Zoology Notes: 004 Chapter 1

Chapter 1: An Introduction Zoology is the scientific study of animal life. Animals differ from one another in size, structure, manner of life, and other features and over the years, man has accumulated tremendous amounts of information about them. Yet, we have so much more to learn about them. Zoology will enrich your own life by helping you understand the fascinating diversity of creatures that share this planet with us. But, before we go into any actual study of any animal, it is first essential for us to understand this definition of zoology. What do we mean by “scientific”? What are the characteristics of living things? What constitutes life? What are animals? What differentiates them from other living things? Science Science is a process for evaluating experimental and observed knowledge (the scientific method), a global community of scholars, and the organized body of knowledge gained by this process and carried by this community (and others). Natural sciences study nature; social sciences study human beings and society. The basic commitment of science is to collect objective data (facts that are observable and measurable) and then reach conclusions and formulate generalizations by analyzing such data. Scientists collect data either by observation or by controlled experimentation. When collecting data by observation, scientist must ensure that the data are as free as possible of subjective bias, recorded and analyzed instrumentally when possible, and extensive enough so that such factors as range of variability can be defined, preferably statistically. When collecting data by experimentation, scientists begin by asking questions, which they then try to answer. A testable question is called a hypothesis. Hypotheses are often tested by means of a controlled experiment, in which one or more experimental groups are compared with one or more control groups, under conditions that are held standard except for one factor, the variable. The number of organisms used is important: an experiment based on only a few test organisms is apt to be non-predictive and unreliable. Upon reaching a conclusion, the scientist tries to form a generalization and compares this generalization to others. A generalization that represents a cohesive statement of principle is known as a theory. It should be pointed out that no matter how firm the database upon which a scientific theory rests, the theory must always remain subject to revision in the light of additional data. The Scientific Method “Scientific” study involves a system that scientists use to get to the bottom of things. The observation of living things has generated a lot of questions about them. How they came to be? How are plants constructed? How do animals move? Why are animals and plants important? Scientists answer these and other questions by using an experimentbased process called the scientific method. The scientific method is a systematic way to describe and explain phenomena based on observing, comparing, reasoning, predicting, testing concluding, and interpreting. This is what science is all about. Rather than just being a set of facts that describe and explain the universe, science is a dynamic process wherein the excitement lies in the intriguing 2

observations and carefully crafted experiments devised to help us learn more about the world around us. The scientific method begins with observations that prompt us to ask the cause of these observations. These causal questions lie at the heart of the scientific method. Science is fundamentally about finding answers to these kinds of questions. To find answers to these questions, scientists use past experiences, ideas, and observations to propose hypotheses that may produce predictions. To determine if these predictions are accurate, scientists perform experiments. If the experimental results match the predictions of a hypothesis, the hypothesis is accepted; if they don’t, the hypothesis is rejected. The effect of this is to make scientific progress by revealing answers piece by piece. By testing a single hypothesis, a scientist has not ruled out other possible causes for an observation. To do so, he would have to devise alternative hypotheses, make predictions for them, and obtain experimental results to compare with the predictions. By this process, he may be able to reject all his hypotheses. Either way, he makes progress by testing several hypotheses, not just one. Although the scientific method is a powerful tool for answering some kinds of question, it is not foolproof. Most experiments do not distinguish other possible interpretations. Most of the time it is impossible to recreate conditions in the laboratory or consider all factors that influence the occurrence of events. Any conclusion marks an end to the scientific method for a particular experiment but it seldom ends the process of scientific inquiry. To the curious scientific mind, a conclusion is never the final answer. There is always something more to study, something new to learn. A Short History of Animal Life Although it is impossible to replicate conditions that happened billions of years ago, scientists through experimentation and deductive reasoning have come up with theories regarding the origin of life. The most ancient rocks found to date occur in western Australia and are about 4.2 billion years old. The oldest well-preserved fossils known to date are unicellular prokaryotes buried in silt that became the 3.4 billion-year-old sedimentary strata of the Fig Tree Group formation in South Africa. The earliest organic molecules probably formed abiotically, at a time when oxygen was lacking in the atmosphere. The heat of thermal springs may well have provided the first bonding energy for generating organic molecules and the first molecular or cellular unit that could be considered living in the sense of reproducing itself and taking up additional materials as nutrients from the environment. The sun provided an enduring and reliable flow of free energy for early organisms to tap. The simplest modern organisms that carry on photosynthesis are indeed prokaryotes, mostly known as cyanophytes (bluegreen algae). When eukaryotic cells evolved, some are thought to have acquired as internal symbionts cyanophytes that survived ingestion to become chloroplasts of these new autotrophic organisms. The entire period from 3.5 to 1 billion years ago may be referred to as the “age of blue-green algae” because during their long reign the blue-greens not only flourished but changed forever the composition of the earth’s atmosphere. They gave off great quantities of oxygen gas as waste product of photosynthesis. 3

Cells with nuclei first appear in sedimentary strata about 1.5 billion years old. The 1 billion-year-old Bitter Springs Formation of Australia contains beautifully preserved green algae (chlorophytes) showing nuclei and even nuclear division. To date, no evidence of multi-cellular life (other than algae) has been found in rocks other than about 700 million years. The best-preserved fossil assemblage of this age comes from the Ediacara Hills of Australia and includes a variety of soft-bodied metazoans: jellyfish, corals, segmented worms, together with a number of puzzling forms of unknown affinities. The scarcity of fossiliferous strata older than this seems to have been caused by a series of Precambrian glaciations that deeply eroded most continental surfaces. The Paleozoic Era. The Paleozoic Era spans 370 million years, from the beginning of the Cambrian Period to the end of the Permian Period. At the beginning of the Cambrian Period, a remarkable proliferation and diversification of invertebrate life took place within what appears to have been only a few million years. As a result, all of today’s major animal phyla, and several long extinct ones, are present in rocks of that age. The cause of this proliferation and diversification remains obscure. For one thing, the build-up of atmospheric O2 may have reached the concentrations necessary for this gas to diffuse downward throughout water masses so that bottom-dwelling creatures could begin to flourish. Then too, perhaps the extensive erosion caused by Precambrian glaciations raised marine concentrations of dissolved minerals, especially calcium, to some critical threshold necessary for the optimal functioning of nerves and muscles and the deposition of shells and skeletons. Over the ensuing 75 million years of the Ordovician Period, invertebrates and multicellular plants colonized the land, and vertebrate fishes appeared. During the Silurian Period, the first jawed fishes appeared and so flourished that the 50 million years of the ensuing Devonian Period are known as the “age of the fishes.” Amphibian fossils first appeared in rocks of later Devonian age, as the earliest land vertebrates. These animals characterized the 65 million-year Carboniferous Period (sometimes called the “age of amphibians”) but declined during the 50 million years of cooling and drying climates that marked the Permian Period. Reptiles, which diverged from early amphibian stock during the Carboniferous, were not so disadvantaged by these changes and began to proliferate and spread. Towards the end of the Paleozoic, drifting continents caused by expanding seafloors collided to form a supercontinent we call Pangaea. The continental collisions that ended the Paleozoic obliterated intervening marine habitats, allowed the terrestrial biota of previously isolated land masses to come into competition, and triggered a period of mountain building that affected climate and drained continental seas. Nearly half the known families of animal life became extinct. The Mesozoic Era. The Mesozoic is known as the “age of reptiles” for these became the dominant vertebrates and diversified into many forms, including the largest creatures that have ever walked the earth, marine species as massive as whales, and the most spectacular animals ever to soar the skies. The geographic spread of reptiles was facilitated by the fact that the land they so successfully invaded was the “one world” of Pangaea. The Jurassic Period saw the advent of birds and mammals and the start of the breakup of Pangaea, carrying terrestrial organisms apart, to new climates and destinies. The drifting continents fragmented further during the Cretaceous Period, while reptiles 4

continued to dominate the earth. Then something catastrophic took place: almost suddenly some 25 percent of all existing animal families disappeared, not only the dinosaurs and other still-successful reptilian groups, but a wide variety of marine invertebrates from giant ammonites (a type of mollusk) down to microscopic zooplankton. So far as we can tell, no Mesozoic land animal with a body mass over 25 kg survived into the Cenozoic. Time Scale

Era Cenozoic

Periods and Epochs Tertiary whales carnivores

humans horses elephants

65 Cretaceous primitive mammals crocodiles

13 5 18 0

turtles snakes

dinosaurs

Jurassic Mesozoic

plesiosaurs

winged reptiles

toothed birds ichthyosaurs

insects

Triassic ammonites

23 0 Permian 28 0

primitive reptiles

C arboniferous blastoids

34 5 40 5 42 5

amphibians crinoids

sharks bony fishes

Devonian sea stars, etc

Paleozoic

Silurian Ordovician

lobefins

cystoids

placoderms

scorpions arachnids limulus

ostracoderms coelenterates

50 0

mollusks

eurypterids

echinoderms

Cambrian brachiopods

trilobites

60 crustaceans annelids 0 Proterozic Archeozoic protozoans sponges Fig. 1.1. Distribution of major animal groups in the geologic record. Solid curving lines commence at time when each group first appeared, with broken lines indicating presumed earlier origins. Lines terminating with a  indicate when certain groups became extinct.; those ending in an arrow indicate that the group contains modern descendants. The time scale is in millions of years. (Modified from Jessop, 1995)

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In various parts of the world a thin layer of clay rich in iridium (an element common in meteorites) and soot separates a rich fossil record from a very sparse fossil record marking the onset of the Cenozoic Era. This suggests that a large meteorite impacted the earth, ejecting into the stratosphere such an enormous amount of particulate matter that months of darkness ensued, with plummeting temperatures and suppression of photosynthesis. Soot may have come from fire storms caused as continent-wide forests were ignited by the passage through the atmosphere of such a massive extraterrestrial object. Smoke from such fire storms would have intensified and prolonged the crisis of darkness and cold. Many animals would die of starvation or cold during the long darkness. Surprisingly, it appears that such mass extinction events have occurred with a periodicity of about 26 million years for as far back as such events can be traced. Scientists have come up with possible explanations why such events are so regular. If our sun, like many stars, is one of a binary pair, it may have a small companion star (already named “Nemesis”) with an orbit so eccentric that it passes through the solar system only once in 26 million years, towing a mass of comets collected from the dense Oort comet cloud that lies beyond Pluto. Alternatively, the unidentified celestial object may be a planet with a less far-flung orbit, which intersects Earth’s orbit only every 26 million years with its gravitational train of comets. Either way, this unknown celestial object may not be readily found, for it should now be at about its farthest point from the sun, not due to return for another 13 million years. The most profound effect of mass extinctions is that the survivors proliferate in a depopulated world providing opportunity for a great variety of genetic variants. Under these circumstances, evolution of a new biota can take place quite rapidly until the environment is again “saturated” with enough different life forms to maintain stable ecosystems over long periods of time. However, if a mass extinction is excessively severe, little may remain from which new forms can evolve. The Cenozoic Era. The past 65 million years have witnessed the explosive proliferation of birds and mammals. Hominid (human-like) fossils (mostly found in Africa) have been dated at an age of about 3 million years (Australopithecus afarensis), 1.7 million years (Homo habilis), and 1.5 million years for Homo erectus, with skulls transitional to Homo sapiens dating from 250,000 to 350,000 years ago. Although we have named ourselves “wise man,” to most of the living world we are catastrophe personified, for countless animal and plant species have diminished into endangerment or extinction as Homo sapiens has proliferated. Awareness and concern can still turn the tide, if we really are wise enough to conserve our biological heritage and guard ourselves, too, from extinction. Characteristics of Living Things Living things have common themes that separate them from non-living things. All living things have organization, undergo metabolism, growth and reproduction, respond and adapt to changes in the environment. Organization. All living things are made up of cells. Some organisms are made up of only one cell (unicellular) while others are made up of more than one (multi-cellular). In multi-cellular organisms, each cell has specific functions and specific roles in keeping the organism alive. Even within cells, specific structures have their own functions and roles. Even beyond the organism level, we find that organisms often group themselves 6

into populations. Populations of different species make up a community which is part of an ecosystem which makes up the biosphere. Metabolism. All living things undergo metabolism. Metabolism is the collective term for all the essential biochemical processes that go on inside the body. Digestion, respiration, photosynthesis, and the elimination of waste materials are only some of the processes constantly in progress. There are two phases of metabolism. Anabolism is the constructive or building up phase while catabolism is the destructive or breaking down phase. Growth. Living things grow and develop. Growth involves increase in size (increase in the number of cells for multi-cellular animals) and development involves change in shape and form. Reproduction. Living things reproduce. Reproduction is necessary for the perpetuation of the species. Reproduction can be asexual (single parent) or sexual (recombination of genes from two interacting parents). Irritability / Responsiveness. Irritability is defined as the ability of an organism to respond to stimuli. The stimulus may be simple, such as in bacteria moving away from or toward a heat source. It may be complex i.e. a bird responding to a complicated series of signals in a courtship ritual. Adaptation. Adaptation is the ability of an organism to change in response to the environment. The process of changing to promote survival includes: adaptability of the individual organism in direct response to some specific challenge and mutability (alteration) of genes and chromosomes producing a range of variability in offspring. Each species, whether plant or animal, exhibits an adaptation to the environment distinct from other animals. Animals Living things are classified on the basis of evolutionary relationships that exist among them. Modern scientists usually recognize five major kingdoms that represent all known species of living things. The table below shows the five kingdoms and the major differences that exist between them. Kingdom Monera

Type of Cell Prokaryotic

Protista

Eukaryotic

Plantae

Eukaryotic with walls Eukaryotic

Fungi

Cell Organelles No membrane around organelles, no plastids, no mitochondria All cell organelles Present but cells simpler

Cellular Organization Unicellular and/or colonial

Representative Blue-green algae, bacteria

Unicellular and/or colonial Multicellular with tissues Syncytial

Protozoa Higher plants

Lack plastids and Mushrooms, photosynthetic pigments molds Animalia Eukaryotic Lack plastids and Multicellular with Any animal without walls photosynthetic pigments tissues Table 1.1. Characteristics of five kingdoms (Modified from Storer et al, 6th Ed., 1979)

Plants vs. Animals. Although the basic unit of structure and function of both plants and animals is the eukaryotic cell and plant and animal cells are so much alike as to strongly suggest a common ancestor, there are two salient points of difference: (1) animal cells lack chloroplasts; and (2) animal cells are not enclosed in cell walls. 7

Other differences are noted in the table below. Mode of nutrition Extent of Growth Cell Wall Nervous System Mobility Primary Food Reserve

Animals Heterotrophic (do not photosynthesize, lack chloroplasts) Determinate

Plants Autotrophic (carry out photo synthesis, contain chloroplasts) Indeterminate

Absent Present in most

Made up of cellulose, rigid, inert Absent

Mostly mobile Glycogen (multiply branched glucose chain), saturated fats CO2 and nitrogenous wastes, kidneys needed in most animals

Mostly immobile Starch (unbranched glucose chain), unsaturated oils O2 from photosynthesis, CO2 from Waste metabolism, kidneys not needed since Products nitrogenous wastes not generated Table 1.2. Some major differences between animals and plants (Modified from Glinoga)

Importance of Zoology Animals are very important to people. Understanding how they function enables one to make wise decisions about many things that affect the individual, family, and the community. The use of organism to produce consumer needs is called biotechnology. Use of bacteria to turn milk into cheese or the use of live yeast to make bread rise are techniques of biotechnology. Farming, pest control, livestock management, nutrition, food processing, and food preservation also involve biotechnology. Animals provide us with food, non-edible economic products, biomedical products, research material. They also have ecological, aesthetic, and affectional value. Food. Livestock, game, fish, shellfish, honey, eggs, dairy products, exotic fare such as insects, grubs, and highly relished Palolo worms are just some examples of animals and animal products that we eat to nourish our bodies. Transport and Labor. Horses, donkeys, llamas, camels, dogs, oxen, buffalos, and elephants are all still used in different parts of the world for transport and labor. Non-edible Economic Products. Leather, down, fur, silk, wool, ivory, limestone, chalk have various uses as clothing, shoes, accessories and ornaments. Biomedical Products. We use venom from snakes to make anti-venom. Pig heart valves may be used to replace diseased human heart valves. Insulin and antibodies for protective inoculation against various diseases are of animal origin. Research. Laboratory animals are used to create animal models of human diseases and their treatment. Ecological Value. Animals are essential parts of the food chain Plant eaters (herbivores) are a source of food for carnivores (meat eaters) and omnivores (plant and meat eaters). They are also essential for the pollination of most flowers and as agents of biocontrol. Aesthetic Value. Animals have been subjects and inspirations for works of arts, from cave paintings to present day creations. Some cultures revere totem animals and cultivate in themselves the positive attributes they perceive in animals. Affectional Value. Pets and residents in wildlife parks fulfill various non-economic human needs. They are even used by some psychotherapists in their work with patients. 8

Branches of Zoology Since zoology presents a wide range of topics, scientists often choose a specific category to study. Some zoologists, for example, devote their time to studying animals belonging to one particular taxonomic group. Others study one or more aspects of animal structure, function, or behavior, often using a comparative approach. Here are only a few of the branches of science that fall under the scientific study of animal life. Taxonomy – classification and naming of plants and animals Botany – plant life Zoology – animal life Protozoology – animals that are basically unicellular Helminthology – worms (mainly parasitic) Entomology – insects Parasitology – organisms that live and subsist on or in other organisms Ichthyology – fishes Herpetology – amphibians and reptiles Ornithology – birds Biochemistry – chemical compounds and processes in living organisms Molecular biology – molecules and processes in cells Cytology – cell structures and function Histology – microscopic structure of tissues Gross anatomy – non-microscopic structures of organisms Embryology – growth and development of the new individual Physiology – living processes or functions within organisms Nutrition – use and conversion of food substances Genetics – hereditary traits and their transmission Ecology – relationships between biotic (living) and abiotic (physicochemical) environments Important Contributors to Zoology

Our present knowledge about this subject is based on previous works of past scientists. Zoology would not be as advanced as it is today if not for the great pioneers of the science. Here are only some of the scientists who contributed greatly to the scientific study of living things. Aristotle, 350 B.C. – description of plants and animals and theories of hereditary production and evolution 9

Robert Hooke, 1665 – coined the term “cell” describing the texture of cork using magnifying lenses Anton von Leeuwenhoek, 1667 – microscopic discovery of bacteria, protozoa and spermatozoa Fig. 1.2. Carolus Linnaeus

Carolus Linnaeus, 1735 – basis for modern classification of living things; binomial nomenclature Matthias Schleiden (a botanist) and Theodor Schwann (a zoologist), 1839 – put forth the thesis that cells were the units of structure in plants and animals.

Rudolf Virchow, 1855 – stressed the role of the cell in pathology and stated that all cells came from pre-existing cells “Omni cellulae e cullula” Charles Darwin and Alfred Wallace, 1859 – foundations of modern theory of evolution Louis Pasteur, 1860 – conclusive experimental refutation of the theory of spontaneous generation Gregor Mendel, 1865 – foundations of genetics Fig. 1.3. Gregor Mendel

James Watson and Francis Crick, 1953 – discovered the structure of DNA

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