributing to the animal’s intake of amino-acids. Some of the products of the fermentation are absorbed directly by the lining of the rumen. The rest of the food passes into the omasum, or third stomach, which basically functions as a strainer, and then on to the abomasum. This is the true stomach, where digestive enzymes are secreted. Anatomically, the rumen, reticulum and omasum are actually expansions of the oesophagus.
There is also a great deal of variation among animals in the ways that they find and procure their food. Many species locate their food simply by going around looking for it, in much the same way as we would ourselves, using especially the senses of sight, smell and hearing. Clearly, there is a broad distinction between the techniques of herbivores and carnivores, in that carnivores, except in the case of scavengers, have not only to locate their food source, but also to catch it. Some groups of animals have specialised modes of food location and procurement. Bats, for instance, have evolved the mechanism of echolocation for detecting their prey in the night sky. The technique involves the emission of high frequency sounds and the detection, by means of highly specialised listening devices, of echoes of these sounds coming from objects in the environment. When the returning signal indicates that the object detected is of an appropriate size and is moving in the air, the bat flies rapidly and unerringly towards it, and catches it. The bat is able to discern from the signal whether the object is flying towards or away from it. A similar mechanism has evolved independently in dolphins, which also emit ultrasonic pulses, and the pattern of returning echoes provides them with a picture of the world around them.
In some carnivorous animals that feed in water, receptors have evolved that detect small electric impulses generated by the muscular movements of their prey. The platypus, which is effectively blind under water, detects small crustaceans and worms in this way. Frog tadpoles and some fish make use of similar mechanisms.
Any discussion about food acquisition in animals would be incomplete without reference to the farming practices of certain ants that live in tropical and sub-tropical regions on the American continent. Some of these species collect pieces of leaves or flowers from living plants and carry them back to the nest, where they cut them up into smaller pieces and mix them with saliva and faeces. The ants spread out the resulting compost in an underground garden, and then place pieces of mycelium from a certain kind of fungus on top of it. The fungus grows profusely, deriving nourishment and energy from the cellulose in the leaves or flowers. As the mycelium grows, the ants continually make cuts in it and, at the site of each cut, the fungus develops a nodular proliferation. These nodular proliferations are eventually harvested by the ants as a major food source. Some other ants in the same region make use of this principle, but use insect faeces or dead insects as a substrate for the fungal mycelium.
The ability to reproduce and so perpetuate the species is, of course, an essential feature of all forms of life.
Despite the underlying uniformities at the molecular level, the details of the processes of reproduction at the level of whole organisms vary enormously. First, let us note the all-important distinction between sexual and asexual reproduction. In asexual reproduction there is only one parent, which splits, buds or fragments to give rise to two or more new individuals, each of which have hereditary characteristics identical with those of the parent. Asexual reproduction is common among simpler forms of life, including bacteria, algae, fungi, mosses, protozoa, coelenterates and flatworms. In the case of the last group, if the animal becomes fragmented into several pieces, each may develop into a new whole animal. If a starfish is cut in two, each part will regenerate tissue to form a complete new starfish.
Many plants are capable of reproducing asexually. Some species, such as English elms and Lombardy poplars, may propagate by putting out ‘suckers’, so that new trees grow up from the distal roots of the parent trees. Reproduction by rhizomes or ‘creeping rootstocks’ (actually stems growing laterally underground) is common — as in bamboos, hops, asparagus, irises and many other plants. Some species, like potatoes, reproduce asexually by means of tubers.
Propagation of plants by means of cuttings is another example of asexual reproduction.
Indeed, asexual reproduction also occurs in higher animals, including humans, when a newly fertilised egg divides in the uterus to give rise to two or more genetically identical eggs, each of which develops as an independent organism. Today, as an outcome of scientific advances, it is now possible to bring about asexual reproduction artificially in mammals by means of cloning techniques.
Turning to sexual reproduction, we have already noted some basic differences between the simpler, more ancient plants, like mosses and ferns, and the more recent conifers and flowering plants. Let us now look at a few of the adaptations that have evolved in this last group.
The most striking feature of the reproductive processes of the flowering plants is the fact that, while wind sometimes plays a part in transporting pollen from flower to flower, the great majority of species rely entirely on insects or, in some cases, on small birds or mammals, to bring about pollination. For this to work, the insects have first to be attracted to the flowers so that they pick up pollen and later drop it off when they visit other flowers of the same species, where it can bring about fertilisation. The basic attractant for insects in the great majority of plants is food, in the form of nectar, which is produced at the base of the flower solely for this purpose. Another feature of the adaptation of the flowering plant is the development of petals, which are often displayed conspicuously and in bright colours, signalling the presence of nectar.
While this basic pattern is very common, there are many interesting and sometimes bizarre variations on the general theme. As Darwin noted in his remarkable book on orchids, these plants are especially interesting. In one species the shape and colour of the flower bears a strong resemblance to the female of a particular species of wasp, complete with eyes, antennae and wings. It even gives off a similar odour to that emitted by a female wasp that is ready to mate. Male wasps, deceived by this arrangement, attempt to copulate with the flower. In doing so, they pick up pollen, which they inadvertently deposit on the next flower that they mistake for a female wasp.
The dung lily is another interesting example. This plant gives off an odour similar to that of herbivore dung. When a dung beetle happens to fly overhead, it responds to the dung-like stimulus by dropping head first into the funnel-shaped flower. Because the inside of the flower is lined by small hairs pointing downwards, the beetle is unable to climb out and, if it happens to be carrying pollen from a previous encounter with a dung lily, some of this will come off and fertilise the ova. By morning, the flower tips over and the one-way hairs no longer prevent the beetle from escaping. On the way out, the beetle picks up additional pollen that was not available when it entered.
In multicellular animals, two main mechanisms exist for achieving union of egg with sperm. The first operates only in the case of animals that live, or at least mate, in water, and it involves the male liberating sperm into the water in the region where the female has recently laid her unfertilised eggs. Usually this is preceded by certain courtship behaviours to ensure that the male is at the right place at the right time. This method operates in most marine animals, from molluscs to true fishes, as well as in amphibians, which return to the water to mate. In frogs the male arranges himself on the back of the egg-laden female, keeping firmly in place by means of special clasping pads on the front of his forelimbs. He remains in this position until the female begins to lay her eggs, at which time he ejects spermatozoa into the water, some of which find and unite with ova.
The pattern in newts and salamanders is somewhat different. In the common newt of north-western Europe, Triturus vulgaris, mating takes place in the water and the male courts the female with a dance display involving a rapid waving movement of the end of his tail, which is turned back on itself, and so points forward. When the female is appropriately aroused, apparently partly as a result of a hormone discharged into the water from the male cloaca, the male newt deposits a mucilaginous bundle of spermatozoa, which the female picks up with her hind limbs and inserts into her cloaca, so that fertilisation takes place internally.
The main mechanism in land animals for bringing sperm and eggs in contact involves the insertion of the penis of the male into the genital tract of the female, followed by the ejection from the penis of spermatozoa which then swim their way to the ova. This mechanism exists in most insects, in some birds, and in all reptiles and mammals.
Different procedures operate in worms and some arthropods. The reproductive pattern of the earthworm is particularly complicated. These animals are hermaphrodites and during mating the two worms, heading in opposite directions, lie with their ventral surfaces in opposition and are held together by a sticky secretion. Each worm donates sperm to the other, and these are temporarily stored in a seminal receptacle. After the worms have separated, a glandular ring of thickened skin, the clitellum, secretes a membranous cocoon. As the worm frees itself from this cocoon, it discharges into it the ova produced in its own body as well as the sperms contributed by the other worm. As the cocoon slips off the worm its two openings constrict, and the fertilised eggs then develop inside it to produce new worms.
Another interesting mechanism has been observed in certain species of peripatus — caterpillar-like animals that live in moist forests in Africa, Asia, Australia and South America. They have many pairs of legs (13 to 43 pairs depending on the species), and they share characteristics of both the annelid worms and arthropods. In some species of peripatus, males have a special protuberance on their head that is used to carry around a drop of semen while it searches for a female. When a female is found, the male deposits the semen somewhere on the surface of her body, which causes a reaction to take place inside the female and, as a result, some specialised cells in her body transport the sperm to the ova in the uterus.
Reproduction in spiders is rather similar to the first part of this peripatus procedure. The male spider produces a ball of sperm-containing material that he picks up with one of his pedipalps, which are limb-like structures situated just in front of his four sets of legs. He then sets out in search of a female, which, in the case of most species, he must approach with considerable caution, identifying himself by certain species-specific signals in order to avoid being attacked and eaten. On reaching the female, the male inserts the spermatozoa into the female genital tract. In most cases, he then quickly makes his getaway although, in some spider families, the female consumes the male as soon as mating is completed.
A great variety of procedures exist among different animals for ensuring that males and females find each other for mating purposes. In many instances, the female gives off a specific odour that attracts males. In some moths, the males are exquisitely sensitive to these odours, responding when there are only about a hundred molecules of the specific substance per millilitre of air. It has been estimated that, in some kinds of moth, the male can detect a female over 4,000 metres away if a gentle breeze is blowing in the right direction.
In other species, the male attracts the female to a particular place or territory by emitting a distinctive call. This pattern is common among birds and frogs. In some bird species, the peacock and the Australian lyrebird being notable examples, males attract females by extending and displaying their tail feathers. In bowerbirds, the males achieve the same objective by constructing a ‘bower’ and decorating it with all sorts of colourful objects.
In the great majority of mammals, from rats, mice and shrews, to dogs, zebras, elephants and monkeys, females undergo a hormonally controlled cycle during which there are certain periods coinciding with ovulation when they are sexually attractive, or receptive, to males. The important result of this mechanism is that mating takes place only at times when fertilisable ova exist in the female genital tract. An outstanding exception to this generalisation is Homo sapiens, in which females can be sexually attractive to males at all times, and in which female receptivity is not restricted to a short period in the hormonal cycle.
In all mammals, milk produced in the mammary glands of females is, with one exception, the only source of food for their newborn offspring. Only one species is known — the guinea pig — in which newborn animals can survive without milk, eating solid food immediately after birth. Newborn guinea pigs, however, do drink milk from their mothers if it is available. At the other extreme is the young grey kangaroo, which weighs less than one gram when it is born and which, despite making its own way from the urogenital opening of the mother to the pouch, is otherwise completely helpless. Once in the pouch it immediately becomes attached to one of the nipples, and it does not leave the pouch, even for short periods, for nine months.
The examples given above only touch the surface of the vast range of different life forms that exist on Earth. The shelves of science libraries hold countless volumes providing detailed information on the structural, physiological and behavioural diversity encountered among living organisms. And, apart from all that has already been described, there is much more yet to be discovered.
Health and disease
For any living organism, health can be defined as follows:
Health is that physiological and behavioural state most likely to ensure survival and successful reproduction.
In the case of animals, health is thus consistent with optimal performance in terms of procuring food and water, avoiding predators, mating, giving birth and, in many species, successfully raising young. Health is a relative concept, however, in so far as the state of an organism can be anywhere on a continuum from optimum health at one extreme to near death at the other.
The health needs of animals, including Homo sapiens, are determined by their evolutionary background. This is because, through the processes of evolution, species have become well adapted in their innate biological characteristics to the conditions prevailing in the environment in which they are evolving. It follows that these conditions are capable of satisfying their health needs.
If an organism is exposed to conditions of life that differ significantly from those that prevail in its natural environment — that is, the environment in which it evolved — it is likely to be less well adapted to the new and different environment, and it is therefore likely to show signs of maladjustment. It will be less healthy than in its natural environment. This fundamental evolutionary health principle applies both to plants and animals.
Thus, in the case of plants, all species are biologically adapted to the conditions prevailing in the environment in which they evolved. That is, they are adapted to certain kinds of soil (e.g. depth, chemical constitution and water content), a certain intensity and quality of solar radiation, certain atmospheric conditions and a certain temperature range. If they are exposed to conditions that differ significantly from those to which they are adapted, they will not grow well or will die.
The evolutionary health principle is taken for granted by those responsible for the health of animals in zoos. Zoo keepers try to provide creatures in captivity with the same kind of food that they normally eat in the wild and, if possible, to ensure that they are exposed to temperatures similar to those of their natural habitat. If, like hippopotamuses, they naturally spend most of their time in water, they will be provided with water to wallow in. If the animals in the wild live in trees, then they will be provided with branches to climb. In other words, zoo keepers appreciate that the best guide to the health needs of any species is information about their conditions of life in the environment to which they are biologically adapted through evolution.
Clearly, the natural environment does not satisfy the health needs of all creatures all of the time; every animal eventually dies. But, in animal populations in their natural habitats, most of the individuals are in a state of good health most of the time. This applies to all species, including our own.
Parasitism and infectious disease
Many animals and plants live in intimate association with other organisms of different species. In some cases, these associations are of mutual benefit to both organisms as, for instance, in the case of lichens. Each lichen consists of an organised network of filaments of a fungus and cells of algae are entangled in this network. The algae carry out photosynthesis and so contribute large energy-containing food molecules to the complex, while the fungus provides support and absorbs water and soluble nutrients from the environment.
The word symbiosis is used to describe mutually beneficial associations of this kind. Such associations may involve animals or plants, and they are sometimes obligatory, sometimes optional, sometimes permanent and sometimes transient. In the case of the lichens, the algae can grow independently, but the fungus cannot.
Parasitism is a type of association between two organisms in which one of them, the parasite, is dependent on and lives at the expense of the other, the host. The host provides the parasite with a habitat and with nourishment. Internal parasites live within the host’s body, as in the case of the parasitic worms that are found in the intestines of animals, and various bacteria that live and multiply in internal organs. External parasites, like the fleas of mammals and the mistletoes of plants, live on the outside surface of the host, but still derive nourishment from it. Parasitism is extremely common, and there would not be a single species of multicellular animal or plant that does not normally harbour parasites of one kind or another. The parasites of mammals include not only many kinds of single-celled organisms, but also roundworms, tapeworms, hookworms, liver flukes, mange mites, lice, ticks and fleas.
While all parasites feed on nutrients supplied by their hosts, they can also cause varying kinds of damage. Under typical natural conditions animals are not seriously disadvantaged by the parasites they carry; but unnatural crowding or ill health from other causes often leads to damaging levels of parasitic infestation.
Many animal parasites (i.e. parasites that are animals) are highly host-specific, and can only establish themselves in, or on, the particular species to which they have become adapted through evolution. The tapeworm Taenia saginata cannot infect any species other than humans. Some other parasites can live in, or on, a wide range of host species. The adult form of Trichinella spiralis, the tiny worm that causes trichinosis in humans, can live in the small intestines of humans, pigs, walruses, rats, beavers, racoons, skunks, seals, bears, polar bears, wolves, lynx and many other mammals, as well as some birds.
Some animal parasites have complicated life cycles involving two or even three different species of hosts. The small tapeworm, Echinococcus granulosus, which lives in the intestines of members of the dog family, periodically sheds its final segment, which contains fertilised eggs, and these are excreted into the environment with the dog’s faeces. If the eggs are then taken up and swallowed by sheep or cattle, they hatch in the animal’s intestines, giving rise to very small ‘hooked embryos’. These embryos burrow through the walls of the intestine into the bloodstream and eventually become lodged in the lungs or liver, or occasionally some other organ, where each embryo develops into a round, fluid-containing sac called a hydatid cyst. In each of these cysts large numbers, sometimes millions, of minute ‘tapeworm heads’ grow from the cyst lining. The cysts remain in this form until the animal dies. If the affected organ is then eaten by a dog, the cyst is broken, and the tapeworm heads become attached to the wall of the dog’s intestine, to grow into adult tapeworms, so completing the life cycle. Hydatid cysts sometimes develop in humans who have swallowed the eggs of the tapeworm picked up from an infected dog.
While the vast majority of bacteria, protozoa and fungi are free-living and incapable of multiplying in the bodies of living animals and plants, some have become adapted through evolution to a parasitic way of life. In order to do so, they must acquire resistance to the host’s natural defence processes, which normally detect and eliminate foreign cells.
Infection with parasitic microorganisms often causes signs of overt disease. Well-known examples in plants include potato blight and wheat rust, both of which are caused by a fungus. Examples in humans include malaria and dysentery, which are due to protozoa; and tuberculosis and cholera, which are due to bacteria. The mechanisms by which disease-causing microbes cause damage to the host’s tissues are variable. In some infectious diseases the injury is due to toxic substances produced by the invading organism, as in the case of diphtheria and tetanus. In others, the inflammatory response of the host’s tissues is a major cause of distress, as in the case of tuberculosis, plague and pneumonia.
Most disease-producing bacteria, protozoa and fungi are obligatory parasites and they are incapable of multiplying outside the bodies of their hosts. There are, however, a few microorganisms that are normally free-living but which can, under certain circumstances, multiply in animal tissues and cause disease. An example is the bacterium Clostridium tetani, which lives naturally in the soil. If this organism gains access to the body of an animal through a wound and becomes surrounded by dead tissue in a relatively oxygen-free environment, it may be able to multiply. When it does so, it produces a protein that is extremely toxic for most mammals, causing the symptoms of tetanus.
Some of the more severe infectious diseases of both plants and animals are caused not by bacteria, protozoa or fungi, but by viruses, most of which are not visible under the light microscope. They range in size from the virus of foot and mouth disease, which has a diameter of only 21 millimicrons, to cowpox virus, which measures 210 x 260 millimicrons. Most bacteria measure 1,000 to 2,000 millimicrons. Viruses are relatively simple structures, with a central core of nucleic acid that is usually surrounded by a layer of protein. They are only capable of multiplying within living cells.
The presence of a virus in the cells of a host does not necessarily cause any serious harm, and viruses can sometimes lie latent in the body tissues for long periods without giving rise to any symptoms. Some plant viruses can cause a mottling effect on the leaves or on the petals of the flowers, without apparently interfering significantly with the plant’s viability. On the other hand, some viruses cause severe disease. In humans, infectious diseases due to viruses range from relatively mild conditions like the common cold and gastric flu, to herpes, influenza, measles, mumps, poliomyelitis and smallpox. In most virus diseases, the immune response is effective in bringing the infection to an end. In the more severe diseases, like poliomyelitis and smallpox, however, serious and lasting damage, and sometimes death, may come about before the immune response is effective.
When the tissues of a mammal or a bird are invaded by microbes of a kind that the animal has not experienced previously there usually occurs an immediate inflammatory response in which mobile cells known as phagocytes attempt to ingest and digest the intruding organisms. In cases when the microbes are able to withstand these mechanisms, a second phase of the defence process comes into play — the immune response. As a result, after about a week or 10 days, the animal’s tissues become more sensitive to this particular microorganism and its products. This increased sensitivity is associated with the appearance in the blood and other body fluids of antibodies, which are protein molecules that have the property of combining specifically with the macromolecules that the invading microorganism produces. Because of the presence of the antibodies and some other specific changes in the host’s tissues, the cellular reaction to the infectious agent is greatly enhanced and much more effective. In the case of cholera, for example, if infected humans live long enough for the immune response to develop properly, they are likely to survive and overcome the infection.
The specific immunity produced in this way is usually long-lasting, so that if the host becomes infected with the same kind of pathogenic organisms again at some time in the future, the initial response of the tissues is likely to be immediate, vigorous and effective. All artificial immunisation procedures are based on the principle of bringing about an immune response against disease-causing organisms, and so conferring protection against infection at a later date.
The health of ecosystems
The concept of health can be applied not only to individuals and populations, but also to ecosystems and, even, the biosphere as a whole.
A healthy ecosystem can be defined as one in which the rate of plant growth, or photosynthesis, is more or less constant from year to year. An unhealthy ecosystem is one in which the annual photosynthesis is progressively declining. Another characteristic of healthy ecosystems, at least on a regional level, is the maintenance of biodiversity. That is, a healthy ecosystem contains a wide range of interacting and interdependent species of plants, animals and microorganisms.
Box 2.1 Health needs of ecosystems
Source: Stephen Boyden
In most terrestrial ecosystems, organisms living in the soil play an essential role in maintaining the health of the system. Bacteria and fungi, of which there can be up to five tonnes per hectare, play a crucial role in making nutrients available for plant growth. In some productive ecosystems there can be 60 or more earthworms per square metre, and these contribute to the health of the system by breaking down large pieces of organic matter into humus.
Natural events, such as bushfires and unusual droughts, can temporarily interfere with ecosystem health.
As we shall discuss in later chapters, human activities sometimes seriously interfere with ecosystem health. From our understanding of ecosystem function and of the impacts of natural events and of humans on ecosystems, we can put together a working list of the health needs of ecosystems (Box 2.1).
Every species of animal has characteristic behaviour patterns that are appropriate and advantageous in its natural habitat. Some are aimed at procuring food, avoiding predators and building shelters or nests. Others are social, involving interaction with other members of the same species, as in sexual activity, parental behaviour, status-determining behaviour, mutual grooming and simply staying together in a group.
Many animals are fiercely territorial and will vigorously defend their living space against intruders, especially intruders of the same species and sex. In poplar aphids, for example, two females may be locked in a kicking and shoving contest for hours, even days, as each of them attempts to take over a potential gall site at the base of a leaf. Territorial behaviour is common among birds and, in many species, the male selects a territory in which he and his mate will build a nest and raise their young. Occasionally, birds attack not only members of their own kind, but any intruders, regardless of the species to which they belong. Australian magpies are notorious for their attacks on humans, and unprepared people who enter their territories in springtime can receive nasty wounds to their ears or heads. Territorial behaviour is also seen in many kinds of fish, reptiles and mammals.
On the other hand, there are also many species of animal that show no signs of territoriality, permitting other members of the same species to come and go without interference. There is also tremendous variation in other aspects of social behaviour. Some animals spend most of their lives as part of a social group, such as a flock or a herd, while others live continually with just one partner of the opposite sex. Others spend most of their lives in solitude.
There is an important distinction between innate and learned behaviour. Innate behaviour is programmed by the genetic information that the animal inherits from its parents. Learned behaviour is the consequence of previous experience, and of learning ways of achieving pleasurable sensations or experiences, and of avoiding unpleasant ones. Many actions are mixtures of both innate and learned behaviour.
The contribution of learning to behaviour patterns becomes increasingly important in animals higher in the evolutionary scale, and in humans it plays a bigger role than in any other species. But even in humankind there are still some innate or genetically determined tendencies to behave in certain ways in certain situations. Apart from obvious innate behaviours, such as sucking in newborn infants and the tendencies to eat when hungry and to drink when thirsty, it is highly probable that the innate behavioural tendencies of our species lie behind much social behaviour. There is a strong case for the view that the tendency for people to identify with an in-group, to seek the approval of the members of this group and to avoid their disapproval is innate. Environmental factors, however, especially cultural pressures, largely determine the criteria of approval and disapproval.
An aspect of animal behaviour that has received much attention over the years can be summed up in Alfred Tennyson’s famous phrase ‘Nature, red in tooth and claw’. This notion became linked in people’s minds with the phrase ‘struggle for survival’ that was in common use after the publication of Darwin’s theory of evolution. Today there are many television documentaries that focus on the tearing apart and eventual consumption of one animal by another. Indeed, carnivorous animals routinely engage in this kind of behaviour to keep alive.
There is an important perspective, however, that is often ignored in discussions on this topic. The great majority of animals spend most of their lives in a state of good health and relative tranquility, perhaps enjoying themselves most of the time. The really nasty bit — being attacked and eaten by another animal — is only a fraction of their whole life experience. In any case, it might be preferable to a long, drawn-out and painful death from chronic disease. Moreover, in some mammalian species, the release of endorphins during attack by a predator may well significantly reduce the pain and distress experienced by the prey.